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).
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. 74)
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.
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.
Figure 12: Photo of the Sentinel-1A spacecraft during functional tests in Cannes, France (image credit: TAS) 39)
• 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. 96).
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)
Note: As of July 2019, the previously single large Sentinel-1 file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.
As of 30 April 2020, the Sentinel-1 file has been split into three files.
• This article covers the Sentinel-1 mission and its imagery in the period 2020
Mission status and its imagery for the period 2020
• July 10, 2020: The colossus iceberg that split from Antarctica’s Larsen C ice shelf on 12 July 2017 is now in the open waters of the South Atlantic near the South Orkney Islands, about 1050 km from its birthplace. Having lost two chunks of ice, this record berg is a little less huge than it once was – and now that it is in rougher waters, it may break up further. 55)
- When it calved, A-68 was about twice the size of Luxemburg and one of the largest icebergs on record, changing the outline of the Antarctic Peninsula forever. Despite its size, however, it is remarkably thin, just a couple of hundred meters thick.
- Over the last three years, satellite missions such as Copernicus Sentinel-1 have been used to track the berg as it drifted in the Southern Ocean. For the first two years, it remained close to its parent ice sheet, impeded by sea ice.
- However, it lost a chunk of ice almost immediately after being calved, resulting in it being renamed A-68A, and its offspring became A-68B. More recently, in April 2020, A-68A lost another chunk: A-68C.
- Rather unromantically, Antarctic icebergs are named from the Antarctic quadrant in which they were originally sighted, then a sequential number, then, if the iceberg breaks, a sequential letter.
- Although A-68A is a relatively thin iceberg, it has held together reasonably well, but satellites will be key to monitoring how it changes in open waters.
- Captured by the Copernicus Sentinel-1 radar mission, the image of Figure 17 shows the berg on 5 July 2020, a few days before its third birthday. Satellites carrying radar continue to deliver images regardless of the dark and bad weather, which is indispensable when monitoring the remote polar regions which are shrouded in darkness during the winter months.
Figure 17: A huge iceberg called A-68 calved from the Antarctic Peninsula’s Larsen C ice shelf on 12 July 2017. Three years on, it is in open waters near the South Orkney Islands in the South Atlantic Ocean – about 1050 km from its birthplace. The berg has already lost two chunks of ice, which were big enough to be given names: A-68B and A68C. Copernicus Sentinel-1 captured this image of the parent berg, A-68A, on 5 July 2020 (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
- The map (Figure 18) shows the different positions of A-68A during its three-year journey. The map not only highlights how long it remained close to the Larsen C ice sheet, but how, over the last year or so, its pace of drift has increased considerably.
Figure 18: A huge iceberg called A68 calved from the Antarctic Peninsula’s Larsen C ice shelf on 12 July 2017. It is now in open waters near the South Orkney Islands in the South Atlantic Ocean – about 1050 km from its birthplace. The map shows the different positions of the berg during its three-year journey. However, A-68 is now called A-68A as it has lost two chunks of ice, one in July 2017 named A-68B, and one in April 2020 named A-68C. The map also highlights that during its first two years of freedom A-68 drifted slowly, impeded by sea ice. Now that it is in relatively open waters, the pace of this parent iceberg has increased. The map also includes historic iceberg tracks, based on data from a number of satellites including ESA’s ERS-1 and ERS-2 as part of the Antarctic Iceberg Tracking Database, and shows that A-68A is following this well-trodden path (image credit: ESA, the map contains modified Copernicus Sentinel data (2017–20), processed by ESA; Antarctic Iceberg Tracking Database)
- The wider-view image of Figure 19 from the Copernicus Sentinel-3 mission shows A-68A’s position in February 2020.
Figure 19: A huge iceberg called A68 calved from the Antarctic Peninsula’s Larsen C ice shelf on 12 July 2017. This image, which was captured by the Copernicus Sentinel-3 mission, shows its position on 9 February 2020. The berg is now known as A-68A after losing two chunks of ice: A-68B and A-68C. Antarctic icebergs are named from the Antarctic quadrant in which they were originally sighted, then a sequential number, then, if the iceberg breaks, a sequential letter (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• July 10, 2020: The Copernicus Sentinel-1 mission takes us over the busy maritime traffic in the Bay of Naples, in southern Italy. 56)
- The two identical Copernicus Sentinel-1 satellites carry radar instruments to provide an all-weather, day-and-night supply of imagery of Earth’s surface. Here, three years of Sentinel-1 data over the same area, equal to hundreds of images, have been compressed into a single image.
- Many large vessels depart from the Port of Naples, one of the largest Italian seaports visible in the top-center of the image. From here, and the smaller Port of Pozzuoli in the left of the image, small leisure boats and ferries set sail to the nearby islands. Ischia, renowned for its thermal springs, and Procida are visible in the left of the image, in front of Pozzuoli, while the beautiful island of Capri is visible further south.
- Numerous boats are anchored off the island of Capri in the bottom center of the image. The island lies opposite the Sorrento peninsula, to which it was joined in prehistoric times. Two indentations can be seen in its cliff-lined coast: the Marina Grande on the north shore and the Marina Piccola on the south shore.
- Other vessels are docked off the coast of Sorrento, as well as off of the Amalfi coast in the right of the image.
- The Bay of Naples is famous for its scenic beauty, including the steep, volcanic hills surrounding it. The still-active Mount Vesuvius can be seen just inland from the bay, as a circular shape. Cities, such as Naples, as well as the other surrounding towns, are visible in white owing to the strong reflection of the radar signal.
Figure 20: The sea surface reflects the radar signal away from the satellite, and makes water appear dark in the image. This contrasts with metal objects, in this case the ships in the bay, which appear as bright dots in the dark waters of the Tyrrhenian Sea. Boats crossing the bay in 2017 appear in blue, those from 2018 appear in green, and those from 2019 can be seen in red. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2017-19), processed by ESA, CC BY-SA 3.0 IGO)
• April 30, 2020: The notion that rain could lead to a volcanic eruption may seem strange, but scientists from the University of Miami in the USA, have used information from satellites, including the Copernicus Sentinel-1 mission, to discover that a period of heavy rainfall may have triggered the four month-long eruption of Hawaii’s Kilauea volcano in 2018. 57)
Figure 21: The eruption of Hawaii’s Kilauea volcano in 2018 was one of the most destructive in this volcano’s recorded history. Why this happened has remained a mystery until a paper published recently in Nature suggests that rainfall could have been the culprit (image credit: U.S. Geological Survey)
- Producing about 320,000 Olympic-sized swimming pools’ worth of lava that reshaped the landscape, destroyed hundreds of homes and caused the collapse of the summit caldera, the 2018 eruption was one of the most destructive in Kilauea’s recorded history.
Figure 22: Restless Kilauea. Fiery lava continues to pour from the Kilauea volcano on Hawaii’s Big Island. These Copernicus Sentinel-2 images from 23 May, 7 June and 12 June 2018 show the relentless flow of lava and clouds of ash. This is an update of the Hawaii lava flow animation published on 8 June. The eruption, which began in early May, has destroyed more than 600 homes, spread lava over more than 800 ha of land and opened up at least 22 fissures in the ground, according to Hawaii County Mayor Harry Kim. Although this eruption has produced slow-moving lava, which has allowed people to evacuate, it is reported to be the most destructive eruption in the U.S. since that of Mount St. Helen’s in May 1980 (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
- A paper published recently in Nature proposes a new model to explain why this eruption happened. The authors, Jamie Farquharson and Falk Amelung from the University of Miami’s Rosenstiel School of Marine & Atmospheric Science, suggest that heavy rainfall may have been the culprit. 58)
- In the months before the eruption, Hawaii was inundated by an unusually prolonged period of heavy, and at times extreme, rainfall.
- The rainwater would have found its way through the pores of the volcanic rock and increased the pressure within – decreasing the rigidity of the rock and allowing magma to rise to the surface.
- Falk Amelung said, “We knew that changes in the water content in the Earth's subsurface can trigger earthquakes and landslides. Now we know that it can also trigger volcanic eruptions. Under pressure from magma, wet rock breaks easier than dry rock. It is as simple as that.”
- Using a combination of ground-based and satellite measurements of rainfall, Farquharson and Amelung modelled the fluid pressure within the volcano's edifice over time – a factor that can directly influence the tendency for mechanical failure in the ground, ultimately driving volcanic activity.
Figure 23: Pre-eruption ground deformation around Kilauea Volcano (red triangle) in Hawaii. The map was derived from Copernicus Sentinel-1 data between December 2014 and June 2017 (image credit: J. Farquharson/F. Amelung)
- This is not an entirely new theory, but it was previously thought that this could only happen at shallow depths. Here, the scientists conclude that the rain increased pore pressure deep down – at depths of up to 3 km.
- The team’s results highlight that fluid pressure was at its highest in almost half a century immediately prior to the eruption, which they propose facilitated magma movement beneath the volcano. Their hypothesis also explains why there was relatively little widespread uplift around the volcano in the months prior.
- “We would normally see the ground inflate, or ‘uplift’ before an eruption as the magma chamber swells. We used radar information from the Copernicus Sentinel-1 mission to see that the amount of inflation was low.
- “This lack of substantial inflation suggests that the intrusion–eruption could not only have been triggered by an influx of fresh magma from depth, but that it was caused by a weakening of the rift zone. The six-day repeat observations from the Sentinel-1 mission were key to our research.
- “A fact that must be considered when assessing volcanic hazards is that increasing extreme weather patterns associated with ongoing anthropogenic climate change could also increase the potential for rainfall-triggered volcanic activity.”
- The Copernicus Sentinel-1 mission is a constellation of two identical radar satellites offering the capability to monitor ground deformation with the technique of interferometry. The constellation provides the capability to image part of the globe in the same geometry every six days – a repeat that is ensured for the Group on Earth Observation’s Geohazard Supersites, to which Hawaii islands belong.
Figure 24: Sentinel-1 delivers radar imagery for numerous applications. Radar images are the best way of tracking land subsidence and structural damage. Systematic observations mean that ground movement barely noticeable in everyday life can be detected and closely monitored. As well as being a valuable resource for urban planners, this kind of information is essential for monitoring shifts from earthquakes, landslides and volcanic uplift. Moreover, Sentinel-1 is designed specifically to provide images for rapid response to disasters such as floods and earthquakes. — Images acquired over the ocean are essential for generating timely maps of sea ice for safe passage as well as for detecting and tracking oil spills. The mission also offers key information on wind and waves in the open sea for shipping and wave-energy applications (video credit: ESA/ATG medialab)
• March 27, 2020: The Copernicus Sentinel-1 mission takes us over part of the Mekong Delta – a major rice-producing region in southwest Vietnam. 59)
- In Vietnam, rice has been a strategic crop for national food security. Vietnam is the fifth largest producer of rice in the world, the majority of which is grown in the Mekong Delta – a vast flood plain and one of Asia’s most fertile agricultural zones.
- Such an enormous amount of rice is produced in the Mekong Delta that it is often referred to as Vietnam's 'rice bowl'. The rice grown here produces enough to make Vietnam the world's third biggest rice exporter – after India and Thailand.
- Bodies of water reflect the radar signal away from the satellite, making water appear dark. This can be seen in the Bassac River, also known as the Hau river, in the right of the image. Ships in the river can be seen as bright, multi-colored dots.
- The combination of radar images from the Copernicus Sentinel-1 mission can help monitor and map the evolution of rice cultivation. Radar sensors are particularly useful owing to their ability to detect waterlogged ground and penetrate the humid cloud coverage typical of Asian rice-cultivating regions.
Figure 25: This multi-temporal image combines three radar acquisitions from the Copernicus Sentinel-1 mission taken around one month apart to show changes in crop and land conditions over time. The bright colors in the image come from changes on the ground that have occurred between acquisitions. The first image, from 28 October 2019, picks out changes in pink and red, the second from 21 November shows changes in green, and the third image, from 27 December, shows changes in blue. As seen in the image, the majority of growth in the rice fields is visible in December. The grey areas represent either built-up areas or patches of land that saw no changes during this time. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• February 21, 2020: In this week's edition of the Earth from Space program, the Copernicus Sentinel-1 mission takes us over Houston, the most populous city in Texas and the fourth largest in the US. 60)
- The two identical Copernicus Sentinel-1 satellites carry radar instruments to image Earth’s surface. Images acquired with radar are interpreted by studying the intensity of the signal scattered back to the satellite – which is related to the roughness of the ground.
- The colors of this week’s image come from the combination of two polarizations from the Copernicus Sentinel-1 radar mission, which have been converted into a single RGB image (Figure 26). Interpreting polarization can help scientists analyze Earth’s surface.
- With a population of over two million and covering an area of over 1600 km2, Houston is the state’s most populous city and the fourth largest city in the US.
- Houston is dissected by a series of bayous passing. Buffalo Bayou can be seen cutting through Houston, before joining Galveston Bay visible at the bottom of the image. Galveston Bay is around 55 km long and around 30 km wide, making it the largest estuary in Texas. The shallow bay has an average depth of around 2 m, which is unusually shallow for its size.
- The Port of Houston, which spreads across the northwest section of the bay, is one of the world’s largest ports, and many ships can be seen as multi colored dots in the bottom-right of the image.
- Houston is home to the NASA Lyndon B. Johnson Space Center (JSC), which lies west of Galveston Bay. The center acts as NASA’s lead center for astronaut training as well as the International Space Station mission operations. It was identified as mission control or simply ‘Houston’ during the Apollo, Gemini and Space Shuttle flights.
- The center also collaborates with other international facilities in a variety of scientific and engineering programs related to human space flight and planetary exploration. The Johnson Space Center is where many ESA astronauts are sent as part of their training and preparation for future space missions. This is where Luca Parmitano, who recently returned to Earth, trained for his Beyond mission to the International Space Station.
Figure 26: In this composite image, captured on 21 June 2019, the city of Houston appears in shades of white and grey which contrasts with the yellow tones of the surrounding land and the dark blue waters of the Gulf of Mexico. This image is also available in the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• February 11, 2020: As anticipated, Pine Island Glacier, known as PIG for short, in Antarctica has just spawned a huge iceberg. At over 300 sq km, about the size of Malta, this huge berg very quickly broke into many ‘piglet’ pieces the largest of which is dubbed B-49. 61)
Figure 27: Thanks to images the Copernicus Sentinel satellite missions, two large rifts in the glacier were spotted last year and scientists have been keeping a close eye on how quickly these cracks were growing. This animation uses 57 radar images captured by the Copernicus Sentinel-1 mission between February 2019 and February 2020 (the last frame is from today, 11 February 2020) and shows just how quickly the emerging cracks grew and led to this calving event (video credit: ESA, the video contains modified Copernicus Sentinel data (2019-20), processed by ESA, CC BY-SA 3.0 IGO)
- Pine Island Glacier, along with its neighbor Thwaites Glacier, connect the center of the West Antarctic Ice Sheet with the ocean – together discharging significant quantities of ice into the ocean. These two glaciers have been losing ice over the last 25 years. Owing to their extremely remote location, satellites play a critical role in measuring and monitoring Antarctic glaciology – revealing the timing and pace of glacial retreat in Antarctica. Since the early 1990s, the Pine Island Glacier’s ice velocity has increased dramatically to values which exceed 10 m a day. Its floating ice front, which has an average thickness of approximately 500 meters, has experienced a series of calving events over the past 30 years, some of which have abruptly changed the shape and position of the ice front.
- These changes have been mapped by ESA-built satellites since the 1990s, with calving events occurring in 1992, 1995, 2001, 2007, 2011, 2013, 2015, 2017, 2018, and now 2020.
- Mark Drinkwater, senior scientist and cryosphere specialist remarked, “The Copernicus twin Sentinel-1 all-weather satellites have established a porthole through which the public can watch events like this unfold in remote regions around the world. What is unsettling is that the daily data stream reveals the dramatic pace at which climate is redefining the face of Antarctica.”
• February 7, 2020: The Copernicus Sentinel-1 mission takes us over part of the Dutch province of Flevoland – the newest province in the Netherlands and one of the largest land reclamation projects in the world. 62)
- With almost a third of the country lying below sea level, the Netherlands is famously known as a ‘low country,’ and has a long history of land reclamation.
- One reclamation project resulted in Flevoland. After a major flood it was decided to tame the Zuiderzee, a large, shallow inlet of the North Sea, to improve flood protection and also create additional land for agricultural use.
- The project entailed the creation of land known as polders. The ‘Noordoostpolder,’ or the Northeast polder is the focus of this image. Over the years, the region has developed to become a home to a modern and innovative agricultural industry. The province produces predominantly apples, cereals, potatoes and flowers – with each colorful patch in the image representing a different crop.
- Along the dikes of the Ijsselmeer, west of the Noordoostpolder, lies one of the largest wind farms in the Netherlands. The strong, almost star-shaped, reflections that can be seen near the shore are around 86 wind turbines. The wind farm is said to generate approximately 1.4 billion kWh of clean renewable energy per year – comparable to the power consumption of over 400,000 households.
- Images acquired with radar are interpreted by studying the intensity of the backscatter radar signal, which is related to the roughness of the ground. Cities and towns are visible in white owing to the stronger reflection of the signal. Emmeloord can be seen in the center of the Noordoostpolder, as well as several farms that appear as bright white dots along the roads.
Figure 28: This image combines three radar acquisitions from the Copernicus Sentinel-1 mission taken about two months apart to show change in crop and land conditions over time. The first image from 8 May 2018 is associated with red, the second from 7 July depicts changes in green, and the third from 5 September has been linked to blue. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA)
• January 15, 2020: Since a magnitude-6.4 earthquake struck Puerto Rico last week, aftershocks near its southwestern coast have been relentless. The frequency and intensity of the aftershocks continue to cause damage on this already-vulnerable part of the island. 63)
- NASA scientists are helping local and federal agencies assess the extent of that damage. Using synthetic aperture radar data from the Copernicus Sentinel-1 satellites operated by the European Space Agency (ESA), the Advanced Rapid Imaging and Analysis (ARIA) team at NASA's Jet Propulsion Laboratory and Caltech in Pasadena, California, created a new damage map that includes the southwestern coast near the main quake's epicenter.
- The ARIA team compared post-quake satellite data acquired on 14 January with data as far back as September 2019 to produce the map. The color variation from yellow to dark red indicates increasingly significant damage. Their analysis shows that Guanica, west of the city of Ponce, was particularly hard hit.
- The NASA Earth Applied Sciences Disasters Program has activated a Tier 1 response in support of this disaster and has been in contact with the Federal Emergency Management Agency, the United States Geological Survey, Health and Human Services and other agencies to provide NASA Earth-observing data in support of response and recovery efforts. A webpage has also been created on the NASA Disasters Mapping Portal to supply relevant geographic information system (GIS) data products.
Figure 29: The map covers an area of 107 by 47 miles (172 x 76 km), shown with the large red polygon, with each pixel measuring about 30 m across. The data is most sensitive to building damage rather than small scale changes or partial structural damage. It is also less reliable over heavy vegetation. Even with these limitations, the map can still serve an important role in identifying the areas that may need help the most (image credit: NASA/JPL-Caltech, ESA, The map contains modified Copernicus Sentinel data processed by ESA and analyzed by ARIA team scientists at NASA JPL and Caltech)
• January 10, 2020: Scientists use satellite data to help response agencies identify damaged areas. Days after a 6.4-magnitude earthquake rocked Puerto Rico, followed by hundreds of aftershocks, the full extent of damage is only beginning to be realized. 64)
- NASA scientists are using satellite data to help federal and local agencies identify areas with potential damage. Earthquakes cause permanent changes to the ground surface. By comparing interferometric synthetic aperture radar (InSAR) data acquired on Jan. 9, 2020, with data acquired on Dec. 28, 2019, from the Copernicus Sentinel-1A satellite, the scientists were able to map where, how much and in what direction those changes occurred.
- Managed by the European Space Agency (ESA), the Copernicus Sentinel-1A satellite was able to see the eastern two-thirds of the island during the January 9 flyover. On the map (Figure 30), red indicates areas where the ground was changed, or displaced, with darker shades corresponding to more significant displacement. The scientists found that the greatest displacement from the flyover area occurred west of the city of Ponce (identified by the green star), not far from the quake's offshore epicenter. They recorded up to 14 cm of ground change there. The ground appeared to shift downward and slightly to the west.
- The quake epicenter and the cluster of quakes and aftershocks in the region identified by the United States Geological Survey (shown as orange circles) fall just west of the satellite's Jan. 9 track. Because of this, scientists also plan to analyze data from Sentinel-1A's forthcoming Jan. 14 flyover, which will include western Puerto Rico.
- The NASA Earth Applied Sciences Disasters Program has activated Tier 1 response in support of this disaster and is in contact with the Federal Emergency Management Agency, the United States Geological Survey and Health and Human Services (HHS) to provide NASA Earth-observing data in support of response and recovery efforts. Products in the process of being produced include Suomi-NPP-based "Black Marble" power outage maps, damage maps, and landslide maps. A webpage has also been created on the NASA Disasters Mapping Portal to supply relevant GIS data products.
- The map contains modified Copernicus Sentinel data processed by ESA and analyzed by scientists at NASA's Jet Propulsion Laboratory, and earthquake location data from the USGS.
Figure 30: This map shows ground changes, or displacement, on the eastern two-thirds of Puerto Rico following a 6.4-magnitude earthquake. The ground shifted up to 5.5 inches (14 cm) in a downward and slightly west direction (image credit: NASA/JPL-Caltech, ESA, USGS)
Copernicus: Sentinel-1 continued
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. 65) 66) 67) 68) 69) 70) 71) 72) 73) 74) 75) 76) 77) 78)
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 31: 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 32: Overview of the Sentinel-1 C-SAR instrument observation scheme and operational support modes (image credit: ESA)
Table 8: Key parameters of the C-SAR instrument
Table 9: 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 10).
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). 79) 80) 81)
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. 82) 83) 84) 85) 86) 87)
TOPS is employing a rotation of the antenna in the azimuth direction as is shown in Figure 33. 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. 88)
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. 96).
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 34: 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. 88).
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. 89) 90)
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 35: 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. 91) 92) 93)
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: 94)
- 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 36: 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 37: 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 39). 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. 95)
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 38: 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 40: 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. 95).
Roll steering mode (Ref: 70): 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 41: 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. 96) 97)
Figure 42 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.
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. 98) 99)
The SAS consists of 14 identical tiles (12.3 m x 0.84 m) in 5 deployable panels as shown in Figure 43. 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.
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.
Legend to Figure 44: 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.
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 46 and 47.
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 45). 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.
Table 10: Performance parameters of the C-SAR instrument in the various operational modes 100)
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. 101) 102) 103) 104) 105) 106)
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. 107)
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 48: In-orbit calibration plan for Sentinel-1 versus 12 days repeat cycles (image credit: (image credit: DLR, Ref. 102)
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 49: 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. 108)
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 50 and the novel transponders as depicted in Figure 51, 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.
• 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 52 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 52: 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 52, 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. 109)
Figure 53: 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. 110)
The EC has also defined an overall Copernicus data policy, declaring the Sentinel mission data free and open. 111) 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 11. 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.
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. 110). 112)
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 54 displays the structure of the PDGS and the outcome of the selection of the providers.
Figure 54: Overall structure of Copernicus Payload Data Ground Segment for Sentinels-1 to -3 (Status May 2013), image credit: ESA (Ref. 110)
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 55: 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. 110).
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. 112): 113)
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 56: Simplified view of the Sentinel-1 Ground Segment (image credit: Astrium SAS)
Figure 57: 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. 112):
• 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 58: Illustration of Sentinel-1 ground segment configuration (image credit: ESA) 114)
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. 115)
Figure 59 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.
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 12: Summary of timeliness and accuracy requirements 116)
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. 115).
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 60 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. 116).
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 61 and 62) 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.
In the case of Sentinel-2A, the accuracy results are similar to those of Sentinel-1A. Figure 63 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.
Figure 63: Sentinel-2A orbit comparisons (3D RMS; cm) between CPOD and external solutions (image credit: GMV, Ref. 116)
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. 117)
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 64: 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. 118)
• 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. 124)
• 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 13: Sentinel-1 Operational products access policy
Table 14: Planned operational ESA Sentinel-1 products - L1 characteristics 125)
• 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. 126)
Figure 65: 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 66: 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. 127) 128)
Figure 67: 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. 129) 130) 131)
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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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).