Copernicus: Sentinel-1 — The SAR Imaging Constellation for Land and Ocean Services
Sentinel-1 is the European Radar Observatory, representing the first new space component of the GMES (Global Monitoring for Environment and Security) satellite family, designed and developed by ESA and funded by the EC (European Commission). The Copernicus missions (Sentinel-1, -2, and -3) represent the EU contribution to GEOSS (Global Earth Observation System of Systems). Sentinel-1 is composed of a constellation of two satellites, Sentinel-1A and Sentinel-1B, sharing the same orbital plane with a 180° orbital phasing difference. The mission provides an independent operational capability for continuous radar mapping of the Earth with enhanced revisit frequency, coverage, timeliness and reliability for operational services and applications requiring long time series.
Table 1: Copernicus is the new name of the former GMES program 1)
The overall objective of the Sentinel-1 mission is to provide continuity of C-band SAR operational applications and services in Europe. Special emphasis is placed on services identified in ESA's GSE (GMES Service Element) program. Additional inputs come from on-going GMES projects funded by ESA, the EU, and ESA/EU member states. The Sentinel-1 mission is expected to enable the development of new applications and meet the evolving needs of GMES, such as in the area of climate change and associated monitoring. 2) 3) 4)
The Sentinel-1 mission represents a completely new approach to SAR mission design by ESA in direct response to the operational needs for SAR data expressed under the EU-ESA GMES (Global Monitoring for Environment and Security) program. The mission ensures continuity of C-band SAR data to applications and builds on ESA's heritage and experience with the ERS and Envisat SAR instruments, notably in maintaining key instrument characteristics such as stability and accurate well-calibrated data products.
The key mission parameters are revisit time, coverage, timeliness combined with frequency band, polarization, resolution and other image quality parameters. Short revisit time demands for an appropriate orbit selection and large swath widths.
The baseline mission concept under development is a two-satellite constellation, with four nominal operational modes on each spacecraft designed for maximum compliance with user requirements. 5) 6) 7) 8) 9) 10)
• Orbit: Sun-synchronous near-polar orbit, repeat cycle of 12 days, cycle length of 175 days
• Operational modes:
- Stripmap mode (SM): 80 km swath, 5 m x 5 m resolution, single-look
- Interferometric Wide Swath mode (IWS): 240 km swath, 5 m x 20 m resolution, single-look
- Extra Wide Swath mode (EWS): 400 km swath, single-look
- Interferometric Wide Swath mode (IWS): 240 km swath, 25 m x 80 m resolution, 3-looks
- Wave mode (WM): 20 km x 20 km, 20 m x 5 m resolution, single-look
• Polarization: Dual polarization for all modes VV+VH or HH+HV
- Consistent, reliable and conflict free mission operations
- Near real-time delivery of data within 3 hours (worst case) with 1 hour as goal
- Data delivery from archive within 24 hours
• Sensitivity: NESZ (Noise Equivalent Sigma Zero), σo = -25 dB
- Stability = 0.5 dB
- Accuracy = 1.0 dB
• Ambiguity ratio: DTAR (Distributed Target Ambiguity Ratio) = -25 dB
In April 2007, ESA selected TAS-I (Thales Alenia Space Italia) as prime contractor for the Sentinel-1 spacecraft (overall satellite design & integration at system and subsystem level, including the design of the SAR antenna's transmit/receive modules). ESA awarded the contract to TAS-I on June 18, 2007 at the Paris International Air Show. EADS Astrium GmbH of Friedrichshafen, was in turn awarded a contract by TAS-I to build the radar imaging payload for Sentinel-1, including the central radar electronics subsystem developed by Astrium UK. The objective of Sentinel-1 is to assure C-band SAR data continuity for the user community currently provided by Envisat and ERS-2. 11)
Three priorities (fast-track services) for the mission have been identified by user consultation working groups of the European Union: Marine Core Services, Land Monitoring and Emergency Services. These cover applications such as: 12)
• Monitoring sea ice zones and the arctic environment
• Surveillance of marine environment
• Monitoring land surface motion risks
• Mapping of land surfaces: forest, water and soil, agriculture
• Mapping in support of humanitarian aid in crisis situations.
Unlike its more experimental predecessors ERS-1, ERS-2 and Envisat that supply data on a best effort basis, operational satellites like Sentinel-1 are required to satisfy user requirements and to supply information in a reliable fashion with the data provider accepting legal responsibility for the delivery of information.
In March 2010, ESA and TAS-I signed a contract to build the second Sentinel-1 (Sentinel-1B) and Sentinel-3 (Sentinel-3B) satellites, marking another significant step in the Copernicus program. 13)
As part of the Copernicus space component, the Sentinel-1 (S1) mission is implemented through a constellation of two satellites (A and B units) each carrying an imaging C-band SAR instrument (5.405 GHz) providing data continuity of ERS and Envisat SAR types of mission. Each Sentinel-1 satellite is designed for an operations lifetime of 7 years with consumables for 12 years. The S-1 satellites will fly in a near polar, sun-synchronized (dawn-dusk) orbit at 693 km altitude. 14)
The Sentinel-1 mission, including both S-1A and S-1B satellites, is specifically designed to acquire systematically and provide routinely data and information products to Copernicus Ocean, Land and Emergency as well as to national user services. These services focus on operational applications such as the observation of the marine environment, including oil spill detection and Arctic/Antarctic sea-ice monitoring, the surveillance of maritime transport zones (e.g. European and North Atlantic zones), as well as the mapping of land surfaces including vegetation cover (e.g. forest), and mapping in support of crisis situations such as natural disasters (e.g. flooding and earthquakes) and humanitarian aid.
In addition, the 12-day repeat orbit cycle of each Sentinel-1 satellite along with small orbital baselines will enable SAR interferometry (InSAR) coherent change detection applications such as the monitoring of surface deformations (e.g. subsidence due to permafrost melt) and cryosphere dynamics (e.g. glacier flow).
Figure 1: Artist's view of the deployed Sentinel-1 spacecraft (image credit: ESA, TAS-I)
The spacecraft is based on the PRIMA (Piattaforma Italiana Multi Applicativa) bus of TAS-I, of COSMO-SkyMed and RADARSAT-2 heritage, with a mission-specific payload module. Attitude stabilization: 3-axis, attitude accuracy = 0.01º (each axis), orbital knowledge = 10 m (each axis, 3σ using GPS).
The spacecraft structure provides the accommodation for all platform and payload units. A box type structure has been adopted using external aluminum sandwich material, with a central structure in CFRP (Carbon Fiber Reinforced Plastic). A modular approach has been taken whereby the payload is mounted to a dedicated part of the structure, allowing separate integration & test of the payload before integration to the main part of the structure carrying the platform units. This has many advantages for the overall AIT (Assembly, Integration and Test) process. 15) 16) 17) 18) 19) 20)
The PRIMA platform comprises three main modules, which are structurally and functionally decoupled to allow for a parallel module integration and testing up to the satellite final integration. The modules are: 21)
1) SVM (Service Module), carrying all the bus units apart from the propulsion ones
2) PPM (Propulsion Module), carrying all the propulsion items connected by tubing and connectors
3) PLM (Payload Module), carrying all the payload equipment including the SAR Instrument antenna.
Figure 2: 3D exploded view of the Sentinel-1 platform (image credit: TAS-I)
TCS (Thermal Control Subsystem): The TCS provides control of the thermal characteristics and environment of the Satellite units throughout all phases of the mission. In general the TCS is passive, with the control provided by means of standard techniques such as heat pipes, radiators and MLI (Multi-Layered Insulation). Survival heaters are provided to prevent units becoming too cold during non-operative phases.
EPS (Electric Power Subsystem): The EPS uses two solar array wings for power generation. Each wing consists of 5 sandwich panels using GaAs triple junction solar cells. The average onboard power is 4.8 kW (EOL), the Li-ion battery has a capacity of 324 Ah. The PCDU (Power Control and Distribution Unit) is designed to provide adequate grounding, bonding & protection for the overall electrical system (e.g. by use of fuses) and must also be integrated into the satellite FDIR concept to ensure that adequate power resources and management are available in the event of on-board failures. Li-Ion battery technology has been selected for the batteries in view of the large benefits offered in terms of mass and energy efficiency.
The spacecraft dimensions in stowed configuration are: 3.4m x 1.3 m x 1.3 m. The Sentinel-1 spacecraft has a launch mass of ~2,200 kg, the design life is 7.25 years (consumables for up to 12 years). 22) 23)
Since the B2 Phase of Sentinel-1, a commonality approach with Sentinel-2 and Sentinel-3 was introduced and deeply investigated, to optimize and minimize as much as possible new developments, HW procurement and operations costs. Besides the differences among payload instruments and their relative required performances, each of these three satellites have its own orbital parameters, as well its own specific requirements. 24) 25)
Table 2: List of some Sentinel-1, -2, -3 characteristics and key requirements impacting on end-to-end performance 26)
The Sentinel-1 spacecraft design is characterized by a single C-band SAR (Synthetic Aperture Radar) instrument with selectable dual polarization, a deployable solar array, large on-board science data storage, a very high X-band downlink rate, and stringent requirements on attitude accuracy and data-take timing. In addition, the spacecraft will embark the LCT (Laser Communication Terminal) unit allowing downlink of recorded data via the EDRS (European Data Relay Satellite). 27) 28)
Table 3: Main parameters of the Sentinel-1 spacecraft
AVS (Avionics Subsystem): The AVS performs both Data Handling & Attitude/Orbit Control functions. This is realized through the concept of an integrated control system that performs the control of the platform and payload. The AVS performs all data management & storage functions for the satellite, including TM/TC reception and generation, subsystem & unit monitoring, autonomous switching actions and synchronization. The AVS includes the AOCS processing and the interfaces to the AOCS sensors Star trackers, fine sun sensors, and fine gyroscope and actuators, 4 reaction wheels, 3 torque rods, 14 thrusters, 2 solar array drive mechanism. 29)
The AOCS comprises all means to perform transfer- and on-orbit control maneuvers and to control all necessary satellite attitude and antenna pointing states during all mission phases, starting at separation from the launcher until de-orbiting of the satellite at end of life. This includes the attitude steering of the LEO satellite to provide both yaw and roll steering capability. At present, a dedicated precise orbit predictor is implemented within the AOCS, in addition to making use of the data uploaded to the payload by the GPS constellation. The AOCS (Attitude and Orbit Control Subsystem) is able to perform some functions autonomously and it is supported by a very reliable FDIR scheme (Ref. 15). Telecommand data will be received from the TT&C subsystem and will be decoded and deformatted in the AVS.
AOCS consists of the following sensors and actuators: fine sun sensors, magnetometers, gyroscopes, star trackers, GPS receivers, magnetic torquers, a reaction wheels assembly and a monopropellant (hydrazine) propulsion system. The propulsion system has 3 pairs of 1 N orbit control thrusters and 4 pairs of reaction control thrusters for attitude correction. Every pair is made up of a prime and a redundant component. The attitude control thrusters are fired when the spacecraft enters RDM (Rate Damping Mode) after separation, damping any residual rotation left by the launcher upper stage and achieving a spacecraft pitch rotation of -8 times the orbital period. In the subsequent AOCS mode, called SHM (Safe Hold Mode), magnetotorquers and reaction wheels maintain the attitude and reduce the pitch rotation rate to twice the orbital period.
The periodic behavior of the Earth's magnetic field in a polar orbit and the polarization of the angular momentum with the loading of the reaction wheels allow the magnetotorquers to maintain this pitch rate while aligning the spacecraft –Y axis with the orbit normal, which in a dusk-dawn orbit coincides with the direction to the Sun (Figure 3). When the appendages deployment commences, the effect of the gravity gradient torque dominates over the magnetic torque, resulting in the alignment of the S/C X axis (appendages axis) with the nadir direction, maintaining thus a pitch rate equal to the orbital period. Upon ground telecommand, a transition into the NPM (Normal Pointing Mode) occurs, where the spacecraft performs a fine attitude control based on the use of reaction wheels in close loop with star trackers, gyroscopes and GPS, and magnetotorquers for wheel unloading (Ref. 20).
Figure 3: Sentinel-1A stowed representation (in RDM and SHM). +X S/C axis points towards the flight direction. S/C Y axis is aligned with the Sun direction. Solar Array –Y illuminated when stowed (image credit: ESA, Ref. 20)
Figure 4: Architecture of the avionics subsystem (image credit: TAS-I)
Table 4: Sentinel-1 attitude steering modes (Ref. 179)
Figure 5: Spacecraft power generation and distribution (image credit: TAS-I)
PDHT (Payload Data Handling & Transmission) subsystem (Ref. 24):
The commonality process is driving the spacecraft design with the objective to satisfy the needs of three different missions within the same product. This involves several Sentinels subsystems: in particular, TAS-I was selected to coordinate the common design of two assemblies: 30)
• TXA (Telemetry X-band transmission Assembly) 31)
• XBAA (X-Band Antenna Assembly)
The objective of the PDHT subsystem is to provide the services of data acquisition, storage and transmission to the ground in X-band. After having acquired observation data from the DSHA (Data Storage and Handling Assembly), the TXA executes encoding, modulation, up-conversion, amplification and filtering; the X-band signal provided at the TXA output is then transmitted by an isoflux, wide coverage antenna, included in the XBAA.
To summarize, the performance requirements on TXA specification took into account the different needs of the Sentinels, allowing a fully recurrent units approach: beside a specific TXA layout due to accommodation needs, the modulator, TWTA, and RF filter are exactly the same for the three Sentinels.
After the selection of the TXA & XBAA suppliers (TAS-España and TAS-I IUEL respectively), an agreement was reached between ESA and the Sentinel prime contractors on the way to handle the common design and procurement for TXA and XBAA.
Besides strong efforts to manage different needs coming from different missions, the commonality activities performed in the frame of Copernicus Sentinels enable an effective optimization of costs and development time for those subsystems selected for a common design.
To provide flexibility in the downlink operation, the PDHT is designed with two X-band independent links. The PDHT provides an overall input/output throughput of about 1950 Mbit/s, with a payload input data rate of 2 x 640 Mbit/s (multi-polarization acquisition) or 1 x 1280 Mbit/s (single-polarization acquisition) and a transmitted symbol rate of 2 x 112 Msample/s. The data storage capacity is > 1410 Gbit at EOL.
The provided antenna isoflux coverage zone is about ±64º with respect to nadir to allow link establishment with the ground starting from the ground antenna elevation angle of 5º above the horizon.
Figure 6: The PDHT (Payload Data Handling & Transmission) subsystem (image credit: TAS-I)
Legend of Figure 6:
• DSHA (Data Storage & Handling Assembly)
• TXA (Telemetry X-band transmission Assembly)
• XBAA (X-band Antenna Assembly)
Table 5: Main performance characteristics of the PDHT
The TXA architecture provides two redundant X-band channels with the same output power (16 dBW) and useful data rate (260 Mbit/s). Cold redundancy is implemented at channel level. The main elements of the assembly are:
- X-band modulators, developed by TAS-F, are fully compliant with ECSS and modulation standard
- TWTA (Traveling Wave Tube Amplifiers), provided by TAS-B (ETCA), deliver up to 60 W RF power
- OMUX (Optical Multiplexer), developed by TAS-F, filters and combines both channels and provides out of band rejection.
To achieve good spectral confinement and especially to ensure that the emission levels in the adjacent deep space band (8400 to 8450 MHz) are respected, both baseband filtering with a roll-off of 0.35 (0.35-SRRC) and filtering techniques have been applied. In addition, 6-pole channel band pass filters have been implemented in the OMUX. The 6-pole solution provides two main advantages in front of other less selective solutions, such as 4-pole:
- It filters our more efficiently the regrowth of baseband filtered 8PSK carrier due to the gain nonlinearity of the TWTA, thus allowing for a better overall DC efficiency
- It is compatible with data rates up to 300 Mbit/s per channel by adjusting the frequency plan (increase of frequency spacing between channels).
Figure 7: Architecture of the TXA (image credit: TAS)
Table 6: Summary of the key performances of the Sentinel TXA
PRP (Propulsion Subsystem): The PRP is based on 14 RCTs (Reaction Control Thrusters) located in 4 different sides of the spacecraft, provides the means to make orbit corrections to maintain the requested tight orbit control throughout the mission. Initially, corrections are required to reach the final orbit position after separation from the launcher. During the mission, some infrequent corrections to the orbit are necessary to maintain the requirements upon the relative and absolute positioning of individual satellite. The thrusters located on the –Z side of the satellite are specifically dedicated to attitude control during the safe mode.
Figure 8: Sentinel-1 satellite block diagram (TAS-I, ESA, Ref. 15)
Figure 9: Stowed satellite views (image credit: TAS-I)
RF communications: Onboard source data storage volume of 900 Gbit (EOL). TT&C communications in S-band at 4 kbit/s in uplink and 16, 128, or 512 kbit/s in downlink (programmable). Payload downlink in X-band at a data rate of 2 x 260 Mbit/s.
The Copernicus Sentinel spacecraft are the first ESA Earth Observation spacecraft to implement communications security on the command link. It has been decided to secure the spacecraft from unauthorised command access by adding a security trailer to the command segments which are sent to the spacecraft. The trailer is composed of a Logical Authentication Counter and a Message Authentication Code. The latter is obtained by performing cryptographic encryption of the hash value of the command segment and the Logical Authentication Counter. Only parties in possession of the right key can perform this operation in a way that the command segment is accepted by the spacecraft. The concept applies to all Copernicus Sentinel spacecraft. 32)
Science data compression: Currently, the most promising solution seems to be the FDBAQ (Flexible Dynamic Block Adaptive Quantization) approach as proposed by ESA; 3 output bits would be sufficient for most of "typical" acquisitions over various targets, while few high reflectivity scenes would need 4 bits, making the expected average output bit rate little higher then 3 bits, thus lower then the estimated 3.7 bits for the ECBAQ (Entropy-Constrained Block Adaptive Quantization) compression. 33) 34) 35) 36)
Data delivery: Sentinel-1 will provide a high level of service reliability with near-realtime delivery of data within 1 hour after reception by the ground station, and with data delivery from archive within 24 hours.
OCP (Optical Communication Payload): In parallel to the RF communications, an optical LEO-GEO communications link using the LCT (Laser Communication Terminal) of Tesat-Spacecom (Backnang, Germany) will be provided on the Sentinel spacecraft. The LCT is based on a heritage design (TerraSAR-X) with a transmit power of 2.2 W and a telescope of 135 mm aperture to meet the requirement of the larger link distance. The GEO LCT will be accommodated on AlphaSat of ESA/industry (launch 2012) and later on the EDRS (European Data Relay Satellite) system of ESA. The GEO relay consists of an optical 2.8 Gbit/s (1.8 Gbit/s user data) communication link from the LEO to the GEO satellite and of a 600 Mbit/s Ka-band communication link from the GEO satellite to the ground. 37)
Since the Ka-band downlink is the bottleneck for the whole GEO relay system, an optical ground station for a 5.625 Gbit/s LEO-to-ground and a 2.8 Gbit/s GEO-to-ground communication link is under development.
Table 7: Technical data of the LCT generations 38)
Ground segment: Spacecraft operations is provided by ESOC, Darmstadt, while the payload data processing and archiving functions (including the planning for SAR data acquisitions) are provided by ESRIN, Frascati. Options are being provided to permit some functions to be outscored to other operating entities.
Figure 10: Isometric views of the deployed satellite (image credit: TAS-I)
Figure 11: SAR antenna deployment test supported by zero gravity deployment device (solar array in stowed position), image credit: TAS-I, (Ref. 21)
Figure 12 shows the fully integrated Sentinel-1A spacecraft with the SAR antenna and the solar array wings in stowed position. The figure shows the Sentinel-1 spacecraft already mounted on the shaker and ready for sine vibration testing after it has successfully passed the Mass Properties measurements (namely center of mass and inertia moments). Successful completion of vibration and acoustic testing has been followed by the deployment tests of both the SAR antenna and the solar array. Each solar array is tied down on four hold down points by dedicated Kevlar cables. Wing deployment is purely passive, driven by springs, and actuated upon activation of specific thermal knives devices. The time to complete deployment of one wing lasts about 3.5 minutes since the last cable cut. In the end position, the solar array panels are mechanically latched.
• Prior to shipment to the launch site in late February 2014, the Sentinel-1 spacecraft has spent the last couple of months at Thales Alenia Space in Cannes, France, being put through a last set of stringent tests. This included suspending the satellite from a structure to simulate weightlessness and carefully unfolding the two 10 m-long solar wings and the 12 m-long radar antenna. 40)
• The first satellite dedicated to Europe's Copernicus environmental monitoring program arrived at Cayenne in French Guiana on 24 February 2014. Sentinel-1A is scheduled to be launched from Europe's spaceport in Kourou on 3 April. By delivering timely information for numerous operational services, from monitoring ice in polar oceans to tracking land subsidence, Sentinel-1 is set to play a vital role in the largest civil Earth observation programme ever conceived. 41)
Figure 13: The Sentinel-1A radar satellite has arrived at Europe's Spaceport in French Guiana to be prepared over the coming weeks for launch in April (image credit: ESA,M. Shafiq) 42)
Launch of S-1A: The Sentinel-1A spacecraft was launched on April 3, 2014 (21:02 UTC) on a Soyuz-STB Fregat vehicle from Kourou, French Guiana (the launch is designated as VS07 by the launch provider Arianespace). After a 617 second burn, the Fregat upper stage delivered Sentinel-1A into a Sun-synchronous orbit at 693 km altitude. The satellite separated from the upper stage 23 min 24 sec after liftoff. 43)
Launch of S-1B: The Sentinel-1B spacecraft, a twin sister of Sentinel-1A, was launched on April 25, 2016 (21:02:13 GMT) into the same orbital plane of Sentinel-1A (phased by 180º). The launcher was a Soyuz-STA/Fregat vehicle (VS 14) of Arianespace and the launch site was Kourou. 44) 45)
The contract between ESA and Arianespace to launch the Sentinel-1B satellite was signed on July 17, 2014 by ESA's Director of Earth Observation Programs, Volker Liebig, and CEO of Arianespace, Stéphane Israël, at ESA headquarters in Paris, France. As part of the Copernicus program, Sentinel-1B will round out the initial capacity offered by Sentinel-1A to offer a comprehensive response to the need for environmental and security monitoring via spaceborne radar systems. 46) 47) 48)
On March 22, 2016, the Sentinel-1B satellite has arrived in French Guiana to be prepared for liftoff on 22 April. 49)
Secondary payloads of Sentinel-1B: 50)
• MicroSCOPE, a minisatellite (303 kg) of CNES (French Space Agency) which will test the universality of free fall (equivalence principle for inertial and gravitational mass as stated by Albert Einstein).
• AAUSAT4, a 1U CubeSat of the University of Aalborg, Denmark to demonstrate an AIS (Automatic Identification System), identifying and locating ships sailing offshore in coastal regions.
• e-st@r-II (Educational SaTellite @ politecnico di toRino-II), a 1U CubeSat from the Polytechnic of Turin, Italy.
• OUFTI-1 (Orbital Utility for Telecommunication Innovation), a 1U CubeSat of the University of Liège, Belgium, a demonstrator for the D-STAR communications protocol.
Tyvak International installed the three CubeSats in the orbital deployer. The three CubeSats are part of ESA's FYS (Fly Your Satellite) student program.
Orbit: Sun-synchronous near-circular dawn-dusk orbit, altitude = 693 km, inclination = 98.18º, orbital period = 98.6 minutes, ground track repeat cycle = 12 days (175 orbits/cycle). An exact repeat cycle is needed for InSAR (Interferometric Synthetic Aperture Radar) support. LTAN (Local Time on Ascending Node) = 18:00 hours.
Orbital tube: A stringent orbit control is required to the Sentinel-1 system. Satellites' position along the orbit needs to be very accurate, in terms of both accuracy and knowledge, together with pointing and timing/synchronization between interferometric pairs. Orbit positioning control for Sentinel-1 is defined by way of an orbital Earth fixed "tube" 50 m (rms) wide in radius around a nominal operational path (Figure 14). The satellite is kept inside such a tube for most of its operational lifetime Ref. 15). 51)
One of the challenges of the Sentinel-1 orbit control strategy is the translation of a statistical tube definition in a deterministic control strategy practically functional to the ESOC (European Space Operations Center) operations.
The second obvious challenge is the very stringent tube diameter which forces the application of frequent and intense maneuvers nevertheless still compatible with S/C request for consumables of up to a 12 years lifetime.
A satellite control strategy has been specifically developed and consists in applying a strict cross-track dead-band control in the most Northern Point and in the ascending node crossing. Controlling the orbit at these 2 latitudes, the satellite is shown to remain in the tube, within the rms (root mean square) criteria, for all other latitudes.
Figure 15: Orbital tube section (image credit: ESA, TAS, Ref. 15)
Orbit knowledge accuracy (< 3 m rms in each axis) in realtime for autonomous operations is not considered as demanding as the on-ground postprocessing requirements (< 5 cm 3D rms) for the detection of (slow) land movements and deformations through the differential interferometry technique. The latter is almost as demanding as for Sentinel-3 and requires dual-frequency receivers. 53)
As both satellites, Sentinel-1A and Sentinel-1B, will fly in the in the same orbital plane with 180º phased in orbit, and each having a 12-day repeat orbit cycle, it will facilitate the formation of SAR interferometry (InSAR) image pairs (i.e., interferograms) having time intervals of 6 days. This, along with the fact that the orbital deviation of each Sentinel-1 satellite will be maintained within a tube of ±50 m radius (rms) will enable the generation of geographically comprehensive maps of surface change such as for measuring ice velocity in the Polar regions, as well as monitoring geohazard related surface deformation caused by tectonic processes, volcanic activities, landslides, and subsidence (Ref. 201).
Figure 16: The main Sentinel-1 mode will allow complete coverage of Earth in six days when operational with the two Sentinel-1 satellites are in orbit simultaneously (image credit: ESA/ATG medialab) 54)
• May 12, 2017: Rapid acceleration of an Arctic glacier over the past year has been detected by the Copernicus Sentinel-1 satellites. Sitting on Norway's Spitsbergen island in the Svalbard archipelago, the Negribreen glacier has recently seen a surge in ice surface speed, increasing from 1 m to 13 m a day over the winter. 55)
- When a glacier 'surges' a large amount of ice flows to the end in an unusually short time. While the causes are not completely understood, they are believed to be linked to changes in the amount of heat or water in the lowest layers of the glaciers.
- The last time Negribreen experienced a surge like this was in the 1930s, as documented in aerial photographs. At that time, it advanced almost 12 km into the fjord in one year along a 15 km-wide section of the front. Since then the front of the glacier had been steadily retreating, with large icebergs breaking off.
- This latest jump in speed began in July 2016 and has been climbing ever since – even over the cold winter months. - Monitoring glaciers in areas prone to bad weather and long periods of darkness – like the Arctic – was difficult before the advent of satellites. Radar satellites can 'see' through clouds and in the dark, and Sentinel-1 offers frequent and systematic coverage of the Arctic.
- A team of scientists working under ESA's Climate Change Initiative in the Glaciers_cci project are using satellite radar and optical coverage to map glaciers at different times and determine their changes in extent, elevation and speed.
Figure 17: Radar images from the Copernicus Sentinel-1 mission show the sudden advance of the Negribreen glacier in Norway in early 2017 [image credit: ESA, the image contains modified Copernicus Sentinel data (2016–17), processed by T. Strozzi (Gamma)]
- A team of scientists working under ESA's Climate Change Initiative in the Glaciers_cci project are using satellite radar and optical coverage to map glaciers at different times and determine their changes in extent, elevation and speed.
- "Sentinel-1 provides us with a near-realtime overview of glacier flow across the Arctic, remarkably augmenting our capacity to capture the evolution of glacier surges," said Tazio Strozzi from Swiss company Gamma Remote Sensing and scientist on Glaciers_cci.
- "This new information can be used to refine numerical models of glacier surging to help predict the temporal evolution of the contribution of Arctic glaciers to sea-level rise."
Figure 18: Radar images from the Copernicus Sentinel-1 mission were used to create these two ice speed maps of the Negribreen glacier in Norway. In October 2015, only the front of the glacier was moving by more than 300 m a year. By late 2016, the entire glacier was advancing at this accelerated rate.
• May 2, 2017: Over two decades of observations by five radar satellites show the acceleration of ice loss of 30 glaciers in Antarctica's Western Palmer Land in the southwest Antarctic. 56)
- Radar is particularly suited for monitoring polar regions that are prone to bad weather and long periods of darkness because it can collect information regardless of cloud cover, day or night. Mapping 30 glaciers in the region, the research team found that between 1992 and 2016, most of the glaciers sped up by 20 and 30 cm per day. This is equivalent to an average 13% increase in flow speed across the area as a whole (Figure 19).
- The study in Geophysical Research Letters combines over 24 years of radar data from satellites including ESA's Envisat and ERS missions, as well as from the Copernicus Sentinel-1 mission. sup>57)
- A decrease in the mass and volume of Western Palmer Land has raised the prospect that ice speed has increased in this marine-based sector of Antarctica. To assess this possibility, we measure ice velocity over 25 years using satellite imagery and an optimized modelling approach. More than 30 unnamed outlet glaciers drain the 800 km coastline of Western Palmer Land at speeds ranging from 0.5 to 2.5 m/day, interspersed with near-stagnant ice. Between 1992 and 2015, most of the outlet glaciers sped up by 0.2 to 0.3 m/day, leading to a 13 % increase in ice flow and a 15 km3/yr increase in ice discharge across the sector as a whole. Speedup is greatest where glaciers are grounded more than 300 m below sea level, consistent with a loss of buttressing caused by ice shelf thinning in a region of shoaling warm circumpolar water.
- The two-satellite Sentinel-1 mission for Europe's Copernicus program routinely monitors polar areas at a high resolution, continuing the long-term data record from European satellites.
Figure 19: Ice speed in the Western Palmer Land measured by the Copernicus Sentinel-1 mission between 2014 and 2016 (image credit: ESA, the image contains modified Copernicus Sentinel data (2014-16), processed by J. Wuite, ENVEO)
• April 20, 2017: Two Sentinel-1 radar images from 7 and 14 April 2017 were combined to create this interferogram showing the growing crack in Antarctica's Larsen-C ice shelf (Figure 20). Polar scientist Anna Hogg said: "We can measure the iceberg crack propagation much more accurately when using the precise surface deformation information from an interferogram like this, rather than the amplitude – or black and white – image alone where the crack may not always be visible." 58)
- When the ice shelf calves this iceberg it will be one of the largest ever recorded – but exactly how long this will take is difficult to predict. The sensitivity of ice shelves to climate change has already been observed on the neighboring Larsen-A and Larsen-B ice shelves, both of which collapsed in 1995 and 2002, respectively.
- These ice shelves are important because they act as buttresses, holding back the ice that flows towards the sea.
Figure 20: Larsen-C crack interferogram of Sentinel-1 showing the growing crack of the Larsen-C ice shelf (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by A. Hogg/CPOM/Priestly Centre, CC BY-SA 3.0 IGO)
• April 21, 2017: Earthquake Emergency Management Service in Italy. — A series of strong earthquakes struck Central Italy starting in August 2016, killing over 300 people, injuring some 360 and leaving over 2000 without homes. The Copernicus Emergency Management service was activated by the Italian Civil Protection authorities during this period and produced a total of 120 maps of the damaged areas, supporting their decision-making, rescue and aid delivery activities during these disastrous events. 59)
- Susceptible tectonics: Italy's tectonic and geological characteristics make it a country which is particularly susceptible to the risk of major earthquakes. It straddles the boundary between the Eurasian and African tectonic plates in the south, and a complex series of fault lines run down the entire length of the country along the Apennine mountain chain, collectively known as the Apennine fault. This geological "spine" is clearly visible in the map below, produced by the Italian National Institute for Environmental Protection and Research, ISPRA (Istituto Superiore per la Protezione e la Ricerca Ambientale).
Figure 21: Map of capable fault lines in Italy. "Capable" faults are defined as having "significant potential for displacement at or near the ground surface"(image credit: ISPRA)
- A series of major earthquakes: During the period between the end of August 2016 and January 2017, a series of significant earthquakes ravaged Central Italy:
1) 24 August 2016: A large earthquake measuring 6.2 Mw (Moment magnitude scale) struck Central Italy in the early hours of the morning.
2) 26 October 2016: Two months later, the Marche region was hit by two major earthquakes on the same day, measuring 5.4 and 5.9 Mw, respectively. Within a week, Umbria suffered a massive 6.5 Mw earthquake.
3) 18 January 2017: A set of large earthquakes struck Lazio and Abruzzo, of which the largest measured 5.7 Mw.
Large earthquakes are usually accompanied by large numbers of smaller shocks of varying intensity, some occurring prior to the main quake. These smaller events can amplify the destruction of the main tremor by further weakening already damaged buildings and infrastructure. The Italian Institute for Geophysics and Volcanology, INGV (Istituto Nazionale di Geofisica e Vulcanologia) maintains a network of sensors across Italy to measure earthquakes. The map (Figure 22) shows the earthquakes recorded by INGV in the regions of Central Italy between August 2016 and February 2017; the total number registered exceeds 45.000, and the sheer quantity and density of the events is overwhelmingly apparent.
Figure 22: Map of the over 45.000 seismological events recorded by INGV over Central Italy between August 2016 and February 2017. In total, these earthquakes claimed the lives of over 300 people, injured around 360, and caused the temporary displacement of some 2000 citizens from their damaged homes (image credit: INGV)
Table 8: Rapidly mapping the damages: the Copernicus Emergency Management Service (Ref. 59)
Figure 23: Copernicus EMS Grading Map (Monitoring) of Accumoli, Italy. The village of Accumoli is shown in the zoomed inset, and the map legend has been enlarged for readability. The small, widely spaced settlements are visible (image credit: Copernicus Emergency Management Service)
• April 7, 2017: EMSA (European Maritime Safety Agency), based in Lisbon, Portugal, provides technical assistance and support to the European Commission and Member States, amongst others, in the development and implementation of EU maritime legislation. Its mission is to ensure a high, uniform and effective level of maritime safety, maritime security, prevention of and response to pollution from ships, as well as response to marine pollution caused by oil and gas installations. 60)
- Satellites, with their sophisticated sensors, provide routine, cost-effective, wide-area surveillance over maritime zones. Furthermore, they can be pointed to a targeted location for monitoring specific operations and gather material in response to intelligence information. Earth observation contributes to maritime surveillance to help manage the actions and events that can have an impact on maritime safety and security, including marine pollution, accident and disaster response, search and rescue, as well as fisheries control.
- EMSA also operates CleanSeaNet, a satellite-based oil spill surveillance and vessel detection service. It offers assistance to participating States for the following activities:
1) identifying and tracing oil pollution on the sea surface
2) monitoring accidental pollution during emergencies
3) contributing to the identification of polluters.
- The CleanSeaNet service is based on radar satellite images, covering all European sea areas, which are analyzed in order to detect possible oil spills on the sea surface. When a possible oil spill is detected in national waters, an alert message is delivered to the relevant country. Analyzed images are available to national contact points within 30 minutes of the satellite passing overhead. Approximately 2,000 images are ordered and analyzed per year.
- The service, which is integrated into national and regional pollution response chains, aims to strengthen operational responses to accidental and deliberate discharges from ships, and assist participating States to locate and identify polluters in areas under their jurisdiction.
- Each coastal State has access to the CleanSeaNet service through a dedicated user interface, which enables them to view ordered images. Users can also access a wide range of supplementary information through the interface, such as oil drift modelling (forecasting and backtracking), optical images, and oceanographic and meteorological information.
- SAR satellite images cannot provide information on the nature of a spill (for instance whether it is mineral oil, fish or vegetable oil, or other), but spills from vessels often appear as long, linear dark lines (indicating a substance discharging as the vessel is moving), with a bright spot (the vessel) at the tip. Vessel detection is also available through the CleanSeaNet service. If a vessel is detected in a satellite image, its identity can often be determined by correlating the satellite data with vessel positioning reports from the European monitoring systems operated at EMSA, such as SafeSeaNet.
- The Sentinel-1 mission supports detection of illegal oil spills. 61)
Table 9: Urban monitoring boosted by new data processor 62)
Figure 24: Sentinel-1 TimeScan product: of Germany derived from more than 1500 scans by Sentinel-1 between May 2014 and July 2016 (image credit: DLR, the image contains modified Copernicus Sentinel data (2014-16), processed by DLR) 63)
Legend to Figure 24: In the false-color image, the temporal average, minimal and maximal backscattering values are represented for every pixel in the red, green and blue bands. Urban conglomerations, for example, reflect strongly and appear as prominent, bright areas. Water bodies deflect a large proportion of the oblique synthetic aperture radar beams from the satellite and are therefore dark. Vegetated regions are distinguished by comparatively high minimal backscatter, which causes forests and meadows to appear in green tones. Temporally dynamic land cover types, such as crop acreage, have changed considerably during the acquisition period and appear lilac in the data product.
Legend to Figure 25: This animation shows the TimeScan Landsat data derived for the Pearl River delta in China for 2002–03 and 2014–15. The illustrated TimeScan RGB images are composed of the temporal maximum built-up index in red, the maximum vegetation index in green and the temporal mean value of the water index in blue. A specific image analysis algorithm developed by DLR in ESA's SAR4Urban project uses the TimeScan data to map the extent of the built-up area (highlighted in black in the animation) in order to finally pinpoint the urban growth that took place in the region over the last 10 years.
• March 31, 2017: This Sentinel-1 radar composite image (Figure 26) takes us to the northeastern tip of Ellesmere Island (lower-left), where the Nares Strait opens up into the Lincoln Sea in the Canadian Arctic. 65)
- The image was created by combining three radar scans from Copernicus Sentinel-1 captured in December, January and February. Each image has been assigned a color – red, green and blue – and create this colorful composite when combined. Colors show changes between acquisitions, such as the movement of ice in the Lincoln Sea, while the static landmass is grey. - The obvious distinction between the red and yellow depicts how the ice cover has changed over the three months.
- The maximum extent of Arctic sea ice hit a record low this winter. Scientists attribute the reduced ice cover to a very warm autumn and winter, exacerbated by a number of extreme winter 'heat waves' over the Arctic Ocean.
- In the center-left on the land, we can see a straight, dark link with a circle at its left end. This is the runway for Alert – the northernmost known settlement in the world. Inhabited mainly by military and scientific personnel on rotation, Alert is about 800 km from the North Pole.
- A team of researchers on the CryoVex/Karen campaign was recently in Alert validating sea-ice thickness measurements from the CryoSat-2 satellite and testing future satellite mission concepts. 66)
- Taking off from Alert, the team flew two aircraft equipped with instruments that measure sea-ice thickness at the same time the CryoSat-2 satellite flew some 700 km overhead. The measurements from the airborne campaign will be compared to the satellite measurements in order to confirm the satellite's accuracy.
- A team will also make ground measurements of snow and ice along a CryoSat-2 ground track in April. Ground campaigns like this provide a wealth of data that help scientists better understand how the Arctic is changing and, ultimately, how climate is changing.
Figure 26: Sentinel-1 composite radar image of Ellesmere Island, Canada, acquired in December 2016, and in January and February 2017 (image credit: ESA, the image contains modified Copernicus Sentinel data (2016-17), processed by ESA)
• March 5, 2017: Since the 1920s, excessive pumping of groundwater at thousands of wells has caused land to subside, or sink, by as much as 8.5 meters in sections of California's San Joaquin Valley. This subsidence is exacerbated during droughts, when farmers rely heavily on groundwater to sustain one of the most productive agricultural regions in the United States. 67)
- Subsidence is a serious and challenging concern for California's water managers, putting state and federal aqueducts, levees, bridges and roads at risk of damage. Already, long-term land subsidence has damaged thousands of public and private groundwater wells throughout the San Joaquin Valley. Furthermore, the subsidence can permanently reduce the storage capacity of underground aquifers, threatening future water supplies. It's also expensive. While there is no comprehensive estimate of damage costs associated with subsidence, state and federal water agencies have spent an estimated $100 million on related repairs since the 1960s.
- To determine the extent to which additional groundwater pumping associated with California's recent historic drought has affected land subsidence in the Central Valley, California's Department of Water Resources (DWR) commissioned NASA/JPL ( Jet Propulsion Laboratory) to use its expertise in collecting and analyzing airborne and satellite radar data. An initial report from JPL in August 2015 analyzed radar data from several different sensors collected between 2006 and early 2015. Due to the continuing drought, DWR subsequently commissioned JPL to collect and analyze new radar images from 2015 and 2016 to update DWR.
- Several trouble spots that were identified in 2015 have continued to subside at rates as high as 0.6 m per year. Significant subsidence was measured in subsidence bowls near the towns of Chowchilla, south of Merced; and Corcoran, north of Bakersfield. These bowls cover hundreds of square kilometers and continued to grow wider and deeper between May 2015 and September 2016. Subsidence also intensified near Tranquility in Fresno County, where the land surface has settled up to 51 cm in an area that extends 11 km.
- The map of Figure 27 shows the total subsidence in part of the San Joaquin Valley between May 2015 and September 2016 as observed by the ESA (European Space Agency) Sentinel-1A satellite and analyzed by scientists at JPL. The areas of most extensive subsidence appear in shades of yellow and bright green.
- JPL scientists plotted the history of subsidence of several sites in the mapped areas and found that for some areas in the San Joaquin Valley, subsidence slowed during the winter of 2015-16 when rainfall matched crop water needs. "While we can see the effect that rain has on subsidence, we know that we've run a groundwater deficit for some time, so it will take a long time to refill those reservoirs," said JPL report co-author Tom Farr. 68)
- Cathleen Jones, a co-author and scientist from JPL, said being able to pinpoint where subsidence is happening helps water resource managers determine why it is happening. "If you see a subsidence bowl, then something is going on at the center of the bowl that is causing the land to sink—for example, high levels of groundwater pumping," she said. "We can locate problem spots so the state can focus on those areas, saving money and resources. We find the needle in the haystack, so to speak."
- The researchers compared multiple satellite and airborne interferometric synthetic aperture radar (InSAR) images of Earth's surface to show how subsidence varies over space and time. InSAR is routinely used to produce maps of surface deformation with approximately centimeter-level accuracy. The 2015 and 2017 reports included data from Sentinel-1A, NASA's UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar), Japan's PALSAR on ALOS-2, and Canada's RADARSAT-2.
- "The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable," said DWR Director William Croyle. "Subsidence has long plagued certain regions of California. But the current rates jeopardize infrastructure serving millions of people. Groundwater pumping now puts at risk the very system that brings water to the San Joaquin Valley. The situation is untenable."
Figure 27: NASA Earth Observatory map by Joshua Stevens, using ESA Sentinel-1 SAR data courtesy of Tom Farr and Cathleen Jones, NASA/JPL, caption by Alan Buis of JPL and Ted Thomas of DWR, edited by Mike Carlowicz
• February 24, 2017: California has seen some heavy rains recently after years of drought, filling many of the state's reservoirs. The rising waters are evident in this radar image from the Copernicus Sentinel-1 satellite mission over part of the San Joaquin Valley. 69)
- The three water bodies pictured in Figure 28 are Lake Kawhea (in Tulare County) in the upper right, Bravo Lake to its left and Lake Success (near Porterville on the Tule River) in the lower right. This image was created by combining two scans from Sentinel-1's radar on 15 December and 26 January, and assigning each scan a color. Combined, the colors reveal changes, such as the red coloring in the reservoirs showing the water level increase.
- Officials have begun to release water from Lake Success as heavy rains have nearly filled it to capacity, and the outflow is sometimes exceeding the inflow in these days.
- The problem of too much water is in stark contrast from the situation in previous years, when drought led to water shutoffs and cutoffs, severely hindering yields in the San Joaquin Valley – a major agricultural region. Major crops include grapes, cotton, nuts and fruits, with productivity relying on irrigation from surface water diversions and groundwater pumping from wells.
- Agricultural structures dominate this radar composite image. Like the reservoirs, colors reveal changes between December and January such as vegetation growth or harvests.
- Along the right side of the image, we can see the foothills of the Sierra Nevada mountains.
Figure 28: A Sentinel-1 radar image of water reservoirs in California as of Dec. 15, 2016 and of Jan. 26, 2017 (image credit: ESA, the image contains modified Copernicus Sentinel data, processed by ESA)
• February 16, 2017: Following the appearance of a large crack in the ice shelf close to the Halley VI research station of BAS (British Antarctic Survey) in Antarctica, information from the Copernicus Sentinel-1 and Sentinel-2 satellites helped to decide to close the base temporarily. 70)
- Nourished by an inflow of ice from grounded glaciers and snow accumulating on its surface, Brunt Ice Shelf is a floating ice sheet in the Weddell Sea Sector of Antarctica. The floating ice moves steadily towards the ocean, where it occasionally calves off as icebergs. Cracks often appear on shelves as the ice deforms. However, rapidly expanding cracks indicate impending calving.
- Since the Halley VI base of BAS was only 17 km from the crack that appeared last October, ENVEO – a company that uses satellite data for cryosphere studies – and the Survey used radar images from Sentinel-1 and optical images from Sentinel-2 to monitor the situation.
- Dubbed Halloween Crack, it was lengthening inland as fast as 600 m a day in November and December 2016. Halley was designed to be relocated if the ice becomes dangerous. In fact, it had already been moved 23 km inland during last Antarctica's summer months because another ice chasm had begun to show signs of growth. 71)
- The station sits on Antarctica's 150 m thick Brunt Ice Shelf. This floating ice shelf flows at a rate of 0.4 km per year west towards the sea where, at irregular intervals, it calves off as icebergs. Halley is crucial to studies into globally important issues such as the impact of an extreme space weather event, climate change, and atmospheric phenomena. It was scientific investigations from this location that led to the discovery of the Antarctic Ozone Hole in 1985.
- Normally, around 70 people live and work at the base during the summer and fewer than 20 during the winter. However, this is the first winter that the base has been completely closed.
- Thomas Nagler, ENVEO CEO, said, "We get Sentinel-1 and Sentinel-2 data shortly after acquisition so we are able extract information on the crack's progression and deliver this information to our Survey colleagues very quickly." Hilmar Gudmundsson, Survey lead scientist, added, "The frequency of Sentinel-2 images and Sentinel-1 radar products allows us to follow in detail and almost in real time the development of the crack as it grows week by week. This also provides us with essential information for ice-deformation models, leading to a deeper understanding of such events."
- Mark Drinkwater, head of ESA's Earth observation mission, added "Routine Antarctic summer observations by the combination of Copernicus Sentinel-2A and Sentinel-1A and -1B are now demonstrating their value for monitoring rapid environmental change and providing information crucial to informed decisions on matters of safety and security in Antarctica.
Figure 29: The Sentinel-1A and Sentinel-1B interferogram uses images from 14 and 20 January 2017. It shows a discontinuity all along the Halloween crack and a distinct deformation pattern at its tip. The interferogram is laid on a Sentinel-2 optical image from 5 February 2017 (image credit: ESA, the image contains modified Copernicus Sentinel data (2016–17), processed by ENVEO)
• February 3, 2017: Climate change-driven glacial melt is causing landslides in alpine regions. Data from the Sentinel-1 satellite mission are being inserted into a new cloud computing system to monitor such hazards globally. 72)
- The Aletsch Glacier, the largest in the Alps, is experiencing an average retreat of about 50 m a year. The adjacent rocks that were previously constrained by the ice mass are progressively being released, generating slope instabilities. For this reason, the Aletsch region is a unique place where scientists can investigate how changes in glaciers affect the long- and short-term evolution of rock slope stability.
- To monitor the progressive changes occurring throughout a 2 km2 area southeast of the glacier – called the Moosfluh slope – the Chair of Engineering Geology at the Swiss Federal Institute of Technology in Zurich (ETHZ) installed ground-based instruments in 2013.
- Between September and October 2016, Moosfluh experienced an abnormal acceleration. The deformation generated several deep cracks and rock failures, hindering access to hiking paths visited by tourists, and affecting a cable car station located near the crest of the slope.
Figure 30: Moosfluh slope instability of the Aletsch Glacier (image credit: Geohazards-TEP, the image contains modified Copernicus Sentinel-1 data (2016), processed by Geohazards-TEP)
Legend to Figure 30: Land deformation speed of the Moosfluh area observed by the Sentinel-1 satellite from August to November 2016. The monitoring system installed by the Chair of Engineering Geology at ETHZ includes two Robotized Total Stations (RTS1 and RTS2), which monitor the evolution of surface displacements at point targets located on the unstable area (white and red dots). The results obtained from satellite data helped scientists to define the extension of the most active area and identify locations to place additional monitoring targets (red dots),
• January 30, 2017: A crack in the Larsen-C ice shelf in on the Antarctic Peninsula first appeared several years ago, but recently it has been lengthening faster than before. Carrying radar that can 'see' through the dark, the Copernicus Sentinel-1 satellites are monitoring the situation. The animation shows that the fissure has opened around 60 km since January last year. And, since the beginning of this January it has split a further 20 km so that the 350 m-thick shelf is held only by a thread. The crack now extends around 175 km. 73)
- When the ice shelf calves this iceberg it will be one of the largest ever recorded – but exactly how long this will take is difficult to predict. The neighboring Larsen-A and Larsen-B ice shelves suffered a similar fate with dramatic calving events in 1995 and 2002, respectively. —These ice shelves are important because they act as buttresses, holding back the ice that flows towards the sea.
Figure 31: Larsen Crack evolution in Antarctica: The Sentinel-1 two-satellite constellation is indispensable for discovering and monitoring events like these because it continues to deliver radar images when Antarctica is shrouded in darkness for several months of the year (image credit: ESA, the image contains modified Copernicus Sentinel data (2016–17), processed by ESA)
• November 25, 2016: The Sentinel-1 satellites have shown that the Millennium Tower skyscraper in the center of San Francisco is sinking by a few centimeters a year. Studying the city is helping scientists to improve the monitoring of urban ground movements, particularly for subsidence hotspots in Europe. 74)
- Completed in 2009, the 58-storey Millennium Tower has recently been showing signs of sinking and tilting. Although the cause has not been pinpointed, it is believed that the movements are connected to the supporting piles not firmly resting on bedrock. - To probe these subtle shifts, scientists combined multiple radar scans from the Copernicus Sentinel-1 twin satellites of the same area to detect subtle surface changes – down to millimeters. The technique works well with buildings because they better reflect the radar beam.
- It is also useful for pinpointing displacement hotspots over large areas, thanks to Sentinel-1's broad coverage and frequent visits. Working with ESA, the team from Norut, PPO.labs and Geological Survey of Norway have also mapped other areas in the wider San Francisco Bay Area that are moving. These include buildings along the earthquake-prone Hayward Fault, as well as subsidence of the newly reclaimed land in the San Rafael Bay.
- An uplift of the land was detected around the city of Pleasanton, possibly from the replenishment of groundwater following a four-year drought that ended in 2015.
Figure 32: Data from the Sentinel-1 satellites acquired between 22 February 2015 and 20 September 2016 show that Millennium Tower in San Francisco is sinking by about 40 mm a year in the 'line of sight' – the direction that the satellite is 'looking' at the building. This translates into a vertical subsidence of almost 50 mm a year, assuming no tilting. The colored dots represent targets observed by the radar. The color scale ranges from 40 mm a year away from radar (red) to 40 mm a year towards radar (blue). Green represents stable targets (image credit: ESA, the image contains modified Copernicus Sentinel data (2015–16) / ESA SEOM INSARAP study / PPO.labs / Norut / NGU) 75)
Figure 33: Sentinel-1 radar data show ground displacement of downtown San Francisco. While green indicates no detected movement, points in yellow, orange and red indicate where structures are subsiding, or sinking (image credit: ESA, the image contains modified Copernicus Sentinel data (2015–16) / ESA SEOM INSARAP study / PPO.labs / Norut / NGU) 76)
Figure 34: Sentinel-1 radar data show ground displacement of the San Francisco Bay Area. Hot spots are clearly observed, including the Hayward fault running north–south of the central-right side of the image. Subsidence of the newly reclaimed land in the San Rafael Bay on the left is also visible, while an uplift of land is visible in the lower right, possibly a result of a recovering groundwater level after a four-year long drought that ended in autumn 2015 (image credit: ESA, the image contains modified Copernicus Sentinel data (2015–16) / ESA SEOM INSARAP study / PPO.labs / Norut / NGU) 77)
- European cities experience similar subsidence, and the San Francisco study is helping because it contains a multitude of features (Ref. 74).
- For example, the area around Oslo's train station in Norway is reclaimed land. Newer buildings, such as the nearby opera house, have proper foundation into bedrock, but the older parts of the station experience severe subsidence.
- "Experience and knowledge gained within the ESA's Scientific Exploitation of Operational Missions program give us strong confidence that Sentinel-1 will be a highly versatile and reliable platform for operational deformation monitoring in Norway, and worldwide," noted John Dehls from the Geological Survey of Norway.
- The studies of San Francisco and Oslo are paving the way for moving from targeted case studies to a nationwide or even continental-scale land deformation service.
- "The Copernicus Sentinel-1 mission is, for the first time, making it possible to launch operational national deformation mapping services," said Dag Anders Moldestad from the Norwegian Space Center. The open data policy and regular coverage plan of Copernicus promise cost-efficient and reliable services. "In Norway, we have already initiated a framework project to provide nationwide basic deformation products to the public, with a free and open data policy. Many other countries in Europe are also working towards setting up similar services," noted Dr. Moldestad.
- The Sentinel-1 twins provide 'radar vision' for Europe's Copernicus environment monitoring program. In addition to watching land movements, they feed numerous other services for monitoring Arctic sea ice, routine sea-ice mapping, surveillance of the marine environment, mapping for forest, water and soil management, and mapping to support humanitarian aid and crisis situations.
Figure 35: Data from the Sentinel-1 satellites acquired between 26 December 2014 and 28 October 2016 show that parts of the Oslo train station are sinking by 10–15 mm a year in the 'line of sight' – the direction that the satellite is 'looking' at the building. This translates into a vertical subsidence of 12–18 mm a year. It can also be observed that the new opera house – the white structure located by the fjord south of the subsiding area – has not moved (image credit: ESA, the image contains modified Copernicus Sentinel data (2015–16) / ESA SEOM INSARAP study / PPO.labs / Norut / NGU) 78)
• November 11, 2016. The Sentinel-1 image of Figure 36 shows the Virunga Mountains in East Africa and their volcanoes. While most are dormant, two of the eight volcanoes are active, with the most recent eruptions in 2006 and 2010. 79)
- The mountains are on the Albertine Rift, where the Somali Plate is splitting away from the rest of the African continent. The area is one of Africa's most biologically diverse regions, but high human population density, poverty and conflict pose a challenge to conservation. Across the mountain range, however, a series of national parks has been established to protect the fauna and flora.
- In this image, we can easily identify the delineation between the protected and non-protected lands – the green, orange and yellow dots indicate changes in the surface of non-protected lands between the radar scans that make up this composite image. These changes are primarily in vegetation as the land surrounding the mountains is blanketed with agricultural plots. In particular, we can see the grid-like pattern of agriculture is visible in the green and yellow square at the center of the image.
Figure 36: This Sentinel-1 radar composite image features the Virunga Mountains in East Africa: a chain of volcanoes stretching across Rwanda's northern border with Uganda and east into the Democratic Republic of the Congo (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA)
• November 3, 2016: In the early hours of 30 October 2016, a 6.5 magnitude earthquake struck central Italy. The team od scientists combined several pairs of Sentinel-1 radar images acquired between 25 October and 1 November 2016 to analyze the ground displacements caused by the quake. 80)
- New information on the effects of the 30 October earthquake that struck central Italy continues to emerge as scientists analyze radar scans from satellites. Using radar imagery from the Copernicus Sentinel-1 satellites, Italian experts have identified significant east–west displacements of the ground in the area struck by the earthquake. An eastwards shift of about 40 cm was mapped in the vicinity of Montegallo, while a westwards shift of about 30 cm is centered in the area of Norcia (Figure 37).
- The team of scientists from the Institute for Electromagnetic Sensing of the Environment of the National Research Council and the National Institute of Geophysics and Volcanology combined radar scans taken before and after the event to map centimeter-scale changes. Vertical displacement is also evident, with the ground sinking 60 cm around Castelluccio but rising by about 12 cm around Norcia (Figure 38).
Figure 37: East-West displacements: The results of the analysis show an eastwards shift of about 40 cm in the vicinity of Montegallo, while a westwards shift of about 30 cm is centered in the area of Norcia (image credit: ESA, the image contains modified Copernicus Sentinel data (2016)/ESA/CNR-IREA)
Figure 38: Vertical displacements: The results show ground subsiding or sinking down to 60 cm around Castelluccio. Around Norcia, there is an uplift of about 12 cm (image credit: ESA, the image contains modified Copernicus Sentinel data (2016)/ESA/CNR-IREA)
• October 28, 2016: The Geohazards Exploitation Platform gives users direct access to large Earth-observation datasets for areas at risk of geohazards such as earthquakes and volcanic activity. Radar 'interferometry' involves taking successive radar images of the same location and combining them to produce rainbow-hued 'interferograms'. Like playing spot the difference, the slightest shift between images gives rise to distinct interference fringes, resembling contour lines on a map. 81)
- Pairs of Sentinel-1A radar images from consecutive passes over the same spot on Earth's service are processed continuously and automatically to provide medium-resolution (200 m) images. This, and related interferograms, allows users to identify ground movement and other changes that may have occurred between satellite acquisitions. The service currently covers tectonic areas in Europe. Covering three million km2, this represents 360 interferometric pairs every 12 days.
Figure 39: An overview of seismic zones in Europe provided by the Sentinel-1 twin radar satellites (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by DLR/ESA/Terradue)
- The image of Figure 40 was generated automatically on the Geohazards Exploitation Platform and combines images captured by Sentinel-1A before and after the quakes. The Geohazards Exploitation Platform gives users direct access to large Earth-observation datasets for areas at risk of earthquakes or volcanic activity, for example. 82)
Pairs of Sentinel-1A radar images from consecutive passes over the same spot on Earth's service are processed continuously and automatically to provide medium-resolution (200 m) images. These interferograms allow users to identify ground movement and other changes that may have occurred between satellite acquisitions. Although the service only covers Europe, this illustrates how information on major events can be made available to users. Over the next months, the browse service will be ramped up gradually to cover the global tectonic 'mask'. This mask corresponds to regions where the shape of the ground is deforming because of tectonic activity and prioritised by the geohazards user community.
"The quick-browse service has been under way across European tectonic regions since January, harnessing automated processing developed by the DLR German Aerospace Center," explains Fabrizio Pacini of Terradue, overseeing the Platform. "Our plan next year is to gradually scale up to cover the entire world's tectonic regions, which adds up to a quarter of Earth's land surface. Such wide-area coverage is really unprecedented. It is a crucial step towards empowering society at large to reduce the risk from earthquakes and volcanoes."
Figure 40: This interferogram of Sentinel-1 data shows how the ground moved as a result of the earthquakes that struck Amatrice in Italy on 24 August 2016 (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by DLR/ESA/Terradue)
- Today, it is a well-established technique used to give authoritative snapshots of ground movements following events such as the recent earthquakes in central Italy. — Earth scientists were surprised in the 1980s when centimeter-accuracy GPS networks revealed previously unsuspected motion along tectonic plates, occurring between larger-scale seismic events. 83)
Combined radar scans are sensitive down to a level of millimeters and over wide areas, compared to point-by-point GPS measurements, but the process demands heavy computing power. "The real step change here is that they are being produced across extended areas on an entirely automated basis," adds Fabrizio.
- The service is processing an average 50 image pairs per day across Europe from the Sentinel-1A and Sentinel-1B satellites, with six days between coverage.
- Next year, this will increase to 130 pairs per day, with a maximum of 24 days and potentially 12 days between acquisitions outside Europe, involving the daily processing of 1 terabyte of data, with additional higher resolution produced on request.
Processing on such a grand scale is enabled by the online, cloud-based Geohazards Exploitation Platform, specifically tailored for working with vast amounts of satellite data.
Figure 41: Scientists from Italy's Institute for Electromagnetic Sensing of the Environment combined Sentinel-1 radar acquisitions over central Italy from before and after the 24 August 2016 earthquake: 15 August, 21 August and 27 August 2016. The result shows vertical ground subsidence, reaching about 20 cm in correspondence to the Accumoli area, and lateral movement of up to 16 cm. The blue line indicates the location of the fault trace (image credit: ESA, the image contains modified Copernicus Sentinel data (2016)/ESA/CNR-IREA)
• Sept. 16, 2016: Sentinel-1B was declared operational. On 14 September, project manager Ramón Torres who led the development team, handed over the satellite to the mission manager, Pierre Potin in the presence of Volker Liebig, Senior Advisor to ESA's Director General. — Following liftoff on 25 April 2016, the Copernicus Sentinel-1B satellite has been commissioned and handed over for mission operations. It joins its identical twin, Sentinel-1A, which has been systematically scanning Earth with its radar since October 2014. Orbiting 180° apart, the two satellites optimize coverage and data delivery for the Copernicus services that are making a step change in the way our environment is managed. More than 45,000 users have registered to access Sentinel data, under the free and open data policy data framework of Europe's Copernicus environmental monitoring program. 84)
- Both satellites carry a radar that images Earth's surface through cloud and rain and regardless of whether it is day or night. These images are used for many applications, such as monitoring ice in the polar seas, tracking land subsidence, and for responding to disasters such as floods.
• Sept. 16, 2016: Southern China's Poyang Lake is the largest freshwater lake in the country. Located in Jiangxi province, this lake is an important habitat for migrating Siberian cranes, many of which spend the winter there. The lake is also home to the endangered finless porpoise, a freshwater mammal known for its high level of intelligence. Amid fears that it would soon become extinct, the porpoise made headlines last year when the Chinese government moved eight of them from Poyang Lake to two secure habitats in an effort to increase the population over the coming years. 85)
- One study found that, without action, the current rate of population decrease would likely mean extinction by 2025.
- For the human population, Poyang is one of China's most important rice-producing regions, although local inhabitants must contend with massive seasonal changes in water level.
- Local scientists collaborating with ESA through the Dragon program have identified an overall drop in water level in the lake over the last decade, but the El Niño weather phenomenon earlier this year caused precipitation levels to increase and water levels of the lake to rise.
Figure 42: Radar images from the Copernicus Sentinel-1 mission have been used to monitor the evolution of the lake, including this image which combines two radar scans from 7 and 19 March, 2016 (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA)
• August 31, 2016: ESA engineers have discovered that a solar panel on the Copernicus Sentinel-1A satellite was hit by a millimeter-size particle in orbit on 23 August. Thanks to onboard cameras, ground controllers were able to identify the affected area. So far, there has been no effect on the satellite's routine operations. 86)
- A sudden small power reduction was observed in a solar array of Sentinel-1A, orbiting at 700 km altitude, at 17:07 GMT on 23 August. Slight changes in the orientation and the orbit of the satellite were also measured at the same time. Following a preliminary investigation, the operations team at ESA's control center in Darmstadt, Germany suspected a possible impact by space debris or micrometeoroid on the solar wing.
- Detailed analyses of the satellite's status were performed to understand the cause of this power loss. In addition, the engineers decided to activate the board cameras to acquire pictures of the array. These cameras were originally carried to monitor the deployment of the solar wings, which occurred just a few hours after launch in April 2014, and were not intended to be used afterwards.
- Following their switch-on, one camera provided a picture that clearly shows the strike on the solar panel. The power reduction is relatively small compared to the overall power generated by the solar wing, which remains much higher than what the satellite requires for routine operations.
- "Such hits, caused by particles of millimeter size, are not unexpected," notes Holger Krag, Head of the Space Debris Office at ESA's establishment in Darmstadt, Germany. "These very small objects are not trackable from the ground, because only objects greater than about 5 cm can usually be tracked and, thus, avoided by maneuvering the satellites. In this case, assuming the change in attitude and the orbit of the satellite at impact, the typical speed of such a fragment, plus additional parameters, our first estimates indicate that the size of the particle was of a few millimeters.
- "Analysis continues to obtain indications on whether the origin of the object was natural or man-made. The pictures of the affected area show a diameter of roughly 40 cm created on the solar array structure, confirming an impact from the back side, as suggested by the satellite's attitude rate readings." This event has no effect on the satellite's routine operations, which continue normally.
Figure 43: Solar panel image of the Sentinel-1A spacecraft hit by a space particle (image credit: ESA)
• June 22, 2016: The twin Sentinel-1 satellites have – for the first time – combined to show their capability for revealing even small deformations in Earth's surface. Following its orbital maneuvers, the recently launched Sentinel-1B satellite reached its designated orbit position on 15 June. The satellite is now orbiting Earth 180° apart from its twin, Sentinel-1A, at an altitude of almost 700 km. With both satellites finally in the same orbit, together they can cover the whole globe every six days. — It has now been demonstrated that future images acquired by the pair can be merged to detect slight changes occurring between scans. 87)
- The rainbow-colored patterns (Figure 44) are related to topography, and they demonstrate that the two satellites' identical radars are accurately synchronized, pointing in the same direction and that the satellites are in their correct orbits.
- Once commissioning is completed in mid-September, the pair will be ready to deliver data for the systematic and routine monitoring of Earth surface deformation and ice dynamics.
Figure 44: This 'interferogram' combines a Sentinel-1A radar scan from 10 June 2016 over northwestern Romania with a Sentinel-1B acquisition from 16 June over the same area – shortly before Sentinel-1B reached its designated orbit. The city of Cluj-Napoca is at the center of the image. The color pattern is related to local terrain topography (image credit: the image contains modified Copernicus Sentinel data (2016)/ESA/DLR)
Table 10: Overview of the overall Sentinel-1 mission status in June 2016 with focus on the Sentinel-1A routine operations activities that started in June 2015 following the operational qualification phase, in terms of mission achievements, mission observation scenario, ground segment operations, throughput and data access. 88)
Figure 45: Sentinel-1A production volume evolution (image credit: ESA)
Figure 46: Evolution of number of Sentinel-1A products downloaded by users since opening in Oct 2014 (image credit: ESA)
Figure 47: Evolution of number of Sentinel-1A products available for download since opening in Oct 2014 (image credit: ESA)
Figure 48: Mosaic of Sentinel-1A data acquired on October 27-29, 2015. Daily ice drift is derived from consecutive overlapping scenes (image credit: ESA, the image contains modified Copernicus Sentinel data (2015)/ ESA/DTU/CMEMS)
• June 2016: The Sentinel-1B spacecraft has been launched on 25 April 2016, two years after Sentinel-1A to complete the Copernicus Sentinel-1 constellation. After its launch, the LEOP Phase was successfully completed in less than three days when all subsystems were checked. The SAR instrument was switched on, checked up and acquired the first images that were downloaded and processed by the operational Ground Segment. At present, the four-month Commissioning Phase is on-going; it includes the in-orbit Calibration and Characterization and the verification of the satellite, in order to deliver a full functional and well performing satellite to the mission operations mid September 2016. 89)
- The on-going IOCP (In-Orbit Commissioning Phase) comprises the commissioning of all spacecraft (i.e. platform subsystems, SAR payload, and the Optical Communication Payload interface) and Ground Segment components. It will conclude with the IOCR (In-Orbit Commissioning Review) in September.
- For the SAR instrument switch-on and check-out, some SAR acquisitions in IW (Interferometric Wideswath) mode were performed. These first images were taken just 56 hours after lift-off, two hours after SAR instrument switch-on (see Figure 52) and revealed already an excellent instrument performance. They were processed by the Sentinel-1 operational processor and adjusted to the non-nominal orbital parameters by the Ground Segment Team.
- Commissioning phase: Being the Ground System already commissioned during the Sentinel-1A commissioning phase and subsequent ground segment rump-up phase, the Sentinel-1B System Commissioning comprises only the commissioning of all Spacecraft components. This includes the following activities:
a) Spacecraft in-orbit verification (i.e. platform and payload)
b) SAR calibration and system performance verification
c) SAR cross-calibration with Sentinel-1A
d) Functional verification of the Sentinel-1B basic SAR products (i.e. L0 and L1b products w.r.t. SAR instrument performance and parameter settings
e) Functional verification of the OCP (Optical Communication Payload) interface. OCP is a laser communication terminal that is designed to send and receive data from a similar terminal located on another satellite, such as the geostationary EDRS.
- IOC main activities: The Sentinel-1B commissioning is planned for a period of 6 repeat orbit cycles (12 days each) once the reference orbit is reached. The commissioning phase will, therefore, have two sub-phases corresponding to the period determined to acquire such final reference orbit (Orbit Acquisition Phase) and once the reference orbit is acquired (Reference Orbit Phase).
• June 2016: Performance results of Sentinel-1A & Sentinel-1B repeat-pass SAR interferometry (InSAR) verification, using data acquired with the novel IW (Interferometric Wide-swath) mode. The IW mode, operational for the first time, utilizes ScanSAR-type burst imaging with an additional antenna beam steering in azimuth referred to as TOPS (Terrain Observation with Progressive Scans). 90)
- The Sentinel-1 SAR instrument with its active phased array antenna supports four exclusive imaging modes providing different resolution and coverage: Interferometric Wide Swath (IW), Extra Wide Swath (EW), Stripmap (SM), and Wave (WV). All modes, except the WV mode can be operated in dual polarization.
- Both the IW and EW mode use the TOPS technique to provide large swath width of 250 km at ground resolution of 5 m x 20 m and 400 km at ground resolution of 20 m x 40, respectively with enhanced image performance as compared to the conventional ScanSAR mode.
- The characteristic of the TOPS SAR imaging mode is that, besides the scanning in elevation, the antenna azimuth beam is steered from aft to the fore at a constant rate. As a result and in contrary to ScanSAR, all targets on ground are observed by the entire azimuth antenna pattern. This eliminates almost entirely the scalloping effect and also leads to constant azimuth ambiguities and SNR (Signal-to-Noise Ratio) along azimuth. - However, the fast azimuth beam steering reduces the target dwell time causing a lower spatial azimuth resolution than is achievable in Stripmap mode.
- The Sentinel-1 A& Sentinel-1B SAR constellation mission enables the build-up of long and equidistant InSAR IW mode data time series with a 6-day repeat-pass interval for geophysical applications, such as surface change detection monitoring.
• May 26, 2016: Cyclone Roanu has claimed over 100 lives in Sri Lanka and Bangladesh, and has left tens of thousands in need of aid. Officials are looking to the sky for information on flooded areas to analyze the cyclone's aftermath and support emergency response activities. On May 23, the Sentinel-1A satellite captured about 80% of the country of Bangladesh during a single pass. The images were quickly delivered to the Copernicus EMS (Emergency Management Service) to create flood maps, revealing over 170,000 hectares of land to be inundated. 91)
- The response to the request for data was quick, as was the generation of flood information by EMS: less than 30 hours from activation to the delivery of the first flood map. A 'rush mode' to process and make Sentinel-1 data available for EMS has been set up in order to reduce the response time as much as possible.
- The Sentinel-1 mission for Europe's Copernicus program is revolutionizing the use of satellites in managing risk assessment and emergency response with the provision of large-scale radar data in a systematic fashion. Sentinel-1A can map a 250 km-wide strip while achieving a ground resolution of 20 m per pixel. The radar on Sentinel-1 is able to 'see' through clouds, rain and in darkness, making it particularly useful for monitoring floods, usually connected to bad weather conditions. Images acquired before and after a flood offer immediate information on the extent of inundation and help to assess property and environmental damage.
- The International Charter for Space and Major Disasters also requested data for flood maps in western Sri Lanka – hit by the precursor of the cyclone on 16 May. The Charter was triggered by the Disaster Management Center of Sri Lanka on 17 May, channelled through the Asian Disaster Reduction Center.
- In response to Cyclone Roanu, additional Sentinel-1A radar scans are planned for the coming days to help the relief activities further. Once the twin satellite – Sentinel-1B – is operational, the mission's potential revisit time will be cut in half, further improving the response time to disasters.
Figure 49: Sentinel-1A flood map of Barguna, Bangladesh, acquired on May 23, 2016; showing over 85,000 hectares of flooded areas in light blue (image credit: Copernicus Service information (2016) / Copernicus EMS / e-GEOS)
• May 11, 2016: The Antarctic Peninsula is a narrow mountainous finger or spine of land extending northwards away from the Central Antarctic ice sheet and comprises the northernmost arm of the Antarctic ice sheet (Figure 50). 92)
- The color scale indicates the speed of ice movement in meters per day, ranging from 1 cm/day or less in dark blue to up to 1 m/day in red. The vivid colors trace a complex network of channels along which streams of ice flow from the high mountains down towards the coast where the ice flow speeds up and spreads out into floating ice shelves. The white area on the western flank of the peninsula is where snowfall is likely to have concealed features and so prevented tracking between the image pairs.
- As one of the most dynamic glacial environments on Earth, this region has been experiencing rapid climate warming over recent decades. Since 1991, satellites such as ESA's ERS and Envisat have observed the disintegration of various ice shelves, including the northern portion of the Larsen ice shelf and the Wilkins ice shelf.
- This example shows the spectacular potential of the Sentinel-1 mission for routine mapping and monitoring the surface velocity of glaciers and ice sheets. The combination of Sentinel-1A and -1B will support comprehensive and long-term monitoring of changes in ice sheet velocity and how they respond to climate change.
Figure 50: Successive radar images captured by the Copernicus Sentinel-1A satellite during December 2014 – March 2016 were used to create this spectacular map showing how fast the ice flows on the Antarctic Peninsula. The map was constructed by tracking the movement of ice features in pairs of radar images taken 12 days apart (image credit: ESA, contains modified Copernicus Sentinel data (2015), processed by Enveo)
• May 6, 2016: Figure 51 of the Sentinel-1A SAR instrument shows the Zachariae glacier on the northeast cost of Greenland. The shades of grey on the left side of the image depict the static landmass, while the colors on the right show changes in sea-ice type and cover between the three radar scans. Near the center-left we can see the Zachariae Isstrom (Isstrφm is Danish for ice stream) glacier, which began its "accelerated retreat" in 2012 and is currently losing about five billion tons of ice a year to the ocean. 93)
- Zachariae's dynamics have been changing over the last few years, calving high volumes of icebergs, which will inevitably affect sea levels. It is estimated that the entire Zachariae Isstrom glacier in northeast Greenland holds enough water to raise global sea levels by more than 46 cm.
- Scientists have determined that the bottom of Zachariae Isstrom is being rapidly eroded by warmer ocean water mixed with growing amounts of meltwater from the ice sheet surface. 94)
- Zachariae and the nearby Nioghalvfjerdsfjorden to its north are two of six glaciers being monitored in near-real time by Sentinel-1 through a new web portal by the UK's CPOM (Center for Polar Observation and Modelling). The portal provides frequent maps of ice velocity of key glaciers in both Greenland and Antarctica.
- The polar regions are some of the first to experience and visibly demonstrate the effects of climate change, serving as barometers for change in the rest of the world. It is therefore critical that polar ice is monitored comprehensively and in a sustained manner.
Figure 51: SAR image taken over part of northeast Greenland's coast combines three images from Sentinel-1A's radar observed on 15 February, 10 March and 3 April 2016 (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA)
• April 28, 2016: Sentinel-1B has produced its first images only two hours after the radar was switched on – a record time for a space radar. The first observations were taken a little more than two days after launch, after Sentinel-1B had followed a complicated routine to deploy its 12 m-long radar and two 10-m long solar wings, as well as passing a series of initial checks. The first image (Figure 52), 250 km wide and 600 km long, captured Svalbard, the Norwegian archipelago in the Arctic Ocean, with the Austfonna glacier clearly visible. 95)
- At ESA's operations center in Darmstadt, Germany, mission controllers thoroughly checked the satellite's control, navigation and power systems, among others, during the intense first few orbits. The team also conducted the complex unfolding of the radar wings and solar arrays.
- When Sentinel-1B reaches its final orbit, on the other side of Earth from Sentinel-1A, the radar vision constellation will be complete, meeting the coverage and revisit needs of Copernicus. In the coming months, the satellite will be tested and calibrated before it is declared to be operational.
Figure 52: Sentinel-1B's first data strip stretches 600 km from 80ºN through the Barents Sea. The image, which shows the Norwegian Svalbard archipelago on the left, was captured on 28 April 2016 at 05:37 GMT - just two hours after the satellite's radar was switched on (image credit: ESA, contains modified Copernicus Sentinel data , processed by ESA)
Figure 53: This full resolution subset of the first Sentinel-1B image shows Norway's Nordaustlandet island in the Svalbard archipelago, covered by the Austfonna ice cap ((image credit: ESA, contains modified Copernicus Sentinel data , processed by ESA)
• April 27, 2016: Following liftoff on 25 April from Europe's Spaceport in French Guiana, Sentinel-1B has opened its large solar wings and radar antenna. 96)
- After its launch, Sentinel-1B has followed a carefully planned sequence over 10 hours to open its large radar antenna and solar wings. During ascent, the satellite's 12 m-long radar antenna and two 10 m-long solar wings were folded up to fit into the Soyuz rocket's protective fairing. They opened together in a specific sequence, which also allowed power from the solar panels to be available as soon as possible, so that the satellite no longer depends on batteries.
- ESA's Sentinel-1 project manager, Ramón Torres, said, "The launch last night meant that we were able to forget the disappointment of the delays of the last few days in the blink of an eye. "We had a long night, staying awake to make sure the radar and solar panels deployed properly after the satellite had separated from the rocket. All of this was commanded by the team at ESA 's mission control in Germany. "The deployment is particularly complicated because of the sizes involved but all went well and the teams and I are extremely happy that we now have two Sentinel-1 satellites safely in orbit."
Figure 54: Artist's rendition of the Sentinel-1B satellite delivery into orbit (image credit: ESA)
• April 26, 2016: The launch of Sentinel-1B went according to plan; the spacecraft was placed into its orbit (altitude = 693 km) 23 minutes and 35 seconds after launch (launch mass of 2164 kg). Sentinel-1B is the fourth in a series of Sentinel satellites for the European Copernicus program, a joint project of the EC (European Commission) and ESA (European Space Agency).
- The trio of "Fly Your Satellite!" student-built CubeSats were released into space at 23:50 GMT (2 hours 48 minutes after liftoff). Transmissions from the Fregat upper stage show that the door on the CubeSat deployer opened regularly, around 2 hours and 48 minutes after launch. Now the CubeSats are travelling in their final orbits, and their university teams are waiting for them to establish contact. - In the first hour of flight, the CubeSats will be working autonomously in order to stabilize their motion, perform an internal health check, and deploy their antennae. Then they will establish communication with Earth.
Figure 55: Artist's rendition of the CubeSats orbiting Earth (image credit: ESA, Medialab)
- The CubeSats were deployed into an elliptical orbit of 665 km x 453 km. From this orbit they will re-enter Earth's atmosphere in approximately 8 years, preventing they become space junk after their missions are over.
- The MicroSCOPE minisatellite of CNES was the last deployment in the launch sequence at 01:02 GMT (4 hours into the flight) on April 26.
• April 22, 2016: The swirling landscape of Iran's salt desert, Dasht-e Kavir, is reminiscent of an abstract painting in this Sentinel-1 image (Figure 56). With temperatures reaching about 50ºC in the summer, this area sees little precipitation, but runoff from the surrounding mountains creates seasonal lakes and marshes. The high temperatures cause the water to evaporate, leaving behind clays and sand soils with a high concentration of minerals. The 'brushstroke' patterns are geological layers eroded primarily by wind. 99)
- Iran is one of the world's most important mineral producers. Earth-observing satellites are useful for finding and monitoring natural resources like minerals.
- Along the left side of the image we can see part of an area known as the 'devil's dunes' because it was believed to be haunted by evil spirits. This belief likely originated from its hostile conditions, and the early travellers who did attempt to cross it probably never returned due to starvation or dehydration.
Figure 56: This image combines three scans from Sentinel-1A's radar, acquired on 21 January, 14 February and 9 March 2016. Changes between the acquisitions appear in vibrant bright colors – such as the blues, reds and greens we see primarily on the left half of the image. These areas are salt lakes and the colors show fluctuations in the amount of water present over time (image credit: ESA, contains modified Copernicus Sentinel data , processed by ESA)
• April 21, 2016: Images from the Sentinel-1A satellite are being used to monitor aquaculture in the Mediterranean, in another example of the mission's contribution to food security, as fisheries become the main source of seafood. The satellite counted nearly 4500 fish cages over six months, mainly of mussel racks or finfish, along the western Mediterranean's coastline (Figures 57 and 58). The number of fish hatching cages in the Mediterranean was not known before this survey. Farming finfish, shellfish and aquatic plants is one of the world's fastest growing food sectors, which already provides the planet with about half of the fish we eat. 100)
- Aquaculture surpassed wild fisheries as the main source of seafood in 2014, according to the UN FAO (Food & Agriculture Organization). This reflects the earlier transformation of land from hunting to farming. The EU's aquaculture furnishes a fresh, local supply of healthy seafood, following strict rules that protect the consumer, the fish and the environment.
- ESA is promoting the innovative use of satellite data for aquaculture through its SMART (Sustainable Management of Aquaculture through Remote sensing Technology) project. Remote sensing of oceans is a window into the marine ecosystem, providing essential information for their governance. Satellites can cover site locations, map aquaculture facilities, monitor meteorological events, provide early flood warnings and track water pollution.
- Giuseppe Prioli, President of the European Mollusc Producers' Association, says: "The services being tested in SMART could be very useful for supporting shellfish industry in Italy. In particular, near-realtime data on water quality parameters at each farm, such as sea-surface temperature, chlorophyll content and turbidity, are important for farmers because they affect shellfish growth. "Short-term forecasting of shellfish biomass will also help in planning the harvest. The assessment of the potential biomass yield in a given area, based on Earth observation data, could also be useful for reallocating farms or starting the farming of new species, such as oysters, along the Emilia-Romagna coast."
- The mission benefits numerous services, which relate to the monitoring of Arctic sea-ice extent, routine sea-ice mapping and surveillance of the marine environment. This includes oil-spill monitoring and ship detection for maritime security, monitoring land-surface for motion risks, mapping for forest, water and soil management, and mapping to support humanitarian aid and crisis situations.
- Its twin Sentinel-1B will soon create a constellation of two satellites orbiting 180° apart. The mission will be able to revisit a point on Earth in less than six days, providing more frequent coverage for aquaculture.
Figure 57: The ESA SMART (Sustainable Management of Aquaculture through Remote sensing Technology) poster features the nearly 4500 fish cages detected in the western Mediterranean Sea by Sentinel-1 and other satellites (image credit: ESA, contains modified Copernicus Sentinel data  / ESA / ACRI / Bluefarm / Ifremer)
• April 15, 2016: The Sentinel-1A satellite takes us over to Ireland, in this multi-temporal color composite of land coverage across the island (Figure 59). With a coastline of 7500 km, Ireland is home to some 4.8 million people and a wealth of history and tradition. Stretching 486 km from north to south and 275 km from east to west, Ireland is washed by abundant rainfall all year, coating the country in omnipresent emerald-green grasslands. 102)
- The coastal mountain fringes in the west, northwest and east are composed mainly of granite, while old red sandstone predominates in the south. Many lakes, large bog areas and low ridges make up the very scenic lowland, as seen throughout the image.
Figure 59: This Sentinel-1 image was stitched together from 16 radar scans by the satellite during May 2015, and gives us an idea of the island's land cover and use. Different colors show changes that occurred within the 12 days' coverage. The blues across the entire image represent strong changes in bodies of water or agricultural activities such as ploughing. The yellows represent urban centers, with the capital city of Dublin very distinct on the far middle right. An interesting feature is the many yellow 'spots' scattered throughout the entire island, visible even more clearly when zooming in. These clusters all represent farmhouses. Vegetated fields and forests appear in green. The reds and oranges represent unchanging features such as bare soil or possibly rocks that border the forests, as is clear on the left side of the image, along the tips of the island (image credit: ESA, Contains modified Copernicus Sentinel data , processed by ESA)
• April 14, 2016: Multiple satellites, including Europe's Sentinels, have captured images of two large icebergs that broke away from Antarctica's Nansen ice shelf on 7 April. The icebergs are drifting to the northeast, propelled by wind, tides and currents. Experts say they do not pose any immediate threat of blocking supply routes to research stations such as the Italian Mario Zucchelli and South Korean Jang Bogo Stations in Terra Nova Bay. 103)
- Nonetheless, the icebergs may pose a threat to sea-floor moorings in the region that have been used by Italy's National Antarctic Program since the 1990s, and more recently by New Zealand ocean scientists. The Nansen ice shelf, around 50 km long and 25 km wide, developed a fracture over recent years. Ice shelves are particularly sensitive to climate change because they can melt from warm air at the surface and warming ocean waters below.
- "The crack was first observed during fieldwork in 1999 and was progressively growing, and then accelerating during 2014," said Massimo Frezzotti from Italy's ENEA research organization. "The events following were typical for a cycle of ice-shelf calving. Last century, a first calving event is known to have occurred between 1913 and the 1950s, with a second between 1963 and 1972."
- As winter weather began to set in during early March this year, optical images from Europe's Sentinel-2A satellite and radar images from Sentinel-1A, together with images from the Italian Cosmo-SkyMed mission, indicated that the ice front was only tenuously attached to the shelf.
- By 6 April, the fracture had reached about 40 km long before it severed the portion of the ice front between Inexpressible Island to the north and the Drygalski Ice Tongue – the floating end of the David Glacier – to the south.
- Verified by NASA's Terra satellite, the calving took place on 7 April during persistent strong offshore winds. Two days later, Sentinel-1A's radar confirmed the separation.
- "The area of the fracture was still negligible at the beginning of 2014, but between April 2015 and March 2016 it expanded from 11.68 sq km to 25.87 sq km, signalling a coming calving," said Flavio Parmiggiani of Italy's ISAC-CNR research organization.
- The fracture split the ice shelf along its length, resulting in two large icebergs measuring about 10 km and 20 km in length and 5 km across. Published research indicates that the bergs are likely to be around 250–270 m thick.
- Massimo Frezzotti explained, "History has shown that major calving typically occurs about every 30 years. The crack opened because of a difference in the velocity of ice between the northern Priestley Glacier and southern Reeves Glacier fed portions of the ice shelf, caused by the southern part being hooked and pulled along by the faster moving Drygalski Tongue."
Figure 60: Sentinel-1A captured radar images of the Nansen Ice Sheet on 2 and 9 April 2016, before and after the calving event that gave birth to two large icebergs measuring about 10 km and 20 km in length and 5 km across (ESA, Contains modified Copernicus Sentinel data , processed by ESA)
• April 8, 2016: The SAR image of Figure 61 shows part of the Swiss Alps (Bernese Alps). Near the center of the image are the lakes Thun (left most) and Brienz, with the city of Interlaken between them. In the upper-right section is Lake Lucerne. 104)
- In the lower-central part of the image we can see the Aletsch Glacier, the largest in the Alps. The glacier originates in a large, flat area of snow and ice high in the mountains called Concordia, where three smaller glaciers converge. Switzerland's three famous Eiger, Mönch and Jungfrau mountains rise north of Concordia. The Aletsch Glacier extends south, and its meltwater creates the Massa River in the valley below.
- The glaciers in this region are showing long-term retreat from climate change. The melting ice has given birth to new lakes, which pose risks such as flooding and landslides to communities below.
- The radar imagery can be used to generate precise elevation models, and can also detect deformation over landslide, seismic or subsidence areas. Radar can also support impact assessment for many types of hazard such as geological events.
- Sentinel-1's radar ability to 'see' through clouds, rain and in darkness makes it particularly useful for monitoring floods. Images acquired before and after a flood offer immediate information on the extent of inundation and support assessments of property and environmental damage.
Figure 61: Sentinel-1A image of the Swiss Alps, acquired on September 11, 2015 (image credit: ESA, Contains modified Copernicus Sentinel data , processed by ESA)
• February 5, 2016: Figure 62 is a composite of radar scans by Sentinel-1A on different dates of the Siljan Ring, located in the province of Dalarna in central Sweden. Dalarna, a historical province known for its deep forests, beautiful fishing lakes and lush green landscapes, is a holiday residence for many Swedes and a favorite tourist attraction. 105)
- Sweden's sixth largest lake, Siljan covers an area of 290 km2. Its particular shape comes from the location around the southwestern perimeter of the Siljan Ring, a circular geological formation created almost 400 million years ago by a major meteorite impact.
- Mainly eroded today, the original crater is estimated to have been some 50 km in diameter. It is the largest known impact crater in Europe and one of the 18 largest known impact craters on Earth.
- Given the location and the time of year, it is safe to infer that the landscape was dominated by vegetated areas, partly or completely frozen lakes and lots of snow. It is probably also safe to assume that most changes are related to variations in snow cover and snow condition.
- Focusing on the Siljan Lake, towards the central right part of the image, it is easy to make out the bright red and green lines, which come from changes in the ice edge and cover on the lake, in early spring.
- Southwest of the lake area, the town of Borlänge is clearly visible along the Dal River, appearing as bright white. Famous for its medieval cathedral of Stora Tuna and its important rail and bus junction, it became a commercial center for the surrounding agricultural and industrial areas. The town has paper mills, sawmills, engineering works, foundries, printing establishments and factories.
- The strikingly sharp blue to the east of Borlänge probably represents fresh snow, which fell sometime around the end of March and covered the flat agricultural fields, thus changing the ground properties.
Figure 62: The composite radar images, three scans of the same area of Sentinel-1A, were acquired on different dates and overlaid, with different colors assigned to each: red (7 February 2015), green (15 March 2015) and blue (27 March 2015). The colors represent changes on the ground between the various acquisitions (image credit: Copernicus Sentinel data (2015)/ESA)
• December 15, 2015: ESA has ensured the continuation of the Sentinel-1 Earth observation satellite series for Europe's Copernicus environmental program by ordering two more satellites. The contract was signed today Thales Alenia Space of Italy to build Sentinel-1C and -1D. Thales Alenia Space will lead a consortium of 60 European companies, including Germany's Airbus Defence and Space, responsible for the radar instrument. 106)
- The huge success of Sentinel-1A is highlighted by the number of users and downloads of data: more than 15 000 users have registered on the Sentinels scientific data hub, 2.5 million products have been downloaded – corresponding to 3 million GB of data – and 350 000 products are available online for download. With this new contract, ESA and its partners take on the commitment to prepare the replacements for Sentinel-1A and -1B, ensuring that users will have the data to feed the Copernicus services in the future.
• December 4, 2015: The SAR image of Figure 63 features the Netherlands as observed by Sentinel-1A in March 2015. The Netherlands borders the North Sea to the north and west, Germany to the east, and Belgium to the south. Home to some 17 million inhabitants, the country is extremely low-lying and remarkably flat, with large stretches of lakes, rivers and canals. With over 400 people/km2 , it is one of the most densely populated countries in the world. 107)
- With an eighth of the country lying below sea level,and half of the country below 1 meter above sea level, the North Sea is regarded as a threat to the country and the people. Following the huge flood of 1953, the whole area has been built up with a network of dikes and protective dams, which are clearly visible in the image. If the country were to lose the protection of its dunes and dikes, the most densely populated part of the Netherlands would be inundated, mainly by the sea, but also partly by the rivers.
- Clearly visible in the image are the typical, narrow and long green fields between Utrecht and Rotterdam, scattered with canals and smaller villages. The artificial island of Flevoland, which has been reclaimed from the Zuiderzee, an inland sea, is also visible.
- On the left side of the image is a river delta formed by the confluence of the Rhine, the Meuse and the Scheldt rivers, resulting in a multitude of islands. Rotterdam, Europe's largest harbor, is visible at the northern edge, with ships waiting to enter the port in the North Sea.
Figure 63: This image is a mosaic based on Sentinel-1A satellite coverage of the Netherlands in three scans during March 2015 (image credit: Copernicus Sentinel data (2015)/ESA
• Oct. 30, 2015: The image of Figure 64 is the Manicouagan Crater which was carved out by an asteroid strike some 214 million years ago. This crater in Quebec, Canada is known to be one of the oldest and largest impact craters on the planet. Experts believe that glaciers have since played a large part in its erosion. Its concentric structure results from the shock waves transmitted by the impact. These somewhat resemble the rings that form when a pebble is dropped into water. So big and distinct, the crater can easily be observed from space. 108)
- The multiple-ring structure is some 100 km across, with the 70 km-diameter inner ring its most prominent feature. The annular Manicouagan Reservoir lake stretches more than 550 km from the source of its longest headstream.
- This image was taken by Sentinel-1A, illuminating the landscape with horizontal and vertical radar pulses, from which the artificial color composite was generated. Diverse colors highlight variations of land cover. The varying tones of the same color represent a difference in the land's condition. Hence, while the blue tones represent bodies of ice and some water, the yellow and orange tones denote ageing vegetation of different types, mixed with patches of snow and ice.
Figure 64: This false-color SAR image, featuring the Manicouagan Crater, was captured by the Sentinel-1A satellite on 21 March, 2015 (image credit: Copernicus Sentinel data (2015)/ESA)
• October 9, 2015: This Sentinel-1A radar image of Figure65 was processed to depict water in blue and land in earthen colors. It features some of the Azore islands about 1600 km west of Lisbon, Portugal, including the turtle-shaped Faial, the dagger-like Sao Jorge and Pico Island, with Mount Pico reaching over 2351 m in height. The image highlights the differences in the relief of the islands, with volcanoes and mountains clearly standing out. 109)
- Faial is part of the central group of the Azores. The surface area covers 173 km2. It has some 15,000 inhabitants and its main municipal seat is the city of Horta. Different shades of blue decorate the houses, which divide the fields and line the roads, giving Faial the name 'Blue Island'.- In the 18th century, the development of whale hunting brought whaling fleets to Horta. By the 19th century, Horta had become an important seaport and a layover for a large number of yachts crossing the Atlantic. Along with other islands in the archipelago, Faial is of volcanic origin. In 1957, a big eruption about 1 km from the coast ejected large quantities of lava and ash, forming an islet that later became connected to Faial island by an isthmus.
- Unique among the islands of the Azores, São Jorge is uncharacteristically long and slender, and so susceptible to ocean erosion. The island is 55 km in length, with a mountain range forming its backbone. At 1053 m, Pico da Esperança is its highest peak. The island has an area of 246 km2, with an obvious difference in the relief between the western and eastern sections: the western coast is lined with cliffs, while the east is smoother. Similarly, the northern coast has sharp cliffs, while the southern side is less inclined. The island's 9500 residents have lived in relative isolation for many years. Disturbed only by seldom visits from the authorities, commercial boats from the local islands, or the occasional aristocrat who comes to contemplate the local scenery, life on São Jorge is very relaxed.
- Named after its imposing mountain, Pico Island is one of the most beautiful and underrated islands of the Azores. Its history was forged on whaling and wines. The famous Pico wines and the UNESCO world patrimony designated vineyards, as well as wooden boat building, are contemporary features of Pico. Whale hunting gave way to a movement of study and observation of whales, dolphins and other sea mammals. Since volcanic eruptions ended 300 years ago, Pico is considered dormant, adding to the island's mystery and making it a magnet for scientists.
Figure 65: The Sentinel-1A radar image of the Azore islands was captured on March 16 and release on October 9, 2015 (image credit: Copernicus Sentinel data (2015)/ESA)
There are nine major Azorean islands and an islet cluster, in three main groups. These are Flores and Corvo, to the west; Graciosa, Terceira, São Jorge, Pico, and Faial in the center; and São Miguel, Santa Maria, and the Formigas Reef to the east. They extend for more than 600 km and lie in a northwest-southeast direction.
Figure 66: Distribution of the islands of the Azore archipelago (image credit: Wikipedia)
• Sept. 21, 2015: On 16 September 2015, an 8.3 magnitude earthquake struck the coast of central Chile, triggering tsunami warnings and coastal evacuations. Lasting three minutes, this powerful earthquake occurred along the boundary of the Nazca and South American tectonic plates. 110)
- By combining Sentinel-1A radar scans from 24 August and 17 September, the rainbow-colored patterns in the image show how the surface has shifted as a result of the quake. 'Interferograms' such as these allow scientists to quantify ground movement. By counting the 'fringes', it is estimated that the earthquake caused a displacement of 1.4 m along the viewing direction of the radar observation. In addition, a 0.5 m horizontal movement is estimated along the flight direction of the satellite.
Figure 67: Interferogram of the Chile earthquake on Sept. 16, 2015, generated from two Sentinel-1A radar scans on Aug. 24 and Sept. 17, 2015 (image credit: Copernicus Sentinel data (2015)/ESA SEOM INSARAP study PPO.labs/NORUT)
• August 2015: The Sentinel-1A routine operations are on-going. The observation scenario supports the systematic coverage of a first set of Copernicus Services areas of interest, of European land and coastal waters, of global tectonic/volcanic areas, as well as of other specific targets worldwide for various applications. The observation plan also includes regular mapping of all land areas worldwide. The dedicated campaign for Antarctica ice sheet monitoring is on-going and will run till early October 2015 indicatively. 111)
Table 11: Overview of the Sentinel-1 observation scenario 112)
• August 2015: Sentinel-1A works in a pre-programmed operation mode to avoid conflicts and to produce a consistent long-term data archive built for applications based on long time series. For all existing modes, ESA routinely produces the so-called level 2 ocean product (L2 OCN) which ocean surface wind, ocean surface radial velocities and ocean swell spectra (only for Wave Mode and Stripmap image Modes) geophysical quantities are included. 113)
- Sentinel-1A ensures the continuity of global swell spectra measurements from space already available since 1991 with ERS-1, ERS-2 and ENVISAT/ASAR thanks to the so called Wave Mode. This acquisition mode is dedicated to swell measurements in open ocean. The ESA Ocean Swell product already available at IFREMER/CERSAT (Centre ERS d'Archivage et de Traitement), France, for the past missions will be routinely produced and freely available to the scientific community. On Sentinel-1A, the Wave Mode has been improved to get a larger swath and a better resolution resulting, among other benefits, in extending the range of retrieved swell wavelengths and opening research opportunities for investigations of wave groups parameters. A new "leap frog" acquisition pattern has also been designed for the Wave Mode to maximize both coverage and sampling of swell systems propagating across the ocean. This new capacities will be presented to assess ocean swell spectra performances and discuss its capacity to measure the ocean surface velocities from space in open ocean.
- In coastal areas, acquisitions will be performed in extended and interferometric wide swath modes, where only wind and radial velocities are produced by ESA.
• August 2015: The C-SAR instrument is specifically designed to carry out interferometric analyses over land, thus allowing the user to analyze Earth's surface displacements through the SBAS/DInSAR (Small BAseline Subset/Differential Interferometry SAR) technique, for the generation of Sentinel-1A IWS (Interferometric Wide Swath) deformation time-series. Due to the TOPS (Terrain Observation by Progressive Scans) mode characterizing the IWS acquisitions, the existing SBAS processing chains were properly adapted with new procedures for efficiently handling the Sentinel-1A data. 114)
- The developed DInSAR processing chain has been applied to process IWS Sentinel-1A data acquired during these first months of operation. In Figure 68, some examples of the retrieved interferograms are shown, relevant to Italy, Sakurajima volcano (Southern Japan) and Fogo volcano (Cape Verde). The presented examples demonstrate that the developed DInSAR processing chain is able to process IWS SAR data. In particular, the registration step is correctly performed, and no residual phase ramps or jumps between consecutive bursts are present.
Figure 68: Interferograms retrieved by processing Sentinel-1A IWS data, relevant to A) Italy, B) Sakurajima volcano (Japan), and C) Fogo volcano (Cape Verde), image credit: IREA-CNR, Università degli Studi di Napoli 'Federico II', Vienna University of Technology, Università di Roma 'La Sapienza'
• July 24, 2015: The image of Figure 69 shows the southern part of Germany's state of Bavaria, with the city of Munich on the right and Augsburg at the center. Munich is located on the elevated plains just north of the Alps. Zooming in, one can clearly see the river Isar flowing through the city. 115)
- The landscape in this pre-alpine region was shaped by ice-age glaciers, including the two larger lakes southwest of Munich – Lake Starnberg, which reaches to the bottom of the image, and the Ammersee to the west – that are remnants of the glaciers.
Figure 69: This image combines three radar scans of the Sentinel-1A satellite, acquired in March and April 2015 (image credit: Copernicus Sentinel data (2015)/ESA)
• On June 26, 2015, ESA released the Central California image (Figure 70) of the Sentinel-1A mission. The San Andreas Fault – the border between two tectonic plates – is visible as a somewhat straight line running from the upper-left corner of the image close to the bottom center. The Pacific Plate to the west is moving in a northwest direction, while the North American plate to the east is shifting southeast. This horizontal scraping is happening at up to about 5 cm a year in some parts of the fault. 116)
- The fault is responsible for the high earthquake risk in the area, including the 6.0 magnitude shock that struck the town of Parkfield in 2004.
- Surrounding the fault are the mountains of the Southern Coastal Ranges. With a predominantly Mediterranean climate, these mountains have a range of plant communities including oak woodland, mixed evergreen forests and savannahs.
- East of the Ranges is the San Joaquin Valley, where one can see the geometric shapes of large-scale agricultural production. Major crops include grapes, cotton, nuts and fruits, with productivity relying on irrigation from surface water diversions and groundwater pumping from wells. - Drought in recent years has led to water shutoffs and cutbacks, severely hindering yields in what was once the country's most productive agricultural region.
- Further east along the right side of the image are the foothills of the Sierra Nevada mountains.
Figure 70: This image, captured by the C-SAR instrument of Sentinel-1A on April 1, 2015, shows a central region of California in the US (image credit: Copernicus data (2015)/ESA)
• June 5, 2015: The Chinese city of Tianjin is captured in this Sentinel-1A radar image (Figure 71) created by combining three scans over several months. The city is located to the west of the Bohai Bay within the Bohai Gulf (Yellow Sea). With a population of over 14 million people, this megacity is among China's five largest. 117)
- Urban areas are home to over half of the world's population, and are rapidly changing environments. As more people move from rural areas to cities, this growth needs to be monitored to help it proceed on a sustainable basis. High-resolution satellite data provide essential information for city planning and for the sustainable development of urban regions. Radar in particular can be used to monitor slight ground movements down to a few millimeters – valuable information for urban planners and for risk assessment.
- Zooming in on the upper right of Figure 71, one can see different colors in the geometric agricultural fields, showing changes between the three acquisitions (22 October 2014, 14 January 2015 and 7 February 2015) that make up this image. In this area, the black shapes show divided areas covered in water, which are possibly shrimp or fish farms. Bohai Bay was traditionally home to the country's richest fisheries, but pollution, overfishing and land reclamation has diminished this economic activity.
- Farther south, one can see numerous dots in the water – radar reflections from the boats coming to and from the Port of Tianjin. This massive maritime gateway handles hundreds of millions of tonnes of cargo each year, and is the largest in northern China.
- On the central-left side of the image, the black and bright green areas are water reservoirs. The upcoming Sentinel-2 mission – set for launch on 22 June – will be used to support the sustainable management of water resources by providing measurements of water quality and detecting changes.
Figure 71: Sentinel-1A radar image of Tianjin, China, captured in 3 acquisitions (22 October 2014, 14 January 2015 and 7 February 2015), image credit: Copernicus data (2014/2015)/ESA
• April 30, 2015: Based on data from the Sentinel-1A satellite, this image shows how and where the land uplifted and sank from the 7.8 magnitude earthquake that struck Nepal on 25 April 2015. DLR/EOC (German Aerospace Center/Earth Observation Center) has used data acquired by the ESA radar satellite Sentinel-1 over the earthquake region in Nepal to create a new regional aid map and calculate the ground displacements caused by the earthquake. The data were processed by the EOC to create an interferogram showing the surface displacements. 118) 119)
Figure 72: Based on data from the Sentinel-1A satellite, this image shows how and where the land uplifted and sank from the 7.8 magnitude earthquake that struck Nepal on 25 April 2015 (image credit: DLR/EOC)
Legend to Figure 72: A more than 90 km long and 30 km wide area around Kathmandu is affected. During the earthquake, tensions between the Indian and Eurasian plates were relieved and generated sudden ground movements of several meters. In the newly created map, the deformations are color-coded; near the plate boundary, the surface moved towards the satellite, that is, upwards (blue area). Further to the north there was related subsidence (yellow), a counter-movement that often occurs during earthquakes in subduction zones. In addition, the researchers detected a strong horizontal movement of up to 2 m in a north-south direction in the area. In the image, the locations of numerous aftershocks in recent days are also displayed. Scientists at the EOC are continuing to compare archival data of the area with the latest radar images from Sentinel-1A.
• April 29, 2015: On 25 April, a 7.8 magnitude earthquake struck Nepal, claiming over 5000 lives and affecting millions of people. Satellite images are being used to support emergency aid organizations, while geo-scientists are using satellite measurements to analyze the effects of the earthquake on the land. 120)
- Radar imagery from the Sentinel-1A satellite shows that the maximum land deformation is only 17 km from Nepal's capital, Kathmandu, which explains the extremely high damage experienced in this area.
- By combining Sentinel-1A imagery acquired before and after the quake, changes on the ground that occurred between the two acquisition dates lead to rainbow-colored interference patterns in the combined image, known as an 'interferogram', enabling scientists to quantify the ground movement.
Figure 73: Interferogram over Kathmandu, Nepal, generated from two Sentinel-1A scans on 17 and 29 April 2015 – before and after the 25 April earthquake. Each 'fringe' of color represents about 3 cm of deformation. The large amount of fringes indicates a large deformation pattern with ground motions of 1 m or more (image credit: Contains Copernicus data (2015)/ESA/DLR Microwaves and Radar Institute/GFZ/e-GEOS/INGV–ESA SEOM INSARAP study)
- Under the Copernicus EMS (Emergency Management Service), ESA has been coordinating the collection of optical satellite imagery following the earthquake that struck Nepal on 25 April 2015. The satellite imagery is then used to create maps to support relief efforts, such as this 'grading map' of Kathmandu (Figure 74) showing topographic features and crisis information. The map is based on imagery from the WorldView-3 satellite acquired on 28 April. 121)
Figure 74: Kathmandu grading map (image credit: DigitalGlobe/European Commission)
- The Copernicus EMS was activated on the day the earthquake struck, prompting ESA to begin collecting satellite imagery, which is being made available to support relief efforts. In parallel, the International Charter Space and Major Disasters was activated by India, China and the UN. Partner Agencies of this initiative have been providing data and products over the area to relief organizations.
• April 24, 2015: The Florida peninsula is located between the Gulf of Mexico to the west, and the Atlantic Ocean to the east. The large body of water at the top of the image is the freshwater Lake Okeechobee. Covering about 1900 sq km, the lake is very shallow with a maximum depth of about 4 m. The lake is the largest freshwater lake in the state of Florida with a volume of ~5.2 km3. The southern region of Lake Okeechobee shows geometric patterns of agriculture and infrastructure, while the very lower section of the image shows part of the Everglades National Park. 122)
Figure 75: The Sentinel-1A SAR image shows part of southern Florida, acquired on Nov. 13, 2014 in interferometric wide swath mode. This is the default mode over land, and has a swath width of 250 km and a ground resolution of 5 x 20 m. The image was also acquired in 'dual polarization' horizontal and vertical radar pulses, from which the artificial color composite was generated (image credit: Copernicus data, ESA)
- The Everglades are the United States' largest subtropical wilderness, originally covering some 10 ,000 km2, now the wilderness has a size of about 6000 km2. This area features shallow, slow-moving water and is abundant in a plant called sawgrass that becomes so thick it makes the water barely visible, earning the Everglades the nickname 'River of Grass'. - Home to diverse plant and animal life, including alligator, crocodile, Florida panther and manatees, the Everglades have been designated a Wetland of International Importance by the Ramsar Convention on Wetlands, a World Heritage Site and an International Biosphere Reserve by the UN.
- Other notable locations visible in this image include the cities Miami (east of the Everglades), in the lower right, and the city of Fort Myers, built along the Caloosahatchee River, on the Gulf Coast of Florida.
• On April 3, 2015, the Sentinel-1A mission marked one year on orbit, providing radar vision for Europe's Copernicus program. The Sentinel-1A satellite has completed a successful first year. Just weeks after its launch, its imagery was already being used to assist in emergency responses. Some of its first images were crucial in helping authorities in Namibia and the Balkans decide how to respond to a serious floods – both while the satellite was still in its early commissioning phase. 123)
- Sentinel-1A's began supplying data operationally in October 2014. Within days, experts began using the data to monitor the marine environment. This included the production of ice charts, showing the details of ice conditions in a variety of regions, including the warnings of icebergs drifting in shipping routes to alert vessels. - Over the year, Sentinel-1A has also been used to monitor ice loss from ice caps and ice sheets, such as the Austfonna ice cap in Norway's Svalbard archipelago. The first dedicated campaign observing the Greenland ice sheet was completed in March 2015.
- The flood of results that make Sentinel-1A's first year such a success wouldn't be possible without the rapid data dissemination and the Copernicus open access policy. To date, more than 6000 users have registered to access the 83,000 online data products. Since the data became available in October 2014, over half a million downloads have been made so far – the equivalent of about 680 TB of data. - To assist with data processing, product reading and analysis, the Sentinel-1 Toolbox is being used by over 1000 users in 70 countries.
• April 3, 2015: The image of Figure 76 shows a portion of Lake Baikal in Siberia. Baikal is the largest lake by volume in the world, containing over 23,000 km3 of freshwater, and is considered to be the oldest lake in the world. It sits in the Baikal Rift Zone where Earth's crust is pulling apart. Hot springs are found under and around the lake. The area has over 1000 species of plants and 2500 species of animals. More than 80% of the animals found here are endemic. -Located on the Angara River, Irkutsk is a popular stop on the Trans-Siberian railway because of its close vicinity to Lake Baikal. 124)
Figure 76: The city of Irkutsk (center left) and part of Lake Baikal (right) are pictured in this Sentinel-1A image over Russia's Siberia region (image credit: Copernicus data 2015, ESA)
Legend to Figure 76: This image combines three radar scans from the Sentinel-1A satellite on 11 January, 4 February and 24 March 2015, with each scan being assigned a color: red, green and blue. The colors present in Lake Baikal and the Angara River show the location of ice on the different dates.
• March 27, 2015: Sentinel-1 offers excellent capabilities for observing the velocities of Greenland's outlet glaciers with unprecedented temporal resolution at complete spatial coverage, extending and enhancing the time series of ice-velocity maps available from previous satellite sensors. 125)
- The InSAR (Interferometric Synthetic Aperture Radar) technique is used where two or more images over the same area are combined to detect slight changes occurring between acquisitions. Tiny changes on the ground cause changes in the radar signal and lead to rainbow-colored interference patterns in the combined image, known as an 'interferogram'. Precise measurements – down to a scale of a few millimeters – can be detected across wide areas. Tectonic plates grinding past one another, the slow 'breathing' of active volcanoes, the slight sagging of a city street through groundwater extraction, even the thermal expansion of a building on a sunny day.
- The Fringe Workshop — held at ESA/ESRIN in Frascati, Italy on March 23-27,2015 — takes its name from these colored fringes seen in the interferograms.
Figure 77: This image combines two Sentinel-1A radar scans from 3 and 15 January 2015 to show ice velocities on outlet glaciers of Greenland's west coast (image credit: Copernicus data (2015), ESA, Enveo)
• March 27, 2015: The Aral Sea, located on the border between Kazakhstan and Uzbekistan in Central Asia, is a striking example of humankind's impact on the environment and natural resources. Once the world's fourth-largest inland water body, it has lost around 90% of its water volume since 1960 because of Soviet-era irrigation schemes. 126)
- As the water evaporated, it left behind a dry, white salt terrain now called the Aral Karakum Desert. Each year violent sandstorms pick up salt and sand from the desert and transport it across hundreds of kilometers, causing severe health problems for the local population and making regional winters colder and summers hotter.
- Chemicals in the dry plains from former weapons testing, industrial projects and fertilizer runoff exacerbates the effects of these storms on health. In addition, the area's fishing industry – which once employed tens of thousands of people – has been devastated.
- The World Bank and Kazakhstan has worked together to build the Kok-Aral dyke to stabilize the northern section of the sea. The Aral Sea's southern section – part of which is pictured here – was beyond saving and is projected to dry out completely by the end of this decade.
Figure 78: The Aral Sea, a multispectral image Sentinel-1A radar image (image credit: Copernicus data (2014/2015), ESA)
Legend to Figure 78: This image was created by combining three radar scans from Sentinel-1A, assigning each a color: red (from 17 October 2014), green (from 28 December 2014) and blue (from 14 February 2015). Different colors represent changes between the acquisitions. - In the lower right, the red, yellow and green boomerang shape shows where water flows into the dry seabed from a river, and colors show how the area covered in water increased over time. Along the left side of the image, the large dark area shows where water is still present. Colors along the water's edge show water-level changes between acquisitions. Red shows a lower level than blue, so the water level was lower on 17 October 2014 than on 14 February 2015. - Zooming in on the lower-left corner, one can see the straight line of a road outside of the seabed, with white dots showing where the radar signal has reflected of human-made structures. White dots also appear further east, showing where structures have been built in the seabed.
• March 25, 2015: Sentinel-1A captured the fast-moving Pine Island glacier, which flowed about 100 m in less than two weeks. Pine Island is the largest glacier in the West Antarctic Ice Sheet and one of the fastest ice streams on the continent, with an average of over 4 km per year. About a tenth of the ice sheet drains out to the sea by way of this glacier. With its all-weather, day and night radar vision, the Sentinel-1 mission is an important tool for monitoring polar regions and the effects that climate change has on ice. 127)
Figure 79: Pine Island Glacier on Sentinel-1A's radar [image credit: Copernicus data (2015), ESA, A. Hogg, University of Leeds, Centre for Polar Observation and Modelling (CPOM)]
Legend to Figure 79: This image, combining two scans by Sentinel-1A's radar, shows that parts of the Pine Island glacier flowed about 100 m (in pink) between 3 March and 15 March 2015. Light blue represents stable ice on either side of the stream.
• March 20, 2015: In Figure 80, London appears as a cluster of bright radar reflections along the River Thames in this radar image from Sentinel-1A. The spacecraft captured this image on 4 March 2015 in its Interferometric Wide Swath mode and dual polarization, from which the artificial color composite was generated. 128)
Figure 80: Sentinel-1A SAR image of London captured on March 4, 2015 (image credit: Copernicus data, ESA)
• March 19, 2015: ESA and the UK Space Agency have signed an arrangement that establishes access to data from the Sentinel satellites, marking a significant step in their exploitation. Following the launch of Sentinel-1A in April 2014, the next in the series of satellites, Sentinel-2A, is scheduled for launch in June, 2015. 129)
- The Sentinel-2 mission will provide 'color vision' for Europe's Copernicus environment monitoring program, with data being used to monitor plant health, changing lands, inland water bodies, the coastal environment and support disaster mapping. - Data from the Sentinel satellites and contributing missions to the Copernicus program are freely accessible for Copernicus Services, as well as to scientific and other users.
- The agreement aims to facilitate Sentinel data exploitation in the country. The UKSA (UK Space Agency) will coordinate ground segment activities in the UK – such as hosting, distributing, ensuring access and archiving Sentinel data – and act as an interface between ESA and national initiatives. This will be done through a 'national mirror site' at the Harwell Science, Innovation and Business Campus in Oxfordshire, where ESA's space applications center, ECSAT (European Center for Space Applications and Telecommunications), is also based.
- The agreement also established ESA's role as coordinator of the Copernicus 'space component'. The Agency will ensure direct access to Sentinel data, provide technical advice on the setting up of data acquisition and dissemination, and make data processing and archiving software available to national initiatives. - A total of seven Participating States have now signed the agreement: Greece, Norway, Italy, Finland, Germany, France and the UK.
•March 2015: A Red Alert has been declared in southern Chile after the eruption of the Villarrica volcano early on 3 March. Thousands of residents in the area have been evacuated and the International Charter Space & Major Disasters has been activated by Chile's risk management authority ONEMI. 130)
Figure 81: Surface changes of Villarrica from Sentinel-1A (image credit: Copernicus data/ESA (2015), map produced by the German Remote Sensing Data Center of DLR)
• Figure 82, released on Feb. 20, 2015 in ESA's 'Earth from space video program, shows the Baltic country of Estonia – with the political borders in white – is pictured in this mosaic of eight scans by Sentinel-1A's radar from October to December 2014.
Figure 82: Sentinel-1A SAR mosaic of Estonia acquired in 8 scans from October to December 2014; the scans were recorded in 'dual polarization' horizontal and vertical radar pulses, from which the artificial color composite was generated (image credit: Copernicus data, ESA)
Legend to Figure 82: The flat country has over 1500 islands and 1400 lakes, while forests cover about 40% of the land area. The largest island, Saaremaa, can be seen on the very left side of the image. Although not visible in this image, the island is the site of a group of meteorite craters that, at the time of impact an estimated 4000–7000 years ago, burned forests within a 6 km radius.
The capital and largest city, Tallinn, can be seen by its white radar reflections along the coast near the top of the image. About a third of Estonia's population lives here. East of Tallinn is the Lahemaa National Park. Covering over 470 km2 of land and 250 km2 of the sea, it is the country's largest national park. It includes forests, limestone plateaus, waterfalls, raised bogs, peninsulas and bays along the coastal plain and offshore islands. Wildlife includes lynx, bear and wolves.
Lake Peipus – the country's largest with a total surface area over 3550 km2 – straddles the border with Russia to the east. The population of Estonia is about 1,300,000 in a country of 45,227 km2 in size.
On 4 February 2015, Estonia signed the Accession Agreement to the ESA Convention. Later this year, the Government of Estonia will conclude the ratification process and will become the 21st ESA Member State.
Table 12: Overview of qualification steps of the Sentinel-1 mission as of February 2015
Figure 83: An operational qualification phase leading to the Routine Operations (image credit: ESA)
- The Sentinel-1 data acquisition is performed according to a systematic & pre-defined instrument observation plan (Long Term Plan, LTP) built according to the identified user needs.
- During the operational qualification phase, the observation scenario is gradually increased, in overall sensing time/coverage/modes/timeliness, to reach the full capacity by the Routine Operations Phase.
Figure 84: Sentinel-1A SAR image of the metropolitan area of Portugal's capital, Lisbon, acquired on Oct. 8, 2014 (image credit: ESA)
Legend to Figure 84: Flowing in from the upper-right corner is the Tagus River. Originating in central Spain, the Tagus is the longest river on the Iberian Peninsula, stretching over 1000 km. The river flows southwest through Portugal, emptying into the Atlantic Ocean at Lisbon. Its estuary – visible at the center of the image – is a natural reserve and Ramsar Wetland of international importance.
With extensive mudflats, saltmarshes, reedbeds and human-made salt pans, the area is important for around 16 species of wintering or staging waterbirds, numerous species of breeding birds, and the European otter. Activities outside of the reserve include fishing, shellfish collecting and agriculture, as we can see by the geometric shapes of agricultural plots.
Lisbon's city center sits on the northern shore of the Tagus River, and is visible by the bright radar reflections from buildings and other structures. One can also see the reflections from the suspension bridge that connects Lisbon to Almada on the southern shore. - Further east, though not as clear, is the Vasco da Gama Bridge, the longest in Europe, at 17.2 km.
• January 23, 2015: A research team led by scientists from CPOM (Center for Polar Observation and Modelling) at the University of Leeds in the UK combined observations from eight satellite missions, including Sentinel-1A and CryoSat, with results from regional climate models. These results of the study provide a clear example of just how quickly ice caps can evolve, and highlight the challenges associated with making projections of their future contribution to sea level. 133) 134)
The Austfonna ice cap is located in the northeastern part of the Svalbard archipelago of Norway. Containing approximately 2500 km3 of ice, it is drained by both land- and marine-terminating glacier systems. About 28% of the ice cap bed lies below sea level and over 200 km of its southern and eastern margin terminates in the ocean with parts resting on a retrograde slope. Since 2012, parts of the Austfonna ice cap have thinned by more than 50 m, about a sixth of the ice's thickness.
Figure 85: Austfonna ice loss (image credit: CPOM/GRL)
Legend to Figure 85: The main figure (top) shows the rate of ice cap elevation change between 2010 and 2014 observed by CryoSat, overlaid on an image acquired by Sentinel-1A (in 2014). Red indicates that the ice surface is lowering. In the southeast region (green box) ice thinning far exceeds the color scale of 2 m per year. — A closer look at the southeast region is shown in the four smaller figures at the bottom. These figures show the evolution of ice velocity over the last two decades. Ice velocity in 2014 was mapped using Sentinel-1A and the DLR's (German Aerospace Center's) TerraSAR-X mission.
• Dec. 11, 2014: Five Sentinel-1A radar scans acquired between 3 October and 2 December 2014 were combined to create this interferogram of ground deformation in Mexico City (Figure 86). The deformation is caused by ground water extraction, with some areas of the city subsiding at up to 2.5 cm/month (red). 135)
These preliminary results were presented at the InSARap Workshop at ESA/ ESRIN, Frascati, Italy, for Earth observation, December 10-11, 2014 (Workshop on Sentinel-1 TOPS interferometry). InSARap is a project under ESA's SEOM (Scientific Exploitation of Operational Missions) program.
Figure 86: Radar images from the Sentinel-1A satellite show ground movement in Mexico City, with some areas sinking up to 2.5 cm/month (image credit: Copernicus data (2014)/ESA/DLR Microwave and Radar Institute–SEOM InSARap study)
• Dec. 05. 2014: Figure 87 is a radar image of Romania – with the political borders in red – is a mosaic of 15 scans by Sentinel-1A's radar acquired in October and November. The Carpathian mountain range, 1500 km in length, sweeps down in a wide crescent-shaped arc from the north and across the center of the country. The highest portion within the Carpathians is the Tatra range, on the border of Slovakia and Poland, where the highest peaks exceed 2,600 m. The country of Romania covers an area of 238,391 km2. 136)
- Romania is home to the largest area of virgin forests in Europe, totaling 250,000 hectares, most of them in the Carpathians. These forests are home to brown bears, wolves and other animals, and many thermal and mineral springs can be found in the foothills.
- The longest river in Europe – the Danube – flows along part of western Romania's border with Serbia, as well as its southern border with Bulgaria. The river then flows northward and empties into the Black Sea via the Danube Delta, which lies within Romania and Ukraine – visible on the right side of the image. Designated a UNESCO World Natural Heritage Site in 1991, the Danube Delta is a labyrinth of river channels, lakes, bays, floodplains, marsh and reed beds. This vast triangular delta is home to an extremely rich variety of birds, fish, animals and plants.
- Romania's capital, Bucharest, is visible in the southern part of the country, as a cluster of bright radar reflections expanding outward from the center.
Figure 87: Sentinel-1A radar image of Romania. The scans were acquired in 'dual polarization' horizontal and vertical radar pulses, from which the artificial color composite was generated [image credit: Copernicus data/ESA (2014)]
• Dec. 02.2014: The Pico do Fogo volcano on Cape Verde's Fogo island erupted on 23 November 2014 and showed continuing volcanic activity in the days following. By processing two Sentinel-1A radar images, which were acquired on 3 November and 27 November 2014, this interferogram was generated. Deformation on the ground causes phase changes in radar signals that appear as the rainbow-colored patterns. 137)
- Results like these are being used by Earth scientists to help them map the volcano's subsurface magmatic system, perform geophysical modelling of the volcanic eruption mechanics, and assist the relief efforts on the ground. With this stunning result, the great potential of Sentinel-1 for geophysical applications has been once again unequivocally demonstrated.
Figure 88: Interferogram (combination of 2 radar images) of Cape Verde's Fogo eruption, acquired by Sentinel-1A before and during the eruption (image credit: Copernicus data (2014)/ESA/Norut-PPO.labs–COMET-SEOM InSARap study)
Legend to Figure 88: The Cape Verde island of Fogo (Portuguese for "fire") is located in the Atlantic Ocean (14.93°N, 24.38°W) off the West Coast of Africa. The island has a size of 476 km2 with Pico de Fogo at 2829 m in elevation. — Deformation on the ground causes changes in radar signals that appear as the rainbow-colored patterns. Scientists can use the deformation patterns to understand the subsurface pathways of molten rock moving towards the surface. In this case, the radar shows that the magma travelled along a crack at least 1 km wide. By acquiring regular images from Sentinel-1A, the project is in a position to monitor magma movement in the subsurface, even before eruptions take place, and use the data to provide warnings.
• Nov. 28. 2014: Marking a first in space, Sentinel-1A and Alphasat have linked up by laser stretching almost 36 000 km across space to deliver images of Earth just moments after they were captured (LEO-GEO communication). This important step demonstrates the potential of Europe's new space data highway to relay large volumes of data very quickly so that information from Earth-observing missions can be even more readily available. 138) 139) 140) 141)
Creating a link between the two orbital kinds of satellites (LEO and GEO) means that more information can be streamed to Earth, and almost continuously. Engineers have turned to laser to accomplish this. — Funded by ESA and the DLR (German Aerospace Center), Tesat has developed a laser communications terminal and downlink system that is carried on the geostationary Alphasat, Europe's largest telecommunications satellite. This novel unit's counterpart is flying on Sentinel-1A.
Over the past few weeks the Sentinel-1A operation teams at ESA/ESOC in Darmstadt, Germany, and ESA/ESRIN in Frascati, Italy, and GSOC (German Space Operations Center, in Oberpfaffenhofen, Germany, have been working intensively to prepare for the first laser link tests.
Note: The Alphasat I/Inmarsat 1-XL/Inmarsat-4A F4 spacecraft (6650 kg) was launched on July 25, 2013 into GEO and is positioned at 24.9º East. It carries an LCT (Laser Communication Terminal -TDP1), developed by Tesat Spacecom (Germany) and Ruag Space (Switzerland) with funding from DLR and the Swiss Space Office, and coordinated by ESA. This TDP (Technology Demonstration Payload) is a data relay mission to link observation data from LEO observation satellites towards a ground station through the geostationary Alphasat. The LCT link is operated at 1.8 Gbit/s (bidirectional data link), with a design that could scale up to 7.2 Gbit/s in the future.
Figure 89: Image of Berlin from Sentinel-1A via laser transmission, acquired on Nov. 28, 2014 (image credit: ESA)
Legend to Figure 90: The image of Berlin, Germany, captured by Sentinel-1A is one of the first images delivered via the precursor EDRS (European Data Relay System) carried as a hosted payload on Alphasat, which is in geostationary orbit 36 000 km above Earth. The image is a result of the two satellites using their optical communications instruments to transfer data via laser for fast delivery. Alphasat's 'technology demonstration payload' shows how the EDRS space data highway will make large volumes of data from satellites in LEO available almost instantly. Sentinel-1A transmits data to Earth routinely when passing over ground stations in Norway, Italy and Spain. 142)
- Complementing the Sentinel ground-station network, EDRS connects with the satellite and collects data via laser and then transmits the data in near-realtime to the ground – essential for applications such as maritime safety and for helping respond to disasters such as floods.
- The first EDRS element will be carried on the Eutelsat-9B satellite, which will be launched in 2015. In the meantime, Sentinel-1A will be able to connect through the laser communications terminal on Alphasat. Sentinel-2A, scheduled to be launched in the spring of 2015, also carries the same optical communications payload.
• Nov. 21, 2014: ESA released the image of Figure 91 on Nov. 21, 2014. Greater Tokyo is home to nearly 38 million people, making it the world's largest 'megacity' (a metropolitan area with more than 10 million people). Today, there are over 30 megacities across the globe. Urban areas are home to over half of the world's population, and are rapidly changing environments. As more people move from rural areas to cities, this growth needs to be monitored to help it proceed on a sustainable basis. 143)
Figure 91: Sentinel-1A SAR image of Tokyo Bay, Japan, acquired on July 11, 2014 (image credit: ESA)
Legend to Figure 91: Tokyo's center lies mainly south of the Arakawa River. Other visible rivers on this image are the Edo River to its north and the Tama River just to its south, with all three streaming into Tokyo Bay. At the mouth of the Tama River we can see the runways of Haneda Airport. Note the difference between the area northwest of Tokyo Bay – where bright radar reflections show dense construction – and its southeastern opposite. This area with an overall brownish color is the site of the Minami-Bōsō Quasi-National Park.
• Nov. 10, 2014: With Sentinel-1A, the first Copernicus satellite, now operational, ESA and the DLR German Aerospace Center have signed an arrangement on managing and accessing Sentinel data. The data provided by the Earth-observing missions are freely accessible for Copernicus Services, as well as to scientific and other users. 144)
- At an event held last week at the ESA Headquarters in Paris, France, ESA and Germany signed an Understanding for the Sentinel Collaborative Ground Segment Cooperation, which aims to facilitate Sentinel data exploitation in the country. Under the agreement, DLR will coordinate ground segment activities in Germany – such as hosting, distributing, ensuring access and archiving Sentinel data – and act as an interface between ESA and national initiatives. DLR also plans to cooperate with different European partners and institutions.
- As coordinator of the Copernicus 'space component', ESA supports national initiatives by establishing direct and efficient access to Sentinel data, providing technical advice on the setting up of data acquisition and dissemination, as well as making data processing and archiving software available to national initiatives.
• On 28 October 2014, the European Commission and ESA signed an Agreement of over €3 billion to manage and implement the Copernicus 'space component' between 2014 and 2021. The signature also marks the transfer of ownership of Sentinel-1A to the EU, just weeks after the satellite became operational following intense data quality testing and calibration during its commissioning phase. 145)
• October 22, 2014: Within the first days of its operational life, the Sentinel-1A satellite has provided data for marine services in the Arctic. During the first week of the satellite's operational data supply, experts from the Technical University of Denmark and the Danish Meteorological Institute working under the Horizon 2020 "MyOcean Follow-On project" used the data to alert vessels on marine ice conditions. 146)
- The series of MyOcean projects is the pre-operational precursor of the Copernicus Marine Environment Monitoring Service, to be implemented by the European Commission. Its primary objective is to provide forecasts of the global marine environment and the near-realtime observation data necessary for forecast models.
- Since Sentinel-1A data started to become free and accessible earlier this month with the satellite entering into its operational phase, the DMI (Danish Meteorological Institute) began to use the information to improve observations of the polar regions and forecast maritime conditions. The data are being used to produce ice charts, showing the details of ice conditions in a variety of regions, including the warnings of icebergs drifting in shipping routes.
Figure 92: October 8, 2014 ice chart of Greenland (image credit: DMI)
Legend to Figure 92: Sentinel-1 is the primary source of data for ice charts produced for the Copernicus marine core service MyOcean and PolarView by the Danish Meteorological Institute. This image is an example of an ice chart from 8 October 2014, with red depicting sea ice cover.
Legend to Figure 93: The capability of Sentinel-1A to detect icebergs during all weather conditions is improving maritime safety. The satellite's radar gathers information in either horizontal or vertical radar pulses, and colors can be assigned to the different types. In this image acquired near Greenland's Jakobshavn Glacier on 26 April 2014, sea ice appearing blue-green can be distinguished from icebergs in pink.
• October 6, 2014: ESA announced the end of Sentinel-1A commissioning period. This marks the beginning of the satellite's operational life, delivering radar coverage for an array of applications in the areas of oceans, ice, changing land and emergency response. The operation of the Sentinel-1A spacecraft was formally handed over to the mission management team. The satellite will now begin delivering radar scans for an array of operational services and scientific research. 148)
- In the last few months, during the commissioning phase, Sentinel-1A has been performing exceptionally well, demonstrating its outstanding capabilities and receiving very positive feedback from the user community.
• Sept. 02, 2014: The biggest earthquake in 25 years struck California's Napa Valley in the early hours of 24 August 2014. By processing two Sentinel-1A images, which were acquired on 7 August and 31 August 2014 over this wine-producing region, an interferogram was generated. The two round shapes around Napa valley, which are visible in the central part of the image show how the ground moved during the quake. Deformation on the ground causes phase changes in radar signals that appear as the rainbow-colored patterns. Each color cycle corresponds to a deformation of 28 mm deformation. The maximum deformation is more than 10 cm, and an area of about 30 x 30 km was affected significantly. The image also clearly maps the infamous San Andreas Fault running along the coastline. 149)
Interferograms like these (Figures 94 and 95) are being used by scientists on the ground to help them map the surface rupture and model the earthquake. This interferogram very clearly shows the fault that caused the earthquake, which had not been identified as being particularly hazardous prior to the event.
Despite this interferogram being computed with images acquired in the satellite's 'stripmap mode', which is not going to be the default mode when operational, this result demonstrates the capability of Sentinel-1A and marks the beginning of a new era for our ability to map earthquakes from space.
Scientists collaborating through the UK Natural Environment Research Council's Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), used Sentinel-1A's special capabilities to analyze the quake. Yngvar Larsen from Norway's Northern Research Institute and Petar Marinkovic from PPO.labs in the Netherlands processed this new interferogram from two images: one that Sentinel-1A acquired on 7 August, the day the satellite reached its operational orbit, and another captured on 31 August.
It clearly confirms that part of the West Napa Fault system was responsible for the 6.0 earthquake that rocked California's wine-producing region. However, the fault had not been identified as being particularly hazardous prior to the quake that hit on 24 August.
Importantly, the extent of the ground deformation in the interferogram shows that the fault slip continues further north than the extent of the rupture mapped at the surface.
Figure 94: Sentinel-1A interferogram of the California coast and the Napa Valley released on Sept. 2, 2014 (image credit: ESA/PPO.labs/Norut/COMET-SEOM Insarap study)
Figure 95: Sentinel-1A interferogram of the Napa Valley released on Sept. 2, 2014 (image credit: ESA/PPO.labs/Norut/COMET-SEOM Insarap study)
• Aug. 26, 2014: Although it was only launched a few months ago and is still being commissioned, the new Sentinel-1A radar satellite has already shown that it can be used to generate 3D models of Earth's surface and will be able to closely monitor land and ice surface deformation.
- The satellite reached its operational orbit on August 7, 2014 and just 12 days later, its radar images were used to generate 'interferograms' that map the topography of parts of Italy and Norway. 150)
Figure 96: Sentinel-1A IWS image of the Gulf of Genua, Italy acquired in August 2014 (image credit: ESA/DLR Remote Sensing Technology Institute)
Legend to Figure 96: Produced on 19 August 2014, this is one of the first 'interferograms' generated using radar images from Sentinel-1A. The interferogram combines images acquired on 7 and 19 August in IWS (Interferometric Wide Swath) mode. It covers an area of 250 km x 340 km with two seamless radar acquisitions and shows Italy's Gulf of Genoa towards the top, the Po River Valley in the top right, the island of Elba lower center and the tip of France's island of Corsica in the lower left corner. Although Sentinel-1A is still being commissioned, this new result demonstrates how useful it will be to map the shape of the land and monitor ground movement. Synthetic aperture radar interferometry – or InSAR – is a technique where two or more satellite radar images acquired over the same area are combined to map surface topography and detect surface change.
TOPS (Terrain Observation by Progressive Scans) is the new acquisition mode that Sentinel-1 employs to continuously scan Earth in 250 km swathes with a resolution of 5 m perpendicular to and 20 m along the flight direction. In the future, this technology will allow the two Sentinel-1 satellites (Sentinel-1A and 1B) to regularly image Earth's entire landmass with an unprecedentedly short revisit interval. A great advantage of this type of sensor is that it can systematically image Earth unhindered by cloud cover – both during day and night. The phase and polarization information present in the images will be used for a wide range of applications such as producing up-to-date topographic maps, observing vegetation or measuring the movement of geologically active regions from space with millimeter precision. 151)
Figure 97: Sentinel-1A IWS image of the northern coast of Norway (image credit: ESA/Norut–SEOM Insarap study)
Legend to Figure 97: This interferogram of the northern coast of Norway combines two radar images acquired by Sentinel-1A on August 11 and 23, 2014. This new result demonstrates how useful it will be to map the shape of the land and monitor ground movement.
• In August 2014, the LEO LCT (Laser Communication Terminal) on Sentinel 1A was switched on in orbit and performed successfully all self-tests. The LCT is now ready for the next step, which is the in orbit commissioning. 152)
• Figure 98 of the Sentinel-1A satellite was released on July 11, 2014 in ESA's Earth from Space video program. Guanabara Bay is an oceanic bay located in Southeast Brazil in the state of Rio de Janeiro. On its western shore lies the city of Rio de Janeiro and Duque de Caxias, and on its eastern shore the cities of Niterói and São Gonçalo. Four other municipalities surround the bay's shores. Guanabara Bay covers an area of 412 km2. 153)
Figure 98: C-SAR dual-polarization image of Rio de Janeiro, Brazil, with its Guanabara Bay, acquired on May 13, 2014 (image credit: ESA)
Legend to Figure 98: Rio is connected to the city of Niterói on the east side of the bay by a large bridge which appears as a dotted straight line. To the north, we can see radar reflections from large ships. Governador is the largest island in Guanabara Bay, and the site of Rio de Janeiro's main airport. The runways appear as dark lines. The image was acquired in dual polarization and colors were assigned to the different features of the scene for better distinction.
Part of Rio de Janeiro was designated a UNESCO World Heritage Site in 2012 under the category of 'cultural landscapes'. The Tijuca National Park – the mountainous area in the lower-left – is a hand-planted rainforest covering more than 30 km2. The iconic statue of Christ the Redeemer stands at the eastern end of the forest, overlooking the city from the peak of the 700 m high Corcovado mountain.
At the bottom-center part of the image, the curved coast of the famous Copacabana is visible, while Sugarloaf mountain sits at the mouth of the bay.
• Figure 99 was released on July 4, 2014 in ESA's Earth from Space video program showing the Philippine island of Luzon with Mount Pinatubo. This active volcano experienced a major eruption on 15 June 1991 that injected more particulate matter into the atmosphere than any eruption since Krakatoa in 1883. In the months following, aerosols formed a layer of sulphuric acid haze around the globe, ozone depletion increased and global temperatures dropped by about 0.5ºC. 154)
In the upper-central part of the image, the dark area is Lake Pinatubo, which formed in the summit crater after the 1991 eruption. The water level has been rapidly increasing since its formation, putting pressure on the crater walls, which threaten to collapse and cause flash floods. The Philippine government has taken measures to alleviate the pressure with controlled draining.
South of Lake Pinatubo near the center of the image is Mapanuepe Lake, which also formed as a result of the 1991 eruption. When mud mixed with water and volcanic rock fragments flowed down from Pinatubo, it blocked the drainage of the river. The valley – including the settlements – was inundated. These mud and volcanic debris flows are still visible reaching west towards the South China Sea.
Other features visible in this image include the bright radar reflections from a shipyard on the Subic Bay to the south, and the vast expanse of aquaculture on the edge of Manila Bay in the lower-right corner.
Sentinel-1A is still in the commissioning phase as of early July.
Figure 99: This image from the Sentinel-1A radar satellite, acquired on 6 June, 2014, shows part of the Philippine island of Luzon with Mount Pinatubo (image credit: ESA)
• May 28, 2014: Although not yet operational, the new Sentinel-1A satellite has provided radar data for mapping the floods in Bosnia and Herzegovina. Heavy rainfall leading to widespread flooding and landslides has hit large parts of the Balkans, killing dozens of people and leaving hundreds of thousands displaced. 155)
Figure 100: Flood delineation map over the village of Balatun in northeastern Bosnia and Herzegovina based on Sentinel-1A data. Serbia lies to the north of the Sava river (image credit: ESA)
Legend to Figure 100: Although the radar on Sentinel-1A is still being calibrated, the new information could be integrated into the Copernicus EMS (Emergency Management Service) flood maps of the Sava river in the Balatun area in Bosnia and Herzegovina.
Figure 101: Sentinel-1A image of Lake Constance acquired on May 10, 2014 in 'interferometric wide swath mode' and in dual polarization (image credit: ESA)
Legend to Figure 101: Lake Constance in Central Europe was formed by the Rhine Glacier during the last Ice Age, it covers an area of about 540 km2 and is an important source of drinking water for southwestern Germany. The lake has shorelines in three countries: Germany to the north, Switzerland to the south and Austria at its eastern end. Over the water body, however, there are no borders because there is no legally binding agreement on where they lie.
In the lower-right, one can see where the Rhine river flows into the lake from the south, which then flows out of the lake to the west (left). This and other rivers carry sediments from the Alps, extending the coastline and decreasing the lake's water depth.
The C-SAR radar instrument of Sentinel-1A gathers information in either horizontal or vertical radar pulses, and colors were assigned to the different types. In this image, buildings generally appear pink, while vegetation is green. Areas with lowest reflectivity in all polarizations appear very dark, like the water.
• Sentinel-1A radar acquisition from 22 April 2014 showing Greece's Attica region (Figure 102), with mountainous areas and the capital and largest city of Athens near the center. In the water, different shades of blue indicate different types of sea surface, influenced by currents and waves. The image was acquired in 'interferometric wide swath' mode and with a dual polarization in VV and VH. Colors were assigned to different types of radar polarizations. 157)
The Figures 102 and 103 were released by ESA among other images on the occasion of the European conference in Athens, Greece: 'Copernicus Sentinels Serving Society and the Environment' of May 12-13, 2014. 158)
Figure 102: Sentinel-1A radar image of Athens and mainland Greece (image credit: ESA)
Figure 103: Sentinel-1A radar scan acquired on 22 April 2014 over the Greek Cyclades (Kyklades) island group, located to the southeast of mainland Greece in the Aegean Sea; the image was released on May 12, 2014 (image credit: ESA) 159)
• The sample image of Figure 104, featured on ESA's "Earth from Space video program" and released on May 9, 2014, is an early image of the Sentinel-1A mission acquired less than three weeks after its launch on 3 April. 160)
Figure 104: Radar image of the Salar de Uyuni, Bolivia, acquired by Sentinel-1A on April 20, 2014 (image credit: ESA)
Legend to Figure 104: The Salar de Uyuni in Bolivia is the largest salt flat in the world. With a size of over 10 000 km2, the vast Salar de Uyuni lies at the southern end of the Altiplano, a high plain of inland drainage in the central Andes (elevation of 3,656 m). Some 40 ,000 years ago, this area was part of a giant prehistoric lake that dried out, leaving behind the salt flat.
While the salt flat appears an almost homogenous white in optical satellite imagery, here we see it in shades of grey, and it looks almost like a lake. This has to do with how the radar signal reacts to different surfaces: areas where the radar signal is absorbed appear darker, while areas where the signal is reflected back to the satellite appear lighter. This gives Earth observation experts an indication of how rough or smooth the surfaces are, providing differences in salt density or even the presence of water.
But on the whole, the Salar de Uyuni is very flat, with a surface elevation variation of less than 1 m. This makes the area ideal for calibrating satellite radar altimeters – a different kind of radar instrument that measures the surface topography. The future Sentinel-3 mission will carry a radar altimeter.
The surrounding terrain is rough in comparison to the vast salt flat and is dominated by the volcanoes of the Andes mountains forming part of the Pacific Ring of Fire.
An optical Landsat image of the salt flat is provided in Figure 105 for better distinction between the various features.
Figure 105: A Landsat image of the Salar de Uyuni (image credit: USGS, NASA)
• The image of Figure 106, an early radar scan of Sentinel-1A of the Copernicus program, was released on April 25, 2014; it was featured on the 'Earth from Space video program' of ESA. One of the many application areas of the data will be the surveillance of the marine environment, including monitoring oil spills and detecting ships for maritime security, as well as measuring wave height. 161) 162)
- In this image of the Netherlands and its west coast, one can clearly see the radar reflections from the ships at sea, appearing like stars in a night sky. The two collections of 'stars' are reflections from large-scale offshore wind farms, used to generate electricity. Other visible features include the city of Amsterdam on the center-right side of the image, and the runways of the nearby Schiphol airport. In the lower part of the image, one can see the city of Rotterdam, with Europe's largest port extending to the left.
- The SAR data of Sentinel-1A will also be used for monitoring changes in agricultural land cover – important information for areas with intensive agriculture like the Netherlands.
Figure 106: Sentinel-1A SAR image acquired on April 15, 2014 showing the west coast of the Netherlands (image credit: ESA)
• On April 16, 2014, ESA released initial imagery of the Sentinel-1A mission. The first radar image of Brussels (Figure 107) was acquired in the satellite's 'strip map' mode, which has a swath width of 80 km, and in dual polarization. The image also shows a more detailed view of the city in the 'zoom in'. Antwerp harbor is also visible in the top left. The green colors correspond to vegetation, red–blue to urban areas, white to high-density urban areas and black to waterways and low-reflective areas such as airport runways. 163)
The satellite is not yet in its operational orbit, nor is it calibrated for supplying true data. These tasks will be carried out during the commissioning phase, which will take about three months to complete. 164)
Figure 107: First Sentinel-1A image of Brussels and surrounds in Belgium, acquired on 12 April 2014 at 17:18 GMT, just nine days after launch (image credit: ESA)
Figure 108: Antarctica Peninsula from Sentinel-1A acquired on April 13, 2014 at 23:57 GMT (image credit: ESA)
Legend to Figure 108: This image shows a transect over the northern part of the Antarctica Peninsula. It was acquired in the satellite's 'strip map' mode with a swath width of 80 km and in dual polarization. The colors indicate how the land, ice and water reflect the radar signal differently.
• April 9, 2014: A collision avoidance maneuver during LEOP. - On April 4, 2014, at the end of the first day after launch — all deployments have been executed during the night and completed early in the morning at the beginning of the first 'day shift'. As the first day shift nears its end, a serious alert is received: there is a danger of a collision with a NASA satellite called ACRIMSAT, which has run out of fuel and can no longer be maneuvered. The project is waiting for more information - a collision avoidance maneuver may be needed. — A collision avoidance maneuver during LEOP? This has never been done before, this has not been simulated! 165)
- A recount of the Sentinel-1A LEOP phase events is provided in the following paper: 166)
Table 13: A recount of the events for a collision avoidance maneuver during LEOP of Sentinel-1A (posted by the Sentinel-1A team on April 7, 2014, Ref. 165)
Figure 109: Luisella and Pier-Paolo on shift (image credit: ESA)
• Ref. 20) version of the collision avoidance maneuver during LEOP (October 2015) : At MET 11:00 h on April 4 after having completed the deployment sequence, LEOP operations aimed at commanding from ground the S/C mode transitions to reach NPM (Normal Pointing Mode), the nominal operating mode during the Mission phase. The most relevant operational steps to reach NPM were the on-board update of the input state vectors used by the on-board coarse orbit propagation, switching on and performing checkouts of the hardware required in NPM, namely the GPS receivers and the STTs (Star Trackers) and enabling the on-board precise orbit determination based on the converged least square solution provided by the GPS receivers. Nominally the transition to NPM was planned to be commanded from Kiruna ground station at MET 31:30 h.
- On April 04 at 14:45 UTC (MET 17:45 h) the Space Debris Office at ESOC communicated to the Mission Control Team the results of the determined injection orbit screening against the NORAD TLEs catalog. A series of high risk conjunctions with the NASA operational satellite ACRIMSAT were detected, the first one on April 05 at 06:04 UTC (MET 33:00 h). These results were confirmed within a few hours by the JSpOC (Joint Space Operations Center) of the US Air Force, located at VAFB, CA. Table 14 summarizes the screening results provided by JSpOC.
Table 14: Summary of the Sentinel-1A conjunction risks with ACRIMSAT confirmed by JSpOC. ACRIMSAT 1-sigma uncertainty: radial 10m, along-track 50m and cross-track 10 m. D is the relative position vector of ACRIMSAT with respect to Sentinel-1A.
- At MET 19:00 h on April 4, after getting confirmation from the ACRIMSAT operators that no maneuvering was possible on their side, the ESOC Flight Control Team started working on a new operations timeline which would allow Sentinel-1A to reach NPM and perform a maneuver to mitigate the conjunction risk half a revolution before the first predicted potential conjunction at the latest. In parallel the FD Team and the ESOC Space Debris Office started preparing possible evasion maneuver scenarios. The analysis performed by the ESOC Space Debris Office was showing a series of head-on conjunctions at the same orbit location, namely at 20.082 deg argument of latitude with ACRIMSAT flying above Sentinel-1A in all close approaches. The ACRIMSAT orbit uncertainty in radial component provided by JSpOC was 10 m. With this uncertainty and looking at the fly-by radial distances reported in Table 14, it was clear that the potential conjunctions predicted for 2014/04/05-07:43:04 UTC and 2014/04/05-09:21:39 UTC (Table 14) were posing a real danger to the Sentinel-1A mission.
- Mitigating the risk for these two predicted conjunctions was the target of the collision avoidance maneuver. This risk mitigation had to be achieved by increasing the radial separation at the point of closest approach. The optimal way to achieve this radial separation was to preform an in-plane maneuver 180º away from the argument of latitude of the predicted closest approach. In this case an in-plane maneuver had to be executed at an argument of latitude of 200.082º and before 2014/04/05 07:43:04 UTC (MET 34:46 h), as depicted in Figure 110. The change in radial component introduced by an in-plane maneuver at the Sentinel-1A altitude can be expressed as:
delta-radial (m) = 2 x delta-semi-major axis (m) = 2 x 1885 sec x delta-v (m/s)
- The Sentinel-1A orbit control thrusters are located on the S/C sides +X, -X and –Y. When flying in NPM these directions are aligned with the flight direction, anti-flight direction and orbit normal direction, respectively. The maximum burn duration specified before launch was 300 seconds due to AOCS constraints. However the S/C manufacturer team present at ESOC during LEOP recommended a maximum avoidance maneuver duration of 100 seconds, since it was the first maneuver performed in-flight. A 100 seconds duration at the tank conditions in LEOP translated into a total delta-v of 0.048 m/s, which according to the expression provided above would increase the radial separation by 180 m. This change in radial component was enough to mitigate the risk when performing the maneuver in either direction, in or against the flight direction. Since the launcher injection had been 7.8 km lower than expected, a maneuver in the flight direction would have contributed to the operational altitude acquisition. On the other hand, it was operationally safer to perform the maneuver against the flight direction, given the fact that ACRIMSAT was flying above Sentinel-1A in the two close approaches. In case of a S/C reconfiguration during the maneuver execution, a maneuver in the flight direction could have brought Sentinel-1A closer to ACRIMSAT.
- The Mission Control Team joint decision was therefore to perform the collision avoidance maneuver against the flight direction, activating the thrusters located on the +X S/C face. Three maneuver cases were prepared by the FD Orbit Control Team corresponding to maneuver durations of 30, 60 and 100 seconds which translated into radial separation increase of 54, 108 and 180 m, respectively. The three cases were provided to the ESOC Space Debris Office, which determined that a 40 second burn was sufficient to bring the collision risk probability down to an acceptable level, accounting for some maneuver performance error. Similarly three different maneuver execution times were considered: the latest opportunity to perform the maneuver at 06:53:00 UTC (MET 33:55 h), which corresponds to half an orbital revolution before the first close approach, and two more opportunities one and two revolutions earlier at 05:13 UTC (MET 32:15 h) and 03:33 UTC (MET 30:35 h), respectively.
- The first attempt to enter NPM was commanded on April 04 at 20:36 UTC (MET 23:38 h). Following the entry in NPM a Gyro FDIR monitoring triggered a switch to all redundant equipment and the S/C went to Intermediate Safe Mode. A second attempt to enter NPM on April 05 at 03:00 UTC (MET 30:00 h) was successful, allowing the execution of the collision avoidance burn on April 05 at 05:14:45 UTC (MET 32:15 h).
Figure 110: Collision avoidance scenario for a maximum radial separation increase (image credit: ESA)
- Maneuver performance: The first orbit determination attempts after the execution of the burn showed a large performance error in the along-track component in the order of -20%. This figure was confirmed after collecting sufficient ranging, Doppler and GPS measurements in the hours that followed the maneuver. After retrieving a complete day of almost continuous GPS data a maneuver performance analysis was conducted, aiming at ruling out the possibility of thruster misalignment issues. Two orbit determinations were performed over a determination arc covering from 2014/04/04-05:00 UTC (one day before the maneuver execution) to 2014/04/06-06:00 UTC (one day after the maneuver execution) with different maneuver parameter estimation setups.
- Fortunately, the Sentinel-1A spacecraft avoided a collision with ACRIMSAT - truly, a great relieve for all involved. Naturally, further analyses were conducted by the Flight Dynamics Team to explain the discrepancies in the predicted and achieved performances.
- The Sentinel-1 orbit control S/W was consequently adapted to the new propulsion system features, namely the reduction of burn duration for in-plane orbit correction maneuvers from 300 to 30 seconds. With this maximum in-plane burn duration (providing a delta-v of 0.009 m/s at the end of LEOP tank conditions) and the injection semi-major axis achieved by the launcher, it was clear the reference ground-track acquisition campaign had to be re-engineered based on the execution of in-plane correction batches.
- On April 23, 2014, a maneuver batch approach proof of concept was conducted, performing a sequence of six in-plane corrections in consecutive orbit revolutions. The AOCS response was satisfactory in terms of platform pointing and wheels recovery time. This marked the beginning of a long reference ground-track acquisition campaign which was successfully completed by August 06, 2014. On this day and after the execution of more than four hundred maneuvers, Sentinel-1A started the first control cycle around its reference ground-track (Ref. 20). 167)
• April 4, 2014: Following the launch, Sentinel-1A has performed a carefully a choreographed 10-hour dance routine to open its large radar antenna and solar wings. During the launch, the 12 m long radar and two 10 m long solar wings were folded up to fit into the protective Soyuz rocket fairing. After being lofted to 693 km above Earth and released into orbit, the satellite gently 'tumbled' to stabilize before embarking on its elaborate dance routine. 168)
- The solar wings and radar opened together in a specific sequence that took around 10 hours to complete. As one of most critical stages in the life of the mission, it was choreographed to ensure that both deploy in the safest possible way. The sequence also allowed power from the wings to be available as soon as possible so that the satellite was independent.
• April 3/4, 2014: Once the AOCS achieved SHM (Safe Hold Mode), the deployment of the two Solar Arrays and the two SAR wings began at the fourth ground pass over Troll (MET 02:52 h). A primary target of the sequence was to achieve a S/C Power Positive state, which is the state when the power budget can guarantee a permanent survival and the remaining deployments can go ahead with no risk related to the S/C power consumption (Ref. 20).
- The two appendages are aligned along thespacecraft X-axis when extended. Each SAR wing deployed in two steps called partial and full deployment. As it can be observed in Figure 111 steps 1 and 2, it was important to perform the partial deployment of each SAR wing before deploying the Solar Arrays in order to avoid possible interference during the deployment and rotation of the Solar Array.
- The first step in the sequence was the SAR +X wing release by means of a pyrotechnic actuator. This event was noticeable in TM as a small oscillation in the S/C rates. The partial deployment was not commanded until the following ground pass over Svalbard-Alaska at MET 03:36 h ( Figure 111 step 1). The deployment was successfully completed by the time the S/C entered visibility from Troll at MET 04:28 h. During this pass the SAR –X wing release (not partial deployment yet) was commanded. The +Y Solar Array deployment was telecommanded at the next contact with the S/C from Kiruna-Svalbard-Alaska at MET 05:13 h (Figure 111 step 2). The 210 º +Y Array rotation required to get Sun incidence which was nominally planned for this pass, but was actually commanded and monitored at the next ground pass over Troll at MET 06:04 h, at which point Power Positive Status was achieved (Figure 111 step 3). The deployment of the +Y Solar Array was noticeable in the monitored S/C rates, which experience disturbances of up to 0.5º/s in roll and 2º/s in yaw (Figure 112).
Figure 112: Gyro rates during the deployment of the Solar Arrays (shaded regions). +Y Solar Array (left) and –Y Solar Array (right), image credit: ESA
• Cameras on the Soyuz Fregat upper stage, that released the Sentinel-1A spacecraft into orbit on April 3, acquired superb footage showing the Sentinel-1A satellite separating from the Fregat to start its life in orbit around Earth. The satellite separated from the upper stage 23 min 24 sec after liftoff. The video sequence can be found in the following reference. 169)
- Separation occurred within visibility of KSAT ground station at Svalbard (Figure 113) and three minutes before entering visibility from the SSC/USN antenna in Alaska (Ref. 20). Approximately one minute after physical separation, the on-board automatic sequence started, switching on the S-band transponder and setting the S/C mode to RDM (Rate Damping Mode). The first TM frames were successfully received at the Svalbard station and forwarded to ESOC at 21:27:15 UTC on April 3, 2014 (just 25 minutes after launch).
Figure 114: A spectacular video showing Sentinel-1's release into orbit captured by cameras on the Soyuz Fregat upper stage (image credit: ESA)
Sensor complement: (C-SAR)
C-SAR (C-band SAR instrument):
The C-SAR instrument is designed and developed by EADS Astrium GmbH of Germany. The instrument provides an all-weather, day and night imaging capability to capture measurement data at high and medium resolutions for land, coastal zones and ice observations.
The C-SAR instrument is an active phased array antenna providing fast scanning in elevation (to cover the large range of incidence angle and to support ScanSAR operation) and in azimuth (to allow use of TOPS technique to meet the required image performance). To meet the polarization requirements, it has dual channel transmit and receive modules and H/V-polarised pairs of slotted waveguides.
It has an internal calibration scheme, where transmit signals are routed into the receiver to allow monitoring of amplitude/phase to facilitate high radiometric stability.
Sentinel-1's metalized carbon-fiber-reinforced-plastic radiating waveguides ensure good radiometric stability even though these elements are not covered by the internal calibration scheme. The digital chirp generator and selectable receive filter bandwidths allow efficient use of on-board storage considering the ground range resolution dependence on incidence angle.
The goal of the all-weather imaging capability of the C-SAR instrument is to provide measurement data at high and medium resolutions for land, coastal zones and ice observations in cloudy regions and during night, coupled with radar interferometry capability for detection of small (mm or sub-mm level) ground movements, with the appropriate frequencies and operating modes required to support the Copernicus services. 170) 171) 172) 173) 174) 175) 176) 177) 178) 179) 180) 181) 182) 183)
The Sentinel-1 requirements call for the support of four observation/acquisition modes:
• SM (Stripmap mode): 80 km swath with a spatial resolution of 5 m x 5 m
• IW (Interferometric Wide swath) mode: 250 km swath, 5 m x 20 m spatial resolution and burst synchronization for interferometry. IW is considered to be the standard mode over land masses.
- satisfies most currently known service requirements
- avoids conflicts and preserves revisit performance
- provides robustness and reliability of service
- simplifies mission planning & decreases operational costs
- satisfies also tomorrow's requests by building up a consistent long-term archive.
The IW mode images three subswaths using TOPSAR (Terrain Observation with Progressive Scans SAR). With the TOPSAR technique, in addition to steering the beam in range as in ScanSAR, the beam is also electronically steered from backward to forward in the azimuth direction for each burst, avoiding scalloping and resulting in a higher quality image. Interferometry is ensured by sufficient overlap of the Doppler spectrum (in the azimuth domain) and the wave number spectrum (in the elevation domain). The TOPSAR technique ensures homogeneous image quality throughout the swath.
• EW (Extra Wide Swath) mode: 400 km swath and 25 m x 100 m spatial resolution (3-looks). - Six overlapping swathes have to be foreseen to cover the required access range of 375 km.
• WV (Wave mode): low data rate and 5 m x 20 m spatial resolution. Sampled images of 20 km x 20 km at 100 km intervals along the orbit. The Wave mode at VV polarization is the default mode for acquiring data over open ocean. WV mode is acquired at the same bit rate as SM however, due to the small vignettes, single polarization and sensing at 100 km intervals, the data volume is much lower. The table below shows the main characteristics of the Wave mode.
Figure 115: Alternating WV mode acquisitions (image credit: ESA)
Except for the wave mode, which is a single polarization mode (HH or VV), the SAR instrument has to support operations in dual polarization (HH-HV, VV-VH), requiring the implementation of one transmit chain (switchable to H or V) and two parallel receive chains for H and V polarization. The specific needs of the four different measurement modes with respect to antenna agility require the implementation of an active phased array antenna. For each swath the antenna has to be configured to generate a beam with fixed azimuth and elevation pointing. Appropriate elevation beamforming has to be applied for range ambiguity suppression.
Figure 116: Overview of the Sentinel-1 C-SAR instrument observation scheme and operational support modes (image credit: ESA)
Table 15: Key parameters of the C-SAR instrument
Table 16: Overview of the Sentinel-1 operational concept (Ref. 15)
Introduction of a new SAR imaging mode (new observation technology):
The IW (Interferometric Wide swath) mode is being implemented with a new type of ScanSAR mode called TOPS (Terrain Observation with Progressive Scan) SAR operations support mode (note: the terms TOPS and SAR is simply contracted to TOPSAR). TOPSAR is an ESA-proposed acquisition mode (Francesco De Zan and Andrea Monti Guarnieri) for wide swath imaging which aims at reducing the drawbacks of the ScanSAR mode. The basic principle of TOPSAR is the shrinking of the azimuth antenna pattern (along-track direction) as seen by a spot target on ground. This is obtained by steering the antenna in the opposite direction as for Spotlight support. The TOPSAR signal includes particularities of both ScanSAR and Spotlights modes, but existing processing algorithms do not provide an efficient processing of TOPSAR data. - The EW (Extra Wide swath) mode is also implemented with the TOPS capability (Table 17).
The TOPSAR mode is intended to replace the conventional ScanSAR mode. The technique aims at achieving the same coverage and resolution as ScanSAR, but with a nearly uniform SNR (Signal-to-Noise Ratio) and DTAR (Distributed Target Ambiguity Ratio). 184) 185) 186)
The TOPSAR mode will be implemented on the Sentinel-1 mission due to the performance advantages compared to ScanSAR. The TOPSAR technique has already been demonstrated on the TerraSAR-X spacecraft during its commissioning phase (fall 2007) and showed very promising results. The measured values of the intensity variation of the analyzed images corresponded very well with the expected theoretical values. Scalloping in the TOPSAR image is 0.3 dB against 1.2 dB in the ScanSAR image. Additionally, fewer bursts are required in TOPSAR, which also positively affects the image quality. 187) 188) 189) 190) 191) 192)
TOPS is employing a rotation of the antenna in the azimuth direction as is shown in Figure 117. Like in ScanSAR, several subswaths are acquired quasi simultaneously by subswath switching from burst to burst. The increased swath coverage is as in ScanSAR achieved by a reduced azimuth resolution. However, in TOPS the resolution reduction is obtained by shrinking virtually the effective antenna footprint to an on-ground target, rather than slicing the antenna pattern, as it happens for ScanSAR. 193)
Concerning the implementation of TOPS InSAR, the Sentinel-1 C-SAR system is designed to enable TOPS burst synchronization of repeat-pass datatakes supporting the generation of TOPS interferograms and coherence maps. Specifically, for the IW and EW modes the TOPS burst duration is 0.82 s and 0.54 s (worst case), respectively, with a requirement for achieving a synchronization of less than 5 ms between corresponding bursts (Ref. 201).
Furthermore, a critical issue for TOPS InSAR performance is the accuracy that is required for TOPS image co-registration. A small co-registration error in azimuth can introduce an azimuth phase ramp due to the SAR antenna azimuth beam sweeping causing Doppler centroid frequency variations of 5.5 kHz.
Figure 117: Sketch of the TOPSAR acquisition geometry. TB is the burst duration and ω r is the steering angle rate (image credit: DLR)
Figure 118: Alternate view of the TOPSAR subswath acquisition (image credit: ESA)
The TOPS demonstration, conducted for ESA in 2007 with TerraSAR-X data, was conducted with the ETT (Experimental TerraSAR-X TOPS) processor; the test was based on sub-aperture processing, the Extended Chirp scaling algorithm and BAS (Baseband Azimuth Scaling). - The SPT (Sentinel-1 Prototype TOPS) processor is based on a pre-processing stage to unfold the azimuth spectrum, a standard ω-k focusing block and an azimuth one-dimensional 'unfolding' processing block. It is an extension of the standard ω-k based ScanSAR processor developed for the Envisat ASAR instrument (Ref. 193).
Comparison of both processors: The comparison of the SPT (Sentinel-1 Prototype TOPS) processor with the ETT (Experimental TerraSAR-X TOPS) processor turned out to be a complex task. The results are confirming both processing approaches mutually.
TOPS implementation on RADARSAT-2: The objective of simulating Sentinel-1 TOPS mode image data products using RADARSAT-2 is to support the implementation of the TOPS mode, specifically the Interferometric Wide swath (IW) mode, on ESA's Sentinel-1 mission. The use of real C-band RADARSAT-2 TOPS image data enables the preparation for the Sentinel-1 exploitation phase (i.e. GMES Initial Operations (GIO)). In particular, the provision of Sentinel-1 like TOPS image products to operational Copernicus/GMES services and other users will help the user community to prepare and verify their SAR data post-processing chains including ingestion tools etc., prior to the launch of Sentinel-1A. 194) 195)
The Canadian RADARSAT-2 mission operates at the same C-band frequency (5.405 GHz) as the Sentinel-1 mission. The RADARSAT-2 SAR instrument with its phased array antenna has multiple SAR imaging modes, including ScanSAR modes, as well as it has quad-polarization and repeat-pass SAR Interferometry (InSAR) capabilities. The RADARSAT-2 mission has been implemented as a public-private partnership between the Government of Canada and MDA (MacDonald, Dettwiler and Associates Ltd.), whereby MDA has the commercial data rights.
The design and implementation of the experimental TOPS mode on RADARSAT-2 resembles as closely as possible the performance characteristics of the Sentinel-1 IW mode, within the constraints imposed by the design and implementation of RADARSAT-2. The RADARSAT-2 TOPS image data sets have been processed to Level 1 SLC (Single Look Complex) data with the Sentinel-1 Image Processing Facility (IPF) and are provided in the official Sentinel-1 Level (SLC) product format.
The experimental RADARSAT-2 TOPS mode is referred to as PSNB (Progressive ScanSAR Narrow B). It is based on the existing RADARSAT-2 SCNB (ScanSAR Narrow B) mode. This mode uses 3 sub-swaths, like the Sentinel-1 IW mode, and covers a comparable range of incidence angles.
The Sentinel-1 satellites carry a single payload consisting of a C-band Synthetic Aperture Radar (SAR) instrument. The instrument is composed of two major subsystems:
• SES (SAR Electronics Subsystem)
• SAS (SAR Antenna Subsystem).
Figure 119: The block diagram of C-SAR (image credit: EADS Astrium, ESA)
The radar signal is generated at baseband by the chirp generator and up-converted to C-band within the SES. This signal is distributed to the HPA (High Power Amplifiers) inside the EFE (Electronic Front End) Transmit/Receive Modules (TRMs) via the beam forming network of the SAS. Signal radiation and echo reception are performed with the same antenna using slotted waveguide radiators. When receiving, the echo signal is amplified by the low noise amplifiers inside the TRM and summed up using the same network as for transmit signal distribution. After filtering and down conversion to baseband inside the SES (SAR Electronics Subsystem), the echo signal is digitized and formatted for recording. 196) 197) 198)
The key design aspects of the C-SAR instrumentation can be summarized as follows:
• Active phased array antenna providing fast scanning in elevation (to cover the large range of incidence angle and to support ScanSAR operation) and in azimuth (to allow use of the TOPS technique to meet the required image performance)
• Dual channel TRM (Transmit & Receive Modules) and H/V-polarized pairs of slotted waveguides (to meet the polarization requirements)
• Internal Calibration scheme, where transmit signals are routed into the receiver to allow monitoring of amplitude/phase to facilitate high radiometric stability
• Metalized CFRP (Carbon Fiber Reinforced Plastic) radiating waveguides to ensure good radiometric stability even though these elements are not covered by the internal calibration scheme
• Digital chirp generator and selectable receive filter bandwidths to allow efficient use of on board storage considering the ground range resolution dependence on incidence angle
• FDBAQ (Flexible Dynamic Block Adaptive Quantization) to allow efficient use of on-board storage and minimize downlink times with negligible impact on image noise.
SES (SAR Electronics Subsystem)
The SES forms the core of the radar instrument connecting the SAS for transmission of Tx pulses and receiving of backscattered pulses from ground targets. The SES provides all radar control, IF/RF signal generation and receive data handling functions comprising: 199)
- radar command and control, timing control, and redundancy control
- transmit chirp generation, frequency generation, up-conversion/down-conversion, modulation/demodulation and filtering
- digitization, data compression and formatting.
A digital chirp generator and selectable receive filter bandwidths allow an efficient use of on board storage capacity considering the ground range resolution dependence on the incidence angle. The radar signal is generated at base band by the chirp generator and up-converted to C-band within the SES. This signal is distributed to the high-power amplifiers inside the EFE TRMs via the beam-forming network of the SAS. Signal radiation and echo reception are performed with the same antenna using slotted waveguide radiators. When receiving, the echo signal is amplified by the low noise amplifiers inside the EFE TRMs and summed up using the same network as for transmit signal distribution. After filtering and down-conversion to base band inside the SES, the echo signal is digitized and formatted for recording. Flexible dynamic block adaptive quantization allows the efficient use of on-board storage and to minimize downlink times with negligible impacts on image noise.
The SES hardware comprises the following units:
• ICE (Integrated Central Electronics) unit
• MDFE (Mission Dependent Filter Equipment)
• TGU (Transmit Gain Unit)
The ICE unit is the principal module of the SES providing the radar with its core functionality, control and monitoring. The subsystem uses a fully digital design approach for both the derivation of the C-band chirped radar signal and the digital receiver which samples the echo signal at an IF close to base band. The ICE maintains and manages a database of operational parameters such as transmit pulse and beam characteristics for each swath of each mode, and timing characteristics like pulse repetition frequencies and window timings. The MDFE is passive, providing a set of RF filters for the Tx path (to control out-of-band transmissions) and for the Rx path (to limit out-of-band interference). The TGU provides the final RF amplification of the Tx pulse signal before sending it to the SAS. The TGU receives its own dedicated primary power supply from the platform. Switching the TGU on and off is performed by the ICE.
Figure 120: Photo of the SES device (image credit: EADS Astrium Ltd.)
To augment this design and provide mission variable compatibility, the SES also includes mission dependent units that comprise amplification and filtering to provide an ideal signal level and match to the antenna subsystem to be supported.
Figure 121: SES context diagram (image credit: EADS Astrium Ltd)
The SES configuration is implemented as a fully cold redundant pair of chains. The TGU being common to both chains only in as much as the two amplifier chains are mounted in a single physical equipment unit before the paths do combine within a passive hybrid device which in turn permits dual outputs to the antenna supplying both fore and aft antenna segments.
The ICE (Integrated Central Electronics) unit is the principal module of SES consisting in turn of highly-integrated modules (Figure 123). ICE is being produced at Astrium UK (Portsmouth). This equipment provides the radar with its core functionality, control and monitoring. The subsystem uses a fully digital design approach for both the derivation of the (up to 100 MHz) C-band chirped radar signal and the digital receiver which samples the echo signal at an IF close to baseband. With single up-conversion and down-conversion stages and data processing using efficient digital filtering and data compression algorithms it is anticipated that this equipment will provide a highly stable core electronics base for this new exciting Copernicus utility. 200)
The Astrium UK ICE design is aimed at providing not only a solution for the Sentinel-1 system but also aims to provide a modern solution for the complex electronics at the heart of radars and particularly that of a SAR. The architecture is designed for adaptability using the inherent flexibility of the digital approach. It is therefore able to adapt easily to the needs of different missions.
Figure 122: Configuration of SES (image credit: EADS Astrium Ltd.)
The ICE modular design makes use of the integrated RF and digital design technologies now commonly available. High speed ADC (Analog Digital Converter)) and DAC (Digital Analog Converter) components along with flexible design of digital processing through the use of large scale FPGAs and dedicated ASICs, as well as the use of MMIC (Modular Microwave Integrated Circuitry) has allowed the design to respond to the demands of the Sentinel-1 mission.
The design of ICE is comprised of the following elements:
• ICM (Instrument control Module): A Leon ll processor based module, developed by Syderal of Switzerland with:
- PROM for boot software and EEPROM for the application software and the radar characterization database
- Multiple interface formats allowing 1553B communication with the platform, SpaceWire for the internal modules (using the Atmel AT7910/SpW_10X SpaceWire Router ASIC) and CAN for external equipments TGU and SAS.
• Ty module: This RUAG designed and built module uses a direct digital synthesis chirp generation method at an IF of 150 MHz, with up-conversion in a single stage to the nominal 5.405 GHz center frequency to deliver the radars a fully programmable chirp transmission chain. This requires only a further amplification stage provided by the externally provided DAD designed TGU (Transmit Gain Unit) to drive the SAS (SAR Antenna Subsystem).
• Rx modules: The dual polarization approach required by the C-SAR instrument necessitates a pair of matched receive modules to be implemented within the ICE. The receive path is band filtered externally to the ICE by a MDFE (Mission Dependent Filter Equipment) provided by DAD of Finland prior to its input to the ICE Rx modules Here the signal is down-converted directly to an IF of 75 MHz before being digitized in the ADC which sample's at approx 300 Msamples. This in turn feeds the digital processing chain of a decimation filter followed by, compression and packetization stages before the output is piped to the on board mass memory via a Wizard link interface in a standard CCSDS format at 640 Mbit/s.
• TCM (Timing Control Module): TCM represents the timeline control element for the system. Implementing ECC program driven FPGA logic to provide the necessary timing waveforms required to define and control the within pulse precise timing relationships of all the required timing signals used by the instrument. These timing pulses and the PRI rate communication bus to the antenna are the means whereby the radar establishes the autonomous complex timeline of each mode acquisition with the absolute repeatability required to provide the synthetic aperture quality and the Interferometric property of the system data output.
• PCM (Power Control Modules): These modules are implemented so as to reduce the individual module voltage conversion effort and to reduce the power losses. These modules use modular common DC/DC converters designed by BLU Electronics to provide 3 voltage rails to all internal ICMs. However, It is to be noted that point of load regulation at module level is still expected for more user specific voltages.
• FDM (Frequency Distribution Module) and USO (Ultra Stable Clock): Using FOAMO (Foam Insulated Master Oscillator) of Astrium as the master clock, the FDM generates the timing reference frequencies used by all other signal modules in ICE. To maintain the highest level of phase stability, this unit also takes in the Tx LO (Local Oscillator) and creates from this the offset Rx down convertor LO for both Rx module channels.
Figure 123: Block diagram of SES (only one of two redundancy chains is shown), image credit: EADS Astrium
Figure 124: Illustration of the modular configuration of ICE (image credit: EADS Astrium Ltd.)
The ICE equipment has a mass of 20.6 kg. The modules have been selected to be integrated into an equipment enclosure with a backplane wiring loom rather than a fixed motherboard interface plate. This approach allows greater flexibility for test and diagnosis as well as a mechanical flexibility that offers a simpler solution to the thermal challenges of the mission.
The low internal interface count which also allows this open loom approach is in part due to the use of the SpaceWire interconnect for control which has been implemented on the front face of the unit. This being so implemented to facilitate an ESA objective for the ICM module development aimed at further mission systems (Ref. 200).
Roll steering mode (Ref: 175): The roll steering mode of the spacecraft provides a continuous roll maneuver around orbit (similar to yaw steering in azimuth) compensating for the altitude variation such that it allows usage of a continuous PRF (Pulse Repetition Frequency) and a minimal number of different sample window lengths (SWLs) around the orbit. In addition, the update rate of the sampling window position around orbit is minimized (< 1 /2.5 min), which simplifies instrument operations significantly. Since the instrument can work with a single fixed beam for each swath/sub-swath over the complete orbit, also the number of elevation beams is minimized. The roll steering rate has been fixed to 1.6º/27 km altitude variation. The roll applied to the sensor attitude depends linearly on altitude and varies within the interval -0.8º (minimum sensor altitude) to 0.8º (maximum sensor altitude).
Figure 125: Variation of the roll angle along the orbit (image credit: ESA, TAS)
The attitude steering mode introduces an additional roll angle as a function of latitude to compensate changes in the satellite's altitude around the orbit, hence maintaining a specific, quasi "constant", slant range for each SAR imaging mode. This enables the use of a single PRF per swath or subswath around the orbit, except for SM-5 (i.e. different PRF for SM-5N and SM-5S), and a fixed set of constant elevation antenna beam patterns. 201) 202)
Figure 126 illustrates that for the minimum orbital height (693 km) the mechanical SAR antenna off-nadir angle is more shallow (30.25º) than it is for the maximum orbital height (726 km). In the latter case, the mechanical SAR antenna off-nadir angle is 28.65º.
Figure 126: Schematic view of the Sentinel-1 roll-steering mode (image credit: ESA)
SAS (SAR Antenna Subsystem):
SAS represents the sensor part of the C-SAR instrument and is an active phased array system with Tx and Rx gain and phase control distributed over the antenna area. These functions are provided by so called Transmit Receive Modules (TRMs) as part of the EFE (Electronic Frontend End) assemblies. The SAS is capable of performing rapid electronic beam steering, beam shaping, and also polarization selection. The dual polarized antenna allows at one time either transmission in one single, but selectable polarization (H or V) or simultaneous reception of both H and V polarization. 203) 204)
The SAS consists of 14 identical tiles (12.3 m x 0.84 m) in 5 deployable panels as shown in Figure 127. The electrical functions of the SAS comprise:
- signal radiation and reception
- distributed transmit signal high power amplification
- distributed receive signal low noise amplification with LNA protection
- signal and power distribution (corporate feed, power converter)
- phase and amplitude control including temperature compensation
- internal calibration loop
- deployment mechanisms, including hold down and release
- antenna mechanical structure.
Figure 127: C-SAR antenna in deployed and stowed configuration on the PRIMA bus (image credit: Astrium GmbH)
The instrument is based on a deployable planar phased array antenna carrying TRMs allowing for horizontal and vertical polarizations. The dual polarized antenna allows either transmission in one single but selectable polarization (H or V) or, simultaneous reception of both H and V polarization, at any time. The C-SAR antenna comprises two wings, stowed on the platform's lateral panels during launch, which are deployed once in orbit. Each antenna wing consists of two antenna panels. An antenna panel consists in principle, of a panel frame and a number of antenna tiles. The SAS central panel comprises two antenna tiles, whereas the wing panels comprise three antenna tiles each. The complete antenna is symmetrical around the middle of the central panel.
The active phased array antenna is capable of performing rapid electronic beam steering, beam shaping and polarization selection, providing fast scanning in elevation and azimuth to cover the large range of incidence angles and to meet the image quality requirements for the TOPSAR mode. TRMs are arranged across the antenna such that, by adjusting the gain and phase of individual modules, the transmit and receive beams may be steered and shaped.
The SAS consists of 14 tiles with 20 dual-polarized sub-arrays on each tile. Each subarray is a dual-polarized unit with two parallel slotted resonant waveguides. The vertical polarization is excited by offset longitudinal slots in a ridge waveguide, while the horizontal polarization is generated by transverse narrow wall slots excited by inserted tilted wires.
A SAS tile is composed of 10 'Waveguide 4' assemblies (two vertically and two horizontally polarized waveguides), which form the smallest building block in the tile manufacturing. Each 'Waveguide 4'-assembly is exposed to a kind of RF-incoming / diagnostic inspection consisting of a passive return loss measurement followed by a measurement of the far-field azimuth pattern in a special anechoic test environment at Astrium GmbH.
Figure 128: Photo of the SAS tile (20 HP+20 VP subarrays), image credit: EADS Astrium GmbH
Legend to Figure 128: The instrument comprises three RF networks: Tx network, RxH (H polarization) network and RxV (V polarization) network. During radar operation the Tx network carries the transmit RF signal and the RxH/RxV networks carry the echo signals. The dual polarized antenna allows transmission in one single but selectable polarization (H or V) and simultaneous reception of both H and V polarization.
The tile (size: ~ 0.87 m x 0.84 m) forms the smallest functional entity of the SAS, encompassing all functions necessary to ensure beam shaping / beam steering of the active phased array antenna. The SAS Tile is composed of 10 'Waveguide 4' assemblies and the associated electronics, namely:
- The RF Distribution Network
- 40 Transmit/Receive Modules (20 TRMs for HP & 20 TRMs for VP / supplier: Thales Alenia Space, Italy)
- 2 Tile Controller Units (TCUs)
- 2 Tile Power Supply Units (TPSUs)
allowing signal radiation and reception, distributed transmit signal high power amplification, distributed receive signal low noise amplification with LNA protection, signal and power distribution, phase and amplitude control including temperature compensation and internal calibration.
Figure 129: The SAS tile configuration scheme of the SAR antenna (image credit: EADS Astrium)
The CFRP (Carbon Fiber Reinforced Plastic) waveguide radiator is along with the cross stiffeners the major structural element of a tile. All electronic boxes are either placed onto the rear side of the radiators (e.g. the EFEs) or attached to the inner side of the cross stiffeners (e.g. TCU and TPSU). The low loss CFRP slotted waveguides radiators together with the high performance EFE TRMs ensure to meet the stringent sensitivity requirement of -22 dB. The optimized sizing of the overall SAR antenna and its waveguide radiators ensure further that also the ambitious 2D distributed target ambiguity requirement (DTAR) of -22 dB can be met.
EFE is the main transmitting/receiving section of the Sentinel-1 antenna while 195 EFE are necessary to assure the full functionality of the SAR instrument. Each EFE has been optimized for the best trade-off between integration level and RF performances and is composed of four main sections: Power Supply card, Digital card, RF distribution section and the TRM section.
The EFE is composed of four main sections: Digital card, Power Supply card, RF distribution and the TRM (T/R Module) section. A functional scheme of the EFE architecture is shown in Figures 130 and 131.
Figure 130: Illustration of the EFE architecture (image credit: TAS-I)
Figure 131: Illustration of the EFE prototype (image credit: TAS-I)
The RF networks provide the EFEs with the Tx pulses, and collect the H and the V polarized echoes. On the tile, these networks form the EPDN (Elevation Plane Distribution Network) which is placed on top of the EFEs (Figure 129). Each network (Tx, RxH, RxV) of the EPDN consists of a 1:10 divider to supply the 10 EFEs. Short cables connect the outputs of the 1:10 dividers to the EFEs.
The EFE comprises several TRMs (Transmit/Receive Modules) and associated electronics. It represents the active part of the RF equipment of the antenna. The basic function of each EFE is:
• to transmit Tx pulses in one of two polarizations (either H or V) to the corresponding two radiator elements
• to receive the echoes from the two H and the two V radiator elements independently and simultaneously.
The EFE provides the capability to perform antenna beam steering and forming by electronic means:
• control phase setting in transmit
• control phase and gain settings in receive.
C-SAR instrument calibration:
In contrast to SAR systems already existing in C-band like ASAR/ENVISAT or RADARSAT-2, high demands on the radiometric accuracy are made for C-SAR on Sentinel-1. Thus, product quality is of paramount importance and the success or failure of the mission depends essentially on the method of calibrating the entire Sentinel-1 system in an efficient way. 206) 207) 208) 209) 210) 211)
The most important point with respect to the calibration of this flexible SAR system is the tight performance with an absolute radiometric accuracy of only 1 dB (3σ) in all operation modes. Never before has such a strong requirement (a few tenths of dB) been defined for a SAR system. - With respect to the duration of the Sentinel-1 commissioning phase of three months only, the number of passes and the selection of test sites have to be optimized versus cost and time effort. e.g. calibrating several beams and polarization modes with the same test site. The calibration strategy of Sentinel-1 is based on the experience derived from TerraSAR-X.
Calibration is the process of quantitatively defining the system response to known controlled signal inputs. Calibration tasks are executed throughout the mission to ensure the normalized radar cross-section and the phase of the individual pixels are provided with stability and accuracy. Calibration of the entire SENTINEL-1 system is critical to guaranteeing product quality for operational demands. The SAR system must perform within an absolute radiometric accuracy of only 1 dB in all operation modes. This is a higher radiometric accuracy than any other SAR mission before it.
Calibration can be divided into two forms:
• Internal calibration: Internal calibration provides an assessment of radar performance using internally generated calibrated signal sources, in particular from pre-flight testing.
• External calibration: External calibration makes use of ground targets of known backscatter coefficients to render an end-to-end calibration of the SAR system.
In addition to the commissioning of Sentinel-1B executed by ESA, an independent SAR system calibration will be performed by DLR. For this purpose, the complete calibration chain was developed and established by DLR, starting with an efficient calibration strategy, a detailed in-orbit calibration plan, the SW-tools for analyzing and evaluating all the measurements up to the calibration targets serving as accurate reference. 212)
Internal calibration uses calibration signals which are routed as closely as possible along the nominal signal path. The calibration signals experience the same gain and phase variations as the nominal measurement signals. The ground processing then evaluates the calibration signals to identify gain and phase changes and correct the acquired images accordingly.
Transmit power, receiver gain and antenna gain are subject to instrument noise due to temperature changes or other effects over time. Internal calibration provides corrections for changes in the transmit power and the electronics gain as well as validating the antenna model. The resulting calibration data are used in ground processing to correct image data.
Internal calibration also covers the signal phase. The overall phase of the echo signal depends on two major elements: measurement geometry and instrument internal phase stability. As the hardware cannot generally provide the required phase stability, it is a task of the internal calibration scheme to cover the internal phase variations by adequate measurements. All internal calibration measurements, either for gain or for phase, are used in ground processing to correct data products and achieve the required stability.
Internal calibration uses a PCC (Pulse-Coded Calibration) technique to embed a unique pulse code on a signal such that it can be identified and measured when embedded in other signals. This allows the amplitude and phase of individual signal paths to be measured while operating the complete antenna. The PCC technique is implemented by sending a series of coherent calibration pulses in parallel through the desired signal paths. The individual successive signals are multiplied by factors of +1 or -1. Factor -1 is implemented by adding a phase shift of 180°, while factor +1 means no additional phase. Each path is identified by a unique sequence.
The PCC technique can be applied if:
- the receiver detects the signals coherently
- the whole sequence is executed in a sufficiently short time such that the parameters to be measured are stationary
- the system is linear with respect to the individual signals.
The PCC technique can measure the signal paths via individual Transmit (TX) /Receive (RX) Modules (TRMs) or via groups of TRMs (either TX or RX paths, either polarization).
The average properties of rows or columns of TRMs can be measured by a short PCC sequence. The length of a PCC sequence is always a power of two. There are 20 rows of waveguides, therefore the PCC sequence has a minimum of 32 pulses. Although the 14 columns (14 tiles) could be measured by a PCC sequence of 16 pulses, it is assumed that a sequence length of 32 pulses is also used. All 20 rows are operated together, meaning the antenna is in a full operational state. The overall signal from all rows is received, digitized and packed into calibration packets. These packets are evaluated (on the ground) to determine the properties of the individual rows. The approach for measuring the average azimuth excitation coefficient is similar to the elevation pattern, using columns of TRMs instead of rows.
The PCC-32 measurements described above need approximately 129 pulses. Additional warm-up pulses may also be needed. Such a large number of calibration pulses represent a significant interruption in image generation when operated within the image acquisition of the stripmap mode. For intermediate calibration pulses in stripmap mode, and also for calibration pulses related to each sub-swath measurement in the ScanSAR modes, a shorter sequence is needed. The shortest possible PCC sequence is based on two measurements, however this procedure introduces PCC-inherent error contributions. These latter errors are to be expected, although they are significantly smaller than those due to leakage signals.
For the antenna model, the reference patterns of all beams are derived for radiometric correction of the SAR data. The active antenna of the SAR instrument allows a multitude of different antenna beams with their associated gain patterns. All these patterns are described by the mathematical antenna model which provides the antenna patterns as functions of the commanded amplitudes and phases within the front end EFEs and within the tile amplifiers. The quality of the patterns is ensured by the on-board temperature compensation controlled by the tile control units. The internal calibration signals measure the actual phases and amplitudes and allow verifying the correct function and performance of all included elements. The antenna model is established on-ground, based on pattern tests at various integration levels up to the complete antenna.
RF Characterization Mode:
The RF characterization mode is a self-standing mode and is not associated with the individual imaging data-takes. It is operated at least once per day during a convenient point within the long duration of wave mode.
The RF characterization mode verifies in-flight the correct function and characteristics of the individual TRMs. Operating it two or more times at different temperatures during the cool-down phases between the high dissipating imaging modes can provide in-orbit characterization versus temperature where necessary. The RF characterization mode performs measurements with internal signals and is designed to achieve a number of goals. The RF calibration mode will:
- cover all those measurements needed in-orbit but which are not required for each individual data-take
- provide data sets to assess the instrument health and performance as far as possible
- verify the correct function of the individual TRMs, both within the front-end and the tile amplifiers
- verify the excitation coefficients for the TX and RX patterns to ensure the validity of the antenna model.
This mode is based on the same measurement types as the internal calibration. The mode has to address the individual TRMs while operating the full antenna in representative thermal conditions and with nominal power consumption. This can be achieved using the PCC technique. As a standalone mode, it is not forced to use the signal parameters of a dedicated imaging mode, but instead an optimized set of parameters can be used. The calibration mode is to be operated for both TX polarizations. The receiver will measure both polarizations in any case.
Figure 132: In-orbit calibration plan for Sentinel-1 versus 12 days repeat cycles (image credit: (image credit: DLR, Ref. 207)
External calibration derives the calibration constant by measurement of corner reflector targets and homogeneous areas such as rainforest with exactly known backscatter coefficients. This is necessary as it will not generally be possible to know all parameters with sufficient accuracy prior to the in-flight measurements.
Figure 133: In-orbit external calibration (image credit: ESA)
External calibration comprises five steps as shown below.
1) Radiometric Calibration: Radiometric calibration is applied to correct for the bias of SAR data products. The required absolute calibration factor is derived by measuring the SAR system against reference point targets with well-known radar cross section. Due to the high demand on the radiometric accuracy of 1 dB (3σ) in all four operational modes, it is recommended to measure at least one beam of each mode against the SENTINEL-1 transponders deployed at different locations. Each selected beam will be measured during two passes (ascending and descending). Furthermore, two receive polarization combinations per operation mode are to be measured simultaneously. By measuring SENTINEL-1 against the three transponders for selected beams, the radiometric calibration can be performed within a limited number of repeat cycles. The absolute calibration factor of all other beams is then derived by applying the antenna model.
2) Antenna Model Verification: Antenna model verification ensures the provision of precise reference patterns of all operation modes and the gain offset between different beams. Verification of the antenna model is performed for selected beams, at least one with low, one with mid- and one with high incidence angle, all with the same polarization condition. In addition, some of the beams are selected for measuring the second polarization condition. Assuming acquisitions for each of the selected beams, using the receiver mode of the transponders and by using acquisitions over rainforest, antenna model verification can be performed within a few cycles.
3) Geometric Calibration: Geometric calibration is applied to assign the SAR data to the geographic location on the Earth's surface. Using well surveyed reference targets, the internal delay of the instrument and systematic azimuth shifts can be derived. For this purpose the acquired scenes are measured simultaneously against point targets deployed and precisely surveyed.
4) Antenna Pointing Determination: Antenna pointing determination is performed to achieve correct beam pointing of the antenna. The determination of the antenna pointing by the receiver mode of the transponders is performed using notch patterns in azimuth with different incidence angles (near, mid- and far). Using three transponders with a receiver function within one cycle (two passes) sufficient measurements can be acquired to derive the required accuracy. The appropriate antenna pattern is measured across the rainforest and using ground receivers.
5) Inter-Channel Phase Calibration: As the signal travels through different receive channels for H and V polarization, it may experience different gains, phase offsets and even different time delays. Inter-channel phase accuracy is calibrated using the SENTINEL-1 transponders that return the signal with H and V polarization components, and which therefore allow a direct phase comparison between H and V channels. The antenna model to be derived on the ground describes the antenna patterns with high accuracy. This antenna model is verified for a limited set of elevation beams via measurements over a homogeneous target, i.e. over rainforest. The azimuth beams will be measured using the receiver function of the SENTINEL-1 transponder.
Independent calibration verification:
In addition to the commissioning of Sentinel-1A executed by ESA, an independent verification of the system calibration will be executed for the first time by an external institution. For this purpose, the complete calibration chain was developed and established by DLR, starting with an efficient calibration concept, a detailed in-orbit calibration plan, the SW-tools for analyzing and evaluating all the measurements up to the calibration targets serving as accurate reference. 213)
DLR calibration facility: The Sentinel-1 calibration strategy requires a facility that is well-equipped with ground calibration hardware as well as software tools for analyzing and evaluating all the measurements. For this purpose, DLR/MRI (Microwave and Radar Institute) has been developed and established the following reliable and accurate ground equipment:
• Accurate and remote controlled ground targets like the DLR's novel corner reflectors as depicted in Figure 134 and the novel transponders as depicted in Figure 135, precisely surveyed for geometric and radiometric calibration. Using the receiver unit of the transponder, the pointing and the pattern of the antenna in azimuth direction can be measured during an overflight.
Figure 134: DLR's novel corner reflector which is remote controlled (image credit: DLR)
Figure 135: DLR's novel transponder, designed and developed for Sentinel-1A (image credit: DLR)
• Different analysis and evaluation tools have been modified w.r.t. the Sentinel-1A characteristics, like:
- Internal Calibration Module for analyzing the stability of the instrument and deriving several instrument offsets like the channel imbalance.
- Antenna Characterization Module for deriving the actual pointing of Sentinel-1A and verifying the antenna model in-flight
- CALIX, a software tool for point target analysis and deriving the absolute calibration factor. Furthermore by geometric analysis of accurately surveyed targets the internal delay of the instrument and the dating of the SAR data can be determined.
- TAXI, the Institute's experimental TanDEM-X interferometric processor, which will be used for interferometric analysis and for phase analysis of the TOPS mode of Sentinel-1A.
Test site selection: The next important point is concerned with the coverage of Sentinel-1A, because the coverage defines the number of visible measurements across a test site and drives consequently the schedule. Considering all aspects described before, the coverage of Sentinel-1A across the DLR calibration field in South Germany has been investigated for all beams being selected for in-flight measurements, as depicted in Figure 136 by the blue hatched swathes. The red framed area indicates a region covering all beams. Hence, deploying the transponders within this area, reference targets are available providing simultaneously a point target for both polarization channels of Sentinel-1A. The position of the corner reflectors is mainly driven by the edges of the swaths and the small vignettes of the wave mode (indicated by the black framed boxes).
Figure 136: Coverage of Sentinel-1A for all beams being selected for in-flight measurements (SM1, SM2, IW1, EW3, IW3, SM5, WV1) across the DLR calibration field deployed in South Germany (image credit: DLR)
Hence, the test site shown in Figure 136, composed of three transponders and three corner reflectors, enclosing an area of about 85 km x 20 km, is sufficient to cope with the tight requirements of commissioning Sentinel-1A, i.e all measurements required for calibrating the whole Sentinel-1A system can be performed within the commissioning phase of three months.
Copernicus Program Ground Segment
The ground segment is composed of the CGS (Core Ground Segment), the" Collaborative Ground Segment" and the Copernicus contributing missions' ground segments.
The Core ground segment monitors and controls the Sentinels spacecraft, ensures the measurement data acquisition, processing, archiving and dissemination to the final users. In addition, it is responsible for performing conflict-free mission planning according to a predefined operational scenario, and it ensures the quality of the data products and the performance of the space borne sensors by continuous monitoring, calibration and validation activities, guaranteeing the overall performance of the mission. 214)
Figure 137: Copernicus Ground Segment Architecture (image credit: ESA)
The Copernicus Ground Segment is complemented by the Sentinel Collaborative Ground Segment, which was introduced with the aim of exploiting the Sentinel missions even further. This entails additional elements for specialized solutions in different technological areas such as data acquisition, complementary production and dissemination, innovative tools and applications, and complementary support to calibration & validation activities.
For Copernicus operations, ESA has defined the concept and architecture for the Copernicus Core ground segment, consisting of a FOS (Flight Operations System) and a PDGS (Payload Data Ground Segment). Whereas the flight operations and the mission control of Sentinel-1 and -2 is performed by ESOC (ESA's European Space Operations Center in Darmstadt, Germany), the operations of Sentinel-3 and the Sentinel-4/-5 attached payloads to meteorological satellites is performed by EUMETSAT.
The EC (European Commission), supported by its agencies, is in charge to implement the Copernicus Services. The Commission is defined to be the owner and financing organization of Copernicus. The technical implementation is granted to other European organizations, namely ESA, EUMETSAT, EEA (European Environment Agency), ECMWF (European Centre for Medium-Range Weather Forecasts) and others. - Complemented by ESA programs and national contributions, ESA has the responsibility to build and operate a dedicated space segment (Sentinels) and the ground segment of Copernicus. 215)
The EC has also defined an overall Copernicus data policy, declaring the Sentinel mission data free and open. 216) This decision is reflecting the experience made with similar missions in the US (e.g. Landsat) and the new possibilities of the Internet. It will stimulate the use of Earth observation data, but also may challenge commercial suppliers, selling data similar to those of the Sentinels. The Copernicus data policy is also based on the European INSPIRE (Infrastructure for Spatial Information in the European Community) directive, which harmonizes the policy and electronic access to geographic information within Europe.
The majority of the Earth observation data for these services will come from a fleet of dedicated Copernicus satellites: the Sentinels. The features of the Copernicus Sentinel missions are depicted in Table 18. The series of satellites guarantees continuity of the ERS/ENVISAT missions and adds further features and parameters. Moreover, always two Sentinels of each series should be in orbit at any time, increasing the coverage for many applications. Applications and services will also benefit from a supply from other European national and commercial GMES Contributing Missions (GCM), managed in Copernicus by the ESA GSCDA (GMES Space Component Data Access) system.
Table 18: Copernicus Sentinel missions characteristics
The management of the payload data from the Sentinel missions is performed by the CGS (Core Ground Segment) defined by ESA. The CGS consists of a series of X-band data acquisition stations, which will capture all global Sentinel-1/-2/-3 mission data (Sentinel-4/-5 will use dedicated EUMETSAT data acquisition facilities for its next generation geostationary and polar orbiting satellites). These stations are designed for dumping all data recorded on the satellites on-board data recorders, as well as generating near real time products (1-3h after sensing) directly at the stations. The PDGS will also make use of the EDRS (European Data Relay Satellite). This PPP (Public Private Partnership) between ESA and ASTRIUM GmbH, operates a communication payload on two geostationary satellites. The primary link between the Sentinels and the geostationary satellites is a LCT (Laser Communication Terminal), built by Tesat Space, Backnang, Germany and provided as contribution-in-kind by Germany to the Sentinel-1 and -2 satellite series. The downlink from the geostationary satellites is performed via Ka-band to dedicated stations and to user terminals (Ref. 215). 217)
The acquired mission data is then transferred to Sentinel PACs (Processing and Archiving Centers). The PACs are designed to take specifically care for a certain Sentinel/instrument project. For security and redundancy reasons, each Sentinel data set is hosted by two PACs (EUMETSAT is assigned to act as the second PAC for the Sentinel-3 mission data). The PACs generate systematically base level products from all acquired data, archive them in a mission archive and electronically distribute them to the Copernicus users.
The products and performance of each Sentinel is monitored by MPCs (Mission Performance Centers). The entire data flow is managed by a PDMC (Payload Data Management Center), hosted by ESA at ESRIN, Frascati, Italy. The transport and circulation of the data is performed via terrestrial networks. The GMES WAN therefore connects all PDGS elements with links, having appropriate bandwidth.
In 2011, ESA started a series of procurement actions to select European providers to offer their facilities for the set-up and operations of the PDGS elements. Figure 138 displays the structure of the PDGS and the outcome of the selection of the providers.
Within this competitive selection process, ESA has awarded the DLR/DFD ( German Remote Sensing Data Center) with the set-up and operations of the PACs for Sentinel-1 and Sentinel-3 (OLCI part). T-Systems, Germany, is assisting DLR in the network parts of the PACs. In addition and under separate procurement, DFD designs, builds and operates the payload data ground segment) for the Sentinel-5 Precursor mission.
High-level tasks of the DLR Sentinel PAC are:
- receive Sentinel data from CGSs via the GMES WAN
- ingest these data into the STA (Short-Term Archive) and MTA (Mid-Term Archive) of the Sentinel PGDS
- ingest these data in a LTA (Long-Term Archive) for a period of more than 7 years
- perform consolidation and re-assembly of level-0 data received from CGS facilities
- perform systematic and request-driven processing of Sentinel data to higher-level products
- host Sentinel data products within a layered architecture of on-line dissemination elements that will facilitate the direct access of end-users via public networks
- share and exchange any locally processed data with a 2nd partner PAC for the purpose of redundancy.
According to the GSC (GMES Space Component) operations concept, the Sentinel PDGS will become operationally embedded in the GSCDA (GMES Space Component Data Access) System that ESA implements in support of data access to GMES/Copernicus Service Projects and their users.
Figure 139: Overall structure of the Sentinel-1 (S1) and Sentinel-3-OLCI PACs at DFD in Oberpfaffenhofen (image credit: DLR)
Sentinel-5P: The Sentinel-5 Precursor satellite will deliver a key set of atmospheric composition, cloud and aerosol data products for air quality and climate applications. The sensing instrument TROPOMI together with the operational level 1 and level 2 processors will bring a significant improvement in the precision as well as temporal and spatial resolution of derived atmospheric constituents. Sentinel-5 Precursor is planned for launch in 2016.
For the Sentinel-5P mission, DLR/DFD was selected by ESA for the development and operation of the entire PDGS (Payload Data Ground Segment), which covers the whole chain of payload data handling on ground: data reception, processing, archiving, near-real-time and offline delivery to end users. In addition, DLR/IMF (Institut für Methodik der Fernerkundung - Remote Sensing Technology Institute) was selected for the development of retrieval algorithms and operational processors for a number of key atmospheric trace gases and cloud products.
Sentinel-5P will continue the strong DFD heritage on development/operations of processors and ground segments for atmospheric missions started with GOME-1/ERS-2, SCIAMACHY/ENVISAT, and GOME-2 on the MetOp series. The Sentinel-5P PDGS - as well as the LTA for Sentinel-1 and Sentinel 3 OCLI - will be based on the DFD development of DIMS (Data and Information Management System). DIMS will be configured for the Sentinel 5P workflows and mission specific extensions for the demanding throughput and storage requirements (Ref. 215).
In summary, DLR/DFD is involved in the Core and collaborative ground segment of the Copernicus program. It has been developing PDGS (Payload Data Ground Segment) elements and will operate PACs (Processing and Archiving Centers) for Sentinels and national data acquisition stations, both in X-band and using EDRS acquisition services.
Sentinel-1 FOS (Flight Operations Segment)
The main responsibilities of the FOS at ESA encompass satellite monitoring and control, including execution of all platform activities and the commanding of the payload schedules. The principal components of the FOS are (Ref. 217): 218)
1) The Ground Station and Communications Network, which performs TT&C (Telemetry, Tracking and Commanding) operations within the S-band frequency. A single S-band ground station will be used throughout all mission phases, complemented by additional TT&C stations as launch and early operations (LEOP) and backup stations.
2) The FOCC (Flight Operations Control Center), which includes:
• the Sentinel Mission Control System, which supports telecommand coding and transfer and housekeeping telemetry (HKTM) data archiving and processing
• the Sentinel Mission Planning System which supports command request handling, the planning and scheduling of satellite operations and the scheduling of payload operations as prepared by the PDGS Mission Planning System
• the specific Sentinel Satellite Simulators, which support procedure validation, operator training and the simulation campaign before each major phase of the mission
• the Sentinel Flight Dynamics System, which supports all activities related to attitude and orbit determination and prediction, the preparation of slew and orbit maneuvers, satellite dynamics evaluation and navigation
• The Sentinel Key Management Facilities, which support the management of the telecommand security functions.
3) A General Purpose Communication Network, which provides the services for exchanging data with any other external system during all mission phases.
Figure 140: Simplified view of the Sentinel-1 Ground Segment (image credit: Astrium SAS)
Figure 141: Overall Layout of the Sentinel-1 Core PDGS (image credit: Astrium SAS)
The selected geographical locations and operators for the Sentinel-1 PDGS centers are (Ref. 217):
• KSAT for X-band reception (CGS) in Svalbard (Norway) and Alaska
• INTA for X-band reception in Maspalomas (Spain)
• E-Geos for X-band reception in Matera (Italy)
• Astrium Services for the PAC (Processing and Archiving Center) in Farnborough (UK)
• DLR for the PAC in Oberpfaffenhofen (Germany)
• CLS for the MPC (Mission Performance Center) operations in Brest (France).
• PDMC (Payload Data Management Center) will be located at ESRIN and be operated by the European Space Agency.
• The POD (Precise Orbit Determination) service will be provided by GMV (Spain).
Figure 142: Illustration of Sentinel-1 ground segment configuration (image credit: ESA, Ref. 131)
CPOD (Copernicus Precise Orbit Determination) Service
The CPOD service is part of the PDGS (Payload Data Ground Segment) of the Sentinel missions. A GMV-led consortium of Spain is operating the CPOD being in charge of generating precise orbital products and auxiliary data files for their use as part of the processing chains of the respective Sentinel PDGS. 219)
Figure 143 shows the different elements that interact with the CPOD service. On top we have the Sentinels satellites, all of them with two GPS Receivers on-board (Sentinel-3 also has a LRR and DORIS). The raw L0 data is downloaded at least once per orbit to one of the Ground Stations used (particularly Svalbard, but also Maspalomas and Matera are used). The raw L0 data that contains the GPS and attitude data is circulated to the Sentinels PDGS and from there it is made available to the CPOD Service Center, which will generate orbital products with different timelines.
Figure 143: Overview of the CPOD service elements (image credit: GMV, ESA)
The first three Sentinel missions require orbital products in NRT (Near Real Time), with latencies as low as 30 minutes, in STC (Short Time Critical), with latencies of 1.5 days and in NTC (Non-time Critical) with latencies of 20-30 days. The accuracy requirements are very challenging, targeting 5 cm in 3D for Sentinel-1 and 2-3 cm in radial direction for Sentinel-3.
Table 19: Summary of timeliness and accuracy requirements 220)
The CPOD Service has been developed and it is being operated by a GMV-led consortium with a system running at GMV premises to provide orbital products for the Sentinel missions with different timeliness: NRT, STC , NTC and reprocessing REP. Additionally the Sentinel-3 POD IPF (Instrument Processing Facility), a software package developed as part of the CPOD Service, will run at the Sentinel-3 PDGS (on both, the Marine Center and Core Ground Station) generating NRT orbital products for the Sentinel-3 mission.
The accuracy of the orbital products is being assessed by a number of external validation institutions, all of them being part of the Copernicus POD QWG (Quality Working Group). The main purpose of the Copernicus POD QWG is to monitor the performance of the operational POD products (both the orbit products as well as the input tracking data) and to define potential and future enhancements to the orbit solutions.
The different Sentinel FOSs (Flight Operation Segments) provide orbital products (restituted and predicted) plus maneuver and mass history information. CNES provides also orbital products and DORIS data for Sentinel-3, and it receives GPS RINEX (Receiver Independent Exchange Format) files from the CPOD Service Center.
Veripos Ltd is the source of accurate GPS orbits and clocks for NRT and STC latencies and IGS for NTC and REP latencies. The CPOD also has an in-house back-up of Veripos based on magicGNSS, which provides NRT GPS orbits and clocks. For Sentinel-3, ILRS (International Laser Ranging Service) and DORIS data will also be used. Finally, the CPOD Service interacts with the CPOD QWG and a number of external validation centers.
The Copernicus POD Service has been developed and it is operated by GMV, but it interacts with different entities, both public and private, that act as clients, users and subcontractors. Following are the current main members of the Copernicus POD Service:
- ESA/ESRIN (European Space Research Institute) Frascati, Italy. This center leads the development of the different PGDSs, and in particular, the Sentinel-1 and -2 PDGS are located here.
- ESA/ESOC (European Space Operation Center), Darmstadt, Germany. This center hosts the FOS (Flight Operations Segment) of Sentinel-1, -2, and -3 missions during the commissioning phase and also during the Routine Operation phase except for Sentinel-3 mission, which is handed over to EUMETSAT.
- EUMETSAT (European Organization for the Exploitation of Meteorological Satellites), Darmstadt, Germany. This center hosts the FOS ( Flight Operations Segment) of Sentinel-3 mission during the Routine Operation Phase, and also the so-called Marine Center PDGS of Sentinel-3.
- CNES (Centre National d'Études Spatiales), Toulouse, France provides accurate orbits and platform data files for the Sentinel-3 mission. CNES has also contributed with the DORIS instrument, and as so, it also provides RINEX DORIS files to the Copernicus POD Service.
- GMV Innovating Solutions is the prime of the Copernicus POD Service. It has developed and it is operating the Service from its headquarters in Tres Cantos, near Madrid (Spain). It is responsible also of the overall management and the evolutions of the system.
- POSITIM UG provides expertise in the LEO POD field, prototypes improvements in algorithms and manages, on behalf of ESA/GMV, the Quality Working Group.
- DLR (Deutsches Zentrum für Luft- und Raumfahrt) Oberpfaffenhofen, Germany provides expertise in the LEO POD and GNSS fields. Additionally, it contributes with orbital products for external validation of products.
- TUM (Technische Universität München) provides expertise in the LEO POD field. Additionally, it contributes with orbital products for external validation of products.
- AIUB (Astronomisches Institut, Universität Bern), Bern, Switzerland contributes with orbital products for external validation of products.
- TU Delft (Technische Universiteit Delft), Delft, The Netherlands contributes with orbital products for external validation of products.
- VERIPOS Ltd, Aberdeen, UK is the provider of accurate GPS orbits and clocks in NRT and STC timeliness for it use in the GNSS POD processing.
There are two places where the operational orbits are computed. The so-called CPOD Service Center, located in GMV's premises, is in charge of computing all orbital products of Sentinel-1 and -2 and all STC and NTC products of Sentinel-3. The Sentinel-3 POD IPF (Instrument Processing Facility) is in charge of computing the Sentinel-3 NRT orbital products and it will be running at two locations, the Marine center (located in EUMETSAT, Darmstadt) and the Core Ground Station (located in Svalbard).
Orbital accuracy results:
• Sentinel-1A was launched on April 3, 2014. After 6 months of commissioning, the CPOD Service started the ROP (Routine Operation Phase) in October 2014. Since then, every four months the quality of the service is assessed, including the accuracy of the orbital products. For this, a specific period of time is selected for re-processing by the external validation institutions (i.e. AIUB, DLR, ESOC, TU Delft and TUM). This exercise has been performed twice since the beginning of the ROP phase.
• Sentinel-2A was launched on June, 23, 2015. The commissioning phase is expected to finish by mid/end October 2015, so the ROP (Routine Operation Phase) is expected to begin in November 2015. During the commissioning phase the orbit accuracy has been assessed by the same means used with Sentinel-1A, selecting a period of time to be re-processed by the external validation institutions (Ref. 219).
In the case of Sentinel-1A, the institutions compute offline accurate orbits and provide them to GMV for cross-comparison. It has been shown that systematically the accuracy requirements are fulfilled without major problems. Figure 144 shows the results of the last comparison campaign of January 2016; it shows the 3D RMS per day, during 10 days in January 2016, between the operational Sentinel-1 NTC solution (CPOD) and the daily solutions provided by each institution. Additionally, there is a combined solution (COMB) computed as a weighted average of all individual solutions. It can be seen that the differences are systematically below the required 5 cm (Ref. 220).
Figure 144: Sentinel-1A orbit comparisons (3D RMS; cm) between CPOD and external solutions; red line is threshold (image credit: GMV)
In addition, all NRT and NTC products are compared routinely against an offline ESOC solution (which used the same POD SW, NAPEOS, but of a different version and using different configuration and inputs). The following plots (Figures 145 and 146) show the differences with respect to ESOC from October 2015 to January 2016, where it can be seen that systematically the differences are well below the threshold. The cases above the threshold are typically due to maneuvers and data gaps.
Figure 145: Sentinel-1A Restituted Orbital Product vs. ESOC (2D RMS) from 1st October 2015 until 31st January 2016 (image credit: GMV)
Figure 146: Sentinel-1A Precise Orbital Product vs. ESOC (3D RMS) from 1st October 2015 to 31st January 2016 (image credit: GMV)
In the case of Sentinel-2A, the accuracy results are similar to those of Sentinel-1A. Figure 147 shows the results of the last comparison campaign of January 2016; it shows the 3D RMS per day, during 10 days in January 2016, between the operational Sentinel-2 NTC solution (CPOD) and the daily solutions provided by each institution. Additionally there is a combined solution (COMB) computed as a weighted average of all individual solution. It can be seen that the differences are systematically below 5 cm, like in the case of Sentinel-1A.
Copernicus / Sentinels EDRS system operations:
EDRS (European Data Relay Satellite) will provide a data relay service to Sentinel-1 and -2 and initially is required to support 4 Sentinels simultaneously. Each Sentinel will communicate with a geostationary EDRS satellite via an optical laser link. The EDRS GEO satellite will relay the data to the ground via a Ka-band link. Optionally, the Ka-band downlink is planned to be encrypted, e.g. in support to security relevant applications. Two EDRS geo-stationary satellites are currently planned, providing in-orbit redundancy to the Sentinels. 221)
EDRS will provide the same data at the ground station interface as is available at the input to the OCP (Optical Communications Payload) on-board the satellites, using the same interface as the X-band downlink. The EDRS transparently adapts the Sentinels data rate and format to the internal EDRS rate and formats, e.g. EDRS operates at bit rates of 600 Mbit/s and higher.
With EDRS, instrument data is directly down-linked via data relay to processing and archiving centers, while other data continues to be received at X-band ground stations. The allocation of the data to downlink via X-band or EDRS is handled as part of the Sentinel mission planning system and will take into account the visibility zones of the X-band station network and requirements such as timeliness of data.
Figure 148: Sentinel missions - EDRS interfaces (image credit: ESA)
Copernicus / Sentinel data policy:
The principles of the Sentinel data policy, jointly established by EC and ESA, are based on a full and open access to the data:
• anybody can access acquired Sentinel data; in particular, no difference is made between public, commercial and scientific use and in between European or non-European users (on a best effort basis, taking into consideration technical and financial constraints);
• the licenses for the Sentinel data itself are free of charge;
• the Sentinel data will be made available to the users via a "generic" online access mode, free of charge. "Generic" online access is subject to a user registration process and to the acceptation of generic terms and conditions;
Following registration, the user will have the possibility to immediately download a test data set that simulates the data products that will be generated by Sentinel-1. Following launch, registered users will be granted early access to Sentinel-1 data samples, even before the full operational qualification of the products is completed.
Registration is open to all users via simple on-line self-registration accessible via the Sentinel Data Hub. 222)
• additional access modes and the delivery of additional products will be tailored to specific user needs, and therefore subject to tailored conditions;
• in the event security restrictions apply to specific Sentinel data affecting data availability or timeliness, specific operational procedures will be activated.
Sentinel-1 operational products:
• Level-0 products: Compressed, unprocessed instrument source packets, with additional annotations and auxiliary information to support the processing. 228)
• Level-1 products:
- Level-1 Slant-Range Single-Look Complex Products (SLC): Focused data in slant-range geometry, single look, containing phase and amplitude information.
- Level-1 Ground Range Detected Geo-referenced Products (GRD): Focused data projected to ground range, detected and multi-looked. Data is projected to ground range using an Earth ellipsoid model, maintaining the original satellite path direction and including complete geo-reference information.
• Level-2 Ocean products: Ocean wind field, swell wave spectra and surface currents information as derived from SAR data.
Table 20: Sentinel-1 Operational products access policy
Table 21: Planned operational ESA Sentinel-1 products - L1 characteristics 229)
• For GRD (Ground Range Detected) products, the resolution corresponds to the mid range value at mid orbit altitude, averaged over all swaths.
• For SLC (Slant-Range Single-Look Complex) products SM/IW/EW products, the resolution and pixel spacing are provided from lowest to highest incidence angle. For SLC WV products, the resolution and pixel spacing are provided for beams WV1and WV2.
• For SLC SM/IW/EW products, the resolution and pixel spacing are provided from lowest to highest incidence angle. For SLC WV products, the resolution and pixel spacing are provided for beams WV1and WV2.
In the context of the Copernicus program, ESA is conducting a number of coordinated preparatory activities (studies, campaigns, etc.) to demonstrate/validate the observation concepts (as well as many other system aspects) that are being planned for the various Sentinel missions. The following campaigns are in particular dedicated in support of Sentinel-1 (and -2, AgriSAR) applications.
The AgriSAR 2009 campaign of ESA took place in April 2009. The objective is to evaluate how frequent multi-polarization acquisitions provided by Sentinel-1 will improve applications such as land-cover mapping and crop monitoring. To accomplish this ambitious task, ESA has asked MDA Geospatial Services to acquire multi-temporal, quad-polarization RADARSAT-2 imagery throughout the 2009 growing season over three test sites. The chosen sites are located in Flevoland in the Netherlands, Barrax in Spain and Indian Head in mid-west Canada.
In addition to the contribution from MDA Geospatial Services, the campaign included also a number of European and Canadian scientists to help with ground activities. These activities included the collection and analysis of information about land cover, crop type, crop condition and other parameters such as soil moisture. Of particular interest were the new algorithms and methods required to extract land-cover information from a dense temporal series of SAR images and follow how the crops develop. 230)
Figure 149: The color composite of a RADARSAT-2 polarimetric radar image acquired over the Flevoland test site in the Netherlands on April 4, 2009 (image credit: ESA)
Figure 150: Polarimetric RADARSAT-2 SAR image of the Barrax test site in Spain on April 9, 2009 (image credit: ESA)
The IceSAR airborne campaign (3 weeks in March 2007), which took place near Longyearbyen in Svalbard (Norway), was conducted by AWI (Alfred Wegner Institut), Bremerhaven, DLR/HR (Microwave and Radar Institute), and ESA. The radar configuration consisted of a C-band instrument with VV and VH polarizations, very similar to the future Sentinel-1 sensor. The C-band SAR was flown on the DO-228 aircraft of DLR (E-SAR instrument with C- and X-band capability). In addition, the Polar-2 aircraft of AWI was flown carrying the AWI infrared line scanner during the IceSAR campaign in coordination with the radar aircraft. 231) 232)
Figure 151: An airborne SAR sea-ice image taken over Storfjorden, Svalbard on March, 16, 2007 (image credit: ESA)
The AgriSAR campaign of ESA took place in the summer of 2006 (Apr. 18 - Aug. 2, 2006) - representing an ambitious large-scale attempt to assess the performance of the Sentinel-1 (C-band SAR) and Sentinel-2 (Optical Multispectral) for land applications. The campaign was unique in scope and scale representing frequent airborne SAR coverage during the entire crop-growing season, from sowing to harvest. 233) 234) 235)
The main test site was Demmin (Durable Environmental Multidisciplinary Monitoring Information Network), an agricultural site located in Mecklenburg-Vorpommern in North-East Germany, approximately 150 km north of Berlin. Main crop types in the area are winter wheat, barley, maize, rape and sugar beet. The DLR (German Aerospace Center) E-SAR system was flown over the Demmin test site more than 14 times between the months of April and July. Weekly in-situ measurements were taken on the ground in selected fields throughout the same period.
In addition to SAR coverage, optical data using the Canadian CASI instrument from ITRES Research and the Spanish AHS from the National Institute for Aerospace Technology (INTA), were acquired during critical phases of the growing season in June and July. The June acquisitions were extended to include a forest and grassland site in the central Netherlands, used by the EU EAGLE project.
In total, over 15 research institutes from Germany, Spain, Italy, Belgium, The Netherlands, Britain, Canada and Denmark participated in the campaign.
The Sentinel series:
Provides data continuity for:
Validation provided by:
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60) "Sentinel-1 supports detection of illegal oil spills," Geospatial World, April 7, 2017, URL: https://www.geospatialworld.net/news-posts/sentinel-1-supports-detection-illegal-oil-spills/
62) "Urban monitoring boosted by new data processor," ESA, April 4, 2017, URL: http://www.esa.int/Our_Activities/Observing_the_Earth
63) "TimeScan product: Germany," ESA, April 4, 2017, URL: http://www.esa.int/spaceinimages/Images/2017/04/TimeScan_product_Germany
64) "TimeScan product: Pearl River Delta," ESA, April 3, 2017, URL: http://www.esa.int/spaceinimages/Images/2017/04/TimeScan_product_Pearl-River_Delta
65) "Alert, Canada,", ESA Earth observation image of the week , March 31, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/03/Alert_Canada
66) "Cryovex and Karen: Catching Up," ESA, March 28, 2017, URL: http://blogs.esa.int/campaignearth/2017/03/28/cryovex-and-karen-catching-up/
67) "San Joaquin Valley is Still Sinking," NASA Earth Observatory, March 5, 2017, URL: http://earthobservatory.nasa.gov/IOTD/view.php?id=89761
68) Tom G. Farr, Cathleen E. Jones, Zhen Liu, "Progress Report: Subsidence in California, March 2015 – September 2016," URL: http://www.water.ca.gov/waterconditions/docs/2017/JPL%20subsidence-20report%20final%20for%20public%20dec%202016.pdf
69) "Lake Success, California," ESA, Earth observation image of the week, February 24, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/02/Lake_Success_California
70) "Sentinels warn of dangerous ice crack," ESA, Feb. 16, 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Copernicus/
71) "Relocation of Halley Research Station," BAS Press Release, 6 Dec. 2016, URL: https://www.bas.ac.uk/media-post/relocation-of-halley-research-station/
72) "Satellites monitor landslide in the Alps," ESA, Feb. 3, 2017, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Satellites_monitor_landslide-in_the_Alps
73) "Larsen Crack," ESA, Jan. 30, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/01/Larsen_crack
74) "Satellites confirm sinking of San Francisco tower," ESA, Nov. 25, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1/
75) "Millennium Tower sinking," ESA, Nov. 24, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/11/Millennium_Tower_sinking
76) "San Francisco displacement," ESA, Nov. 24, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/11/San_Francisco_displacement
77) "Bay Area displacement," ESA, Nov. 24, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/11/Bay_Area_displacement
78) "Oslo train station on the move," ESA, Nov. 24, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/11/Oslo_train_station_on_the_move
79) "Virunga Mountains," ESA Earth observation image of the week, Nov. 11, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/11/Virunga_Mountains
80) "Sentinel satellites reveal east–west shift in Italian quake," ESA, Nov. 3, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
81) "European seismic zones", ESA, Oct. 28, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/10/European_seismic_zones
82) "Earthquake area in Italy," ESA, Oct. 28, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/10/Earthquake_area_in_Italy
83) "Ground displacement from Italy's earthquake," ESA, Aug. 28, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/08/Ground_displacement_from-Italy_s_earthquake
84) "Copernicus Sentinel-1B handed over for operations," ESA, Sept. 16, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/09/Copernicus_Sentinel-1B_handed_over_for_operations
85) "China's Poyang Lake in radar vision," ESA Earth observation image of the week, Sept. 16, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/09/Poyang_Lake
86) "Copernicus Sentinel-1A satellite hit by space particle," ESA, Aug. 31, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
87) "Sentinel-1 satellites combine radar vision," ESA, June 22, 2016, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
88) Pierre Potin, Betlem Rosich, Patrick Grimont, Nuno Miranda, Ian Shurmer, Alistair O'Connell, Ramón Torres, Mike Krassenburg, "Sentinel-1 Mission Status," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
89) Ramón Torres, Svein Løkås, David Bibby, Dirk Geudtner, "Sentinel-1B LEOP and Commissioning Results,"Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
90) Dirk Geudtner, Pau Prats, Nestor Yague-Martinez, Ignacio Navas-Traver, Itziar Barat, Ramón Torres, "Sentinel-1 SAR Interferometry Performance Verification," Proceedings of EUSAR 2016, 11th European Conference on Synthetic Aperture Radar, Hamburg, Germany, June 6-9, 2016
91) "Sentinel-1 helping Cyclone Roanu Relief," ESA, May 26, 2016, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
92) "Antarctic Peninsula ice flow," T. Nagler , H. Rott, M. Hetzenecker, J. Wuite, "Monitoring ice motion of the Antarctic and Greenland ice sheets at high spatial and temporal resolution by means of Sentinel-1 SAR," Living Planet Symposium, 2016, Prague, Czech Republic, May 9-13, 2016, URL of abstract: http://www.esa.int/spaceinimages/Images/2016/05/Antarctic_Peninsula_ice_flow
93) "Zachariae glacier," ESA, image of the week series, May 6, 2016, URL: http://www.esa.int/spaceinimages/Images/2016/05/Zachariae_glacier
94) Alan Buis, "In Greenland, Another Major Glacier Comes Undone," NASA/JPL, Nov. 12, 2015, URL: http://www.jpl.nasa.gov/news/news.php?feature=4771
95) "Sentinel-1B delivers," ESA, April 28, 2016, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1/Sentinel-1B_delivers
96) "Sentinel-1B spreads its wings," ESA, April 27, 2016: URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Sentinel-1B_spreads_its_wings
97) "Student satellites fly freely on their orbit in space," ESA, April 26, 2016, URL: http://www.esa.int/Education/CubeSats_-_Fly_Your_Satellite
98) "Soyuz demonstrates Arianespace mission flexibility," Space Daily, April 26, 2016, URL: http://www.spacedaily.com/reports/Soyuz_demonstrates_Arianespace_mission-flexibility_999.html
99) "Dasht-e Kavir," ESA, Earth observation image of the week, April 22, 2016, URL: http://www.esa.int/spaceinimages/Images/2016/04/Dasht-e_Kavir
100) "Sentinel-1 counts fish," ESA, April 21, 2016, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Sentinel-1_counts_fish
101) "Sentinel-1A detects fishing cages," ESA, April 14, 2016, URL: http://www.esa.int/spaceinimages/Images/2016/04/Sentinel-1A_detects_fishing_cages
102) "Irish mosaic," ESA, Earth observation image of the week, April 15, 2016: URL: http://www.esa.int/spaceinimages/Images/2016/04/Irish_mosaic
103) "Nansen gives birth to two icebergs," ESA, April 14, 2016, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
105) "Colors of Sweden," ESA, Feb. 5, 2016, URL: http://www.esa.int/spaceinimages/Images/2016/02/Colours_of_Sweden
106) "Deal sealed for new Sentinel-1 satellites," ESA, Dec. 15, 2015, URL: http://m.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
107) "Dutch mosaic," ESA, Earth observation image of the week, Dec. 4, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/12/Dutch_mosaic
108) "Manicouagan Crater, Canada," ESA, Earth observation image of the week, Oct. 30, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/10/Manicouagan_Crater_Canada
109) "Azore Islands," ESA, Earth observation image of the week, Oct. 9, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/10/Azore_islands
110) "Chile earthquake on the radar," ESA, Sept. 21, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/09/Chile_earthquake_on_the_radar
111) "Mission Status Report 68 for the period August 11-17, 2015," EC, ESA, URL: https://sentinel.esa.int/documents/247904/1752934/Sentinel-1-Mission_Status_Report_68-Period_11-17_August_2015.pdf
112) "Sentinel-1 Observation Scenario," ESA, 2015, URL: https://sentinels.copernicus.eu/web/sentinel/missions/sentinel-1/observation-scenario
113) Alexis Mouche, Bertrand Chapron, Harald Johnsen, Fabrice Collard, Romain Husson, He Wang, Gilles Guitton, Fabrice Ardhuin, "Sentinel-1 results: Sea state applications," Proceedings of the IGARSS (International Geoscience and Remote Sensing Symposium) 2015, Milan, Italy, July 26-31, 2015
114) R. Lanari, P. Berardino, M. Bonano, F. Casu, C. De Luca, S. Elefante, A. Fusco, M. Manunta, M. Manzo, C. Ojha, A. Pepe, E. Sansosti, I. Zinno, "Sentinel-1 results: SBAS-DInSAR processing chain developments and land subsidence analysis," Proceedings of the IGARSS (International Geoscience and Remote Sensing Symposium) 2015, Milan, Italy, July 26-31, 2015
115) "Southern Bavaria," ESA, Earth observation image of the week, July 24, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/07/Southern_Bavaria
116) "Central California, USA," ESA, Earth observation image of the week, June 26, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/06/Central_California_USA
117) "Tianjin, China," ESA Earth observation image of the week, June 5, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/06/Tianjin_China
118) "Update: Disaster relief – DLR provides aerial images of Kathmandu," DLR/EOC, April 30, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13487/#/gallery/19391
119) "Nepal earthquake displacement," ESA, April 30, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Nepal_earthquake_displacement
120) "Nepal Earthquake on the Radar," ESA, April 29, 2015, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
121) "Kathmandu grading map," ESA, April 29, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Kathmandu_grading_map
122) "Florida, United States of America," ESA, image release on April 24, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Florida_United_States_of_America
123) "happy birthday, Sentinel-1A," ESA, April 3, 2015, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1/Happy_birthday_Sentinel-1A
124) "Irkutsk and Lake Baikal," ESA image, featured on the Earth from Space video program, April 3, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Irkutsk_and_Lake_Baikal
125) "Earth shifts in color," A gallery of InSAR images is presented at the 2015 Fringe Workshop, held at ESA/ESRIN in March 23-27, 2015, ESA, March 27, 2015, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Highlights/Earth_shifts_in_colour
126) "Aral Sea," ESA image in the series 'Earth observation image of the week,' released on March 27, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/03/Aral_Sea
127) "Pine Island Glacier on Sentinel-1A's radar," ESA, March 25, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/03/Pine_Island_Glacier_on_Sentinel-1A_s_radar
128) ESA, March 20, 2015, 'Our week in images,' URL: http://www.esa.int/Highlights/Week_In_Images_16_20_March_2015
129) "UK joins Sentinel collaborative ground segment," ESA, March 19, 2015, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus
131) Pierre Potin, "Sentinel-1 Mission," POLinSAR 2015, ESA/ESRIN, Frascati, Italy, Jan. 26-30, 2015, URL: http://seom.esa.int/insarap/files/INSARAP-2014_Sentinel-1_Mission_Status_PPotin.pdf
132) "Lisbon, Portugal," ESA, Feb. 6, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/02/Lisbon_Portugal
133) "Satellites catch Austfonna shedding ice," ESA, Jan. 23, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Satellites_catch-Austfonna_shedding_ice
134) Malcolm McMillan, Andrew Shepherd, Noel Gourmelen, Amaury Dehecq, Amber Leeson, Andrew Ridout, Thomas Flament, Anna Hogg, Lin Gilbert, Toby Benham, Michiel van den Broeke, Julian A. Dowdeswell, Xavier Fettweis, Brice Noël, Tazio Strozzi, "Rapid dynamic activation of a marine-based Arctic ice cap," Geophysical Research Letters, Vol. 41, Dec. 23, 2014, doi:10.1002/2014GL06225, URL: http://onlinelibrary.wiley.com/doi/10.1002/2014GL062255/full
135) "Mexico City subsidence," ESA, Dec. 11, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/12/Mexico_City_subsidence
136) "Romania," ESA, image in the 'Earth from Space video program' series, Dec. 5, 2014: URL: http://www.esa.int/spaceinimages/Images/2014/12/Romania
137) "Sentinel-1 maps Fogo eruption," ESA, Dec. 2, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/12/Sentinel-1_maps_Fogo_eruption
138) "Laser link offers high-speed delivery," ESA, Nov. 28, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
139) "Super laser revolutionizes data communications in space," DLR, Nov. 28, 2014, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-12327/year-all/#/gallery/17332
140) "Succesful demonstration of Alphasat-Sentinel-1A laser link," ESA, Nov. 30, 2014, URL: http://artes.esa.int/news/succesful-demonstration-alphasat-sentinel-1a-laser-link
141) "First image download over new gigabit laser connection in space," Airbus Defence and Sapce, Nov. 28, 2014, URL: http://airbusdefenceandspace.com/newsroom/news-and-features/first-image-download-over-new-gigabit-laser-connection-in-space/
143) "Tokyo Bay, Japan," ESA, image in the 'Earth from Space video program' series, Nov. 21, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/11/Tokyo_Bay_Japan
144) "DLR and ESA sign the collaborative ground segment cooperation for Sentinel data," ESA, Nov. 10, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/DLR_and-ESA_sign_the_Collaborative_Ground_Segment_Cooperation_for_Sentinel_data
145) "Copernicus Agreement," ESA, Oct. 28, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/10/Copernicus_Agreement
146) "Copernicus Sentinel 1: Making our seas safer," ESA, Oct. 22, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
147) "Monitoring icebergs," ESA, image release on Oct. 22, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/10/Monitoring_icebergs
148) "First Copernicus satellite now operational," ESA, Oct. 6, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
149) "Radar vision maps Napa Valley earthquake," ESA, Sept. 2, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
150) "Sentinel-1 poised to monitor motion," ESA, August 26, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
151) Manuela Braun, Richard Bamler, "Sentinel-1 — Earth's topography as a colored pattern," DLR, August 26, 2014, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-11427/#/gallery/16251
152) Matthias Motzigemba, Herwig Zech, Frank Heine, Stefan Seel, Daniel Tröndle, "Laser Communication in Space becomes operational – a new chapter begins for EO Missions and military surveillance," Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAF-14,B2,1.4
153) "Rio de Janeiro, Brazil," ESA, Earth from Space video program, July 11, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/07/Rio_de_Janeiro_Brazil
154) "Mount Pinatubo, Philippines," ESA, July 4, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/07/Mount_Pinatubo_Philippines
155) "Sentinel-1A aids Balkan flood relief," ESA, May 28, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
156) "Lake Constance (Bodensee)," ESA, May 30, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/05/Lake_Constance
157) "Greece guaranteed access to Sentinel data," ESA, May 12, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus
158) "Copernicus Sentinels Serving Society and the Environment," Athens, Greece, May 12-13, 2014, URL: http://congrexprojects.com/2014-events/Copernicus/program
159) "Copernicus Benefitting Society and the Environment," ESA, May 13, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Copernicus-benefitting_society_and_the_environment
160) "Radar image of the Salar de Uyuni, Bolivia," released on May 9, 2014 on ESA's Earth from Space video program, URL: http://www.esa.int/spaceinimages/Images/2014/05/Radar_image_of_the_Salar-de_Uyuni_Bolivia
161) "Radar image of the Netherlands," ESA, April 25, 2014, URL: http://www.esa.int/spaceinimages/Images/2014/04/Radar_image_of_the_Netherlands
162) "Earth from Space: Sampling Sentinel," ESA, April 25, 2014, URL: http://www.esa.int/spaceinvideos/Videos/2014/04/Earth_from_Space_Sampling-Sentinel
163) "First radar vision for Copernicus," ESA, April 16, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
164) Ramon Rorres, Svein Lokas, Dirk Geudtner, Betlem Rosich, "Sentinel-1A LEOP and Commissioning Results," Proceedings of EUSAR 2014 (10th European Conference on Synthetic Aperture Radar), Berlin, Germany, June 3-5, 2014
165) "A night shift like never before," ESA, April 9, 2014, URL: http://blogs.esa.int/eolaunches/2014/04/09/a-night-shift-like-never-before/
166) I. Shurmer, A. O'Connell, J. Morales, Juan Pineiro, P.P. Emanuelli, "The Sentinel-1A LEOP: Paving the way for the Sentinels LEOP preparation and execution," Proceedings of the 14th International Conference on Space Operations (SpaceOps 2016), Daejeon, Korea, May 16-20, 2016, paper: AIAA 2016-2457, URL: http://arc.aiaa.org/doi/pdf/10.2514/6.2016-2457
167) A. Vasconcelos, M. A. Martín Serrano, J. Sánchez, D. Kuijper, X. Marc, "Sentinel-1a Reference Orbit Acquisition Maneuver Campaign," Proceedings of the 25th International Symposium on Space Flight Dynamics, Munich, Germany, Oct. 19-23, 2015, URL: http://issfd.org/2015/files/downloads/papers/134_Vasconcelos.pdf
168) "Sentinel-1 performs opening dance routine," ESA, April 4, 2014, URL: http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1
169) "Separation in space," ESA, April 04,2014, URL: http://www.esa.int/spaceinvideos/Videos/2014/04/Separation_in_space
170) F. Rostan, S. Riegger, W. Pitz, A. Torre, R. Torres, "The C-SAR Instrument for the GMES Sentinel-1 Mission," Proceedings of IGARSS 2007 (International Geoscience and Remote Sensing Symposium), Barcelona, Spain, July 23-27, 2007
171) F. Rostan, S. Riegger, W. Pitz, R. Croci, R. Torres, "The C-SAR Instrument Design & Performance for the GMES Sentinel-1 Mission," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
172) U. Schönfeldt, H. Braubach, "Electrical Architecture of the Sentinel-1 SAR Antenna Subsystem," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
173) M. von Alberti, "Thermo-Mechanical Architecture of the Sentinel-1 SAR Antenna," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
174) M. Hutchinson, L. Griffiths, E. Mak, "Next Generation CORE Radar Electronics Subsystem for Sentinel-1," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
175) Ramon Torres, Ignacio Navas-Traver, Roberta Bertoni, Paul Snoeij, Evert Attema, Guido Levrini, Renato Croci, Michelangelo L'Abbate, Andrea Pietropaolo, Friedhelm Rostan, Markus Huchler, "Sentinel-1 CSAR Instrument Design," Proceedings of the 2nd Workshop on Advanced RF Sensors and Remote Sensing Instruments 2009, Noordwijk, The Netherlands, Nov. 17-18, 2009
176) Pierre Potin, "Sentinel-1 Mission Overview," ESA/ESRIN, Frascati, Italy, Jan. 19, 2011, URL: http://earth.eo.esa.int/pub/polsarpro_ftp/RadarPol_Course11/Wednesday19/Sentinel-1_overview.pdf
177) F. Rostan, M. Huchler, S. Riegger, R. Croci, R. Torres, "The GMES Sentinel-1 C-SAR instrument: Development Status and Performance Overview," Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011
178) Friedhelm Rostan, Sebastian Riegger, Markus Huchler, Renato Croci, Ramon Torres, "The Sentinel-1 C-SAR Instrument," Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012
179) Paul Snoeij, Ramon Torres, Dirk Geudtner, Michael Brown, Patrick Deghaye, Ignacio Navas-Traver, Allan Østergaard, Bjorn Rommen, Nicolas Floury, Malcolm Davidson, "Sentinel-1 Instrument Overview," Proceedings of SEASAR 2012, The 4th International Workshop on Advances in SAR Oceanography, Tromsø, Norway, June 18-22, 2012, URL: https://earth.esa.int/c/document_library/get_file?folderId=233696&name=DLFE-2149.pdf
180) Paul Snoeij, Dirk Geudtner, Ramón Torres, Malcolm Davidson, David Bibby, Svein Lokas, "GMES Sentinel-1 System Overview," POLinSAR 2013, 6th International Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry, Frascati, Italy, Jan. 28- Feb. 1, 2013, URL: https://earth.esa.int/c/document_library/get_file?folderId=408536&name=DLFE-5605.pdf
181) Friedhelm Rostan, Markus Huchler, Sebastian Riegger, Antonio Bauleo, Ramon Torres, "The Copernicus Sentinel-1 C-SAR instrument on its way to operational service," Proceedings of IGARSS (IEEE Geoscience and Remote Sensing Society) 2014, Québec, Canada, July 13-18, 2014
182) "Sentinel-1 SAR," ESA Technical Guide, URL: https://sentinel.esa.int/web/sentinel/sentinel-1-sar-wiki/-/wiki/Sentinel%-20One/Instrument
183) "Sentinel-1 SAR User Guide Introduction," ESA, URL: https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar
184) Francesco De Zan, Andrea M. Monti Guarnieri, "TOPSAR: Terrain Observation by Progressive Scans," IEEE Transactions on Geoscience and Remote Sensing, Vol. 44, Issue 9, Sept. 2006, pp. 2352-2360
185) D. D'Aria, F. De Zan, D. Giudici, A. Monti Guarnieri, F. Rocca, "Burst-mode SARs for wide-swath surveys," Canadian Journal of Remote Sensing, Vol. 33, No 1, 2007, pp. 27-38, URL: http://home.dei.polimi.it/monti/papers/tucan.pdf
186) Evert Attema, Paul Snoeij, Ramon Torres, Andrea Pietropaolo, Daniele Scaranari, Vanessa Mastroddi, Silvia Occhigrossi, Claudio Bruno, Michelangelo L'Abbate, "Analysis of Sentinel-1 Mission Capabilities," Proceedings of EUSAR 2010, 8th European Conference on Synthetic Aperture Radar, June 7-10, 2010, Aachen, Germany
187) Adriano Meta, Pau Prats, Ulrich Steinbrecher, Josef Mittermayer, Rolf Scheiber, "TerraSAR-X TOPSAR and ScanSAR comparison," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
188) P. Prats, A. Meta, R. Scheiber, J. Mittermayer, A. Moreira, J. Sanz-Marcos, "A TOPSAR Processing Algorithm Based on Extended Chirp Scaling: Evaluation with TerraSAR-X Data," Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
189) Pau Prats, Luca Marotti, Steffen Wollstadt, Rolf Scheiber, "TOPS Interferometry with TerraSAR-X," Proceedings of EUSAR 2010, 8th European Conference on Synthetic Aperture Radar, June 7-10, 2010, Aachen, Germany
190) Rolf Scheiber, Steffen Wollstadt, Stefan Sauer, Elke Malz, Josef Mittermayer, Pau Prats, Paul Snoeij, Evert Attema,, "Sentinel-1 Imaging Performance Verification with TerraSAR-X," Proceedings of EUSAR 2010, 8th European Conference on Synthetic Aperture Radar, June 7-10, 2010, Aachen, Germany
191) Javier Duro, David Albiol, Fifame N. Koudogbo, Alain Arnaud, Céline Tison, Jean-Claude Souyris, "Emulation of TOPSAR data from TerraSAR-X Stripmap mode," Proceedings of EUSAR 2010, 8th European Conference on Synthetic Aperture Radar, June 7-10, 2010, Aachen, Germany
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).