Copernicus: Sentinel-1 — The SAR Imaging Constellation for Land and Ocean Services
Sentinel-1 is the European Radar Observatory, representing the first new space component of the GMES (Global Monitoring for Environment and Security) satellite family, designed and developed by ESA and funded by the EC (European Commission). The Copernicus missions (Sentinel-1, -2, and -3) represent the EU contribution to GEOSS (Global Earth Observation System of Systems). Sentinel-1 is composed of a constellation of two satellites, Sentinel-1A and Sentinel-1B, sharing the same orbital plane with a 180° orbital phasing difference. The mission provides an independent operational capability for continuous radar mapping of the Earth with enhanced revisit frequency, coverage, timeliness and reliability for operational services and applications requiring long time series.
Table 1: Copernicus is the new name of the former GMES program 1)
The overall objective of the Sentinel-1 mission is to provide continuity of C-band SAR operational applications and services in Europe. Special emphasis is placed on services identified in ESA's GSE (GMES Service Element) program. Additional inputs come from on-going GMES projects funded by ESA, the EU, and ESA/EU member states. The Sentinel-1 mission is expected to enable the development of new applications and meet the evolving needs of GMES, such as in the area of climate change and associated monitoring. 2) 3) 4)
The Sentinel-1 mission represents a completely new approach to SAR mission design by ESA in direct response to the operational needs for SAR data expressed under the EU-ESA GMES (Global Monitoring for Environment and Security) program. The mission ensures continuity of C-band SAR data to applications and builds on ESA's heritage and experience with the ERS and Envisat SAR instruments, notably in maintaining key instrument characteristics such as stability and accurate well-calibrated data products.
The key mission parameters are revisit time, coverage, timeliness combined with frequency band, polarization, resolution and other image quality parameters. Short revisit time demands for an appropriate orbit selection and large swath widths.
The baseline mission concept under development is a two-satellite constellation, with four nominal operational modes on each spacecraft designed for maximum compliance with user requirements. 5) 6) 7) 8) 9) 10)
• Orbit: Sun-synchronous near-polar orbit, repeat cycle of 12 days, cycle length of 175 days
• Operational modes:
- Stripmap mode (SM): 80 km swath, 5 m x 5 m resolution, single-look
- Interferometric Wide Swath mode (IWS): 240 km swath, 5 m x 20 m resolution, single-look
- Extra Wide Swath mode (EWS): 400 km swath, single-look
- Interferometric Wide Swath mode (IWS): 240 km swath, 25 m x 80 m resolution, 3-looks
- Wave mode (WM): 20 km x 20 km, 20 m x 5 m resolution, single-look
• Polarization: Dual polarization for all modes VV+VH or HH+HV
- Consistent, reliable and conflict free mission operations
- Near real-time delivery of data within 3 hours (worst case) with 1 hour as goal
- Data delivery from archive within 24 hours
• Sensitivity: NESZ (Noise Equivalent Sigma Zero), σo = -25 dB
- Stability = 0.5 dB
- Accuracy = 1.0 dB
• Ambiguity ratio: DTAR (Distributed Target Ambiguity Ratio) = -25 dB
In April 2007, ESA selected TAS-I (Thales Alenia Space Italia) as prime contractor for the Sentinel-1 spacecraft (overall satellite design & integration at system and subsystem level, including the design of the SAR antenna's transmit/receive modules). ESA awarded the contract to TAS-I on June 18, 2007 at the Paris International Air Show. EADS Astrium GmbH of Friedrichshafen, was in turn awarded a contract by TAS-I to build the radar imaging payload for Sentinel-1, including the central radar electronics subsystem developed by Astrium UK. The objective of Sentinel-1 is to assure C-band SAR data continuity for the user community currently provided by Envisat and ERS-2. 11)
Three priorities (fast-track services) for the mission have been identified by user consultation working groups of the European Union: Marine Core Services, Land Monitoring and Emergency Services. These cover applications such as: 12)
• Monitoring sea ice zones and the arctic environment
• Surveillance of marine environment
• Monitoring land surface motion risks
• Mapping of land surfaces: forest, water and soil, agriculture
• Mapping in support of humanitarian aid in crisis situations.
Unlike its more experimental predecessors ERS-1, ERS-2 and Envisat that supply data on a best effort basis, operational satellites like Sentinel-1 are required to satisfy user requirements and to supply information in a reliable fashion with the data provider accepting legal responsibility for the delivery of information.
In March 2010, ESA and TAS-I signed a contract to build the second Sentinel-1 (Sentinel-1B) and Sentinel-3 (Sentinel-3B) satellites, marking another significant step in the Copernicus program. 13)
As part of the Copernicus space component, the Sentinel-1 (S1) mission is implemented through a constellation of two satellites (A and B units) each carrying an imaging C-band SAR instrument (5.405 GHz) providing data continuity of ERS and Envisat SAR types of mission. Each Sentinel-1 satellite is designed for an operations lifetime of 7 years with consumables for 12 years. The S-1 satellites will fly in a near polar, sun-synchronized (dawn-dusk) orbit at 693 km altitude. 14)
The Sentinel-1 mission, including both S-1A and S-1B satellites, is specifically designed to acquire systematically and provide routinely data and information products to Copernicus Ocean, Land and Emergency as well as to national user services. These services focus on operational applications such as the observation of the marine environment, including oil spill detection and Arctic/Antarctic sea-ice monitoring, the surveillance of maritime transport zones (e.g. European and North Atlantic zones), as well as the mapping of land surfaces including vegetation cover (e.g. forest), and mapping in support of crisis situations such as natural disasters (e.g. flooding and earthquakes) and humanitarian aid.
In addition, the 12-day repeat orbit cycle of each Sentinel-1 satellite along with small orbital baselines will enable SAR interferometry (InSAR) coherent change detection applications such as the monitoring of surface deformations (e.g. subsidence due to permafrost melt) and cryosphere dynamics (e.g. glacier flow).
Figure 1: Artist's view of the deployed Sentinel-1 spacecraft (image credit: ESA, TAS-I)
The spacecraft is based on the PRIMA (Piattaforma Italiana Multi Applicativa) bus of TAS-I, of COSMO-SkyMed and RADARSAT-2 heritage, with a mission-specific payload module. Attitude stabilization: 3-axis, attitude accuracy = 0.01º (each axis), orbital knowledge = 10 m (each axis, 3σ using GPS).
The spacecraft structure provides the accommodation for all platform and payload units. A box type structure has been adopted using external aluminum sandwich material, with a central structure in CFRP (Carbon Fiber Reinforced Plastic). A modular approach has been taken whereby the payload is mounted to a dedicated part of the structure, allowing separate integration & test of the payload before integration to the main part of the structure carrying the platform units. This has many advantages for the overall AIT (Assembly, Integration and Test) process. 15) 16) 17) 18) 19) 20)
The PRIMA platform comprises three main modules, which are structurally and functionally decoupled to allow for a parallel module integration and testing up to the satellite final integration. The modules are: 21)
1) SVM (Service Module), carrying all the bus units apart from the propulsion ones
2) PPM (Propulsion Module), carrying all the propulsion items connected by tubing and connectors
3) PLM (Payload Module), carrying all the payload equipment including the SAR Instrument antenna.
Figure 2: 3D exploded view of the Sentinel-1 platform (image credit: TAS-I)
TCS (Thermal Control Subsystem): The TCS provides control of the thermal characteristics and environment of the Satellite units throughout all phases of the mission. In general the TCS is passive, with the control provided by means of standard techniques such as heat pipes, radiators and MLI (Multi-Layered Insulation). Survival heaters are provided to prevent units becoming too cold during non-operative phases.
EPS (Electric Power Subsystem): The EPS uses two solar array wings for power generation. Each wing consists of 5 sandwich panels using GaAs triple junction solar cells. The average onboard power is 4.8 kW (EOL), the Li-ion battery has a capacity of 324 Ah. The PCDU (Power Control and Distribution Unit) is designed to provide adequate grounding, bonding & protection for the overall electrical system (e.g. by use of fuses) and must also be integrated into the satellite FDIR concept to ensure that adequate power resources and management are available in the event of on-board failures. Li-Ion battery technology has been selected for the batteries in view of the large benefits offered in terms of mass and energy efficiency.
The spacecraft dimensions in stowed configuration are: 3.4m x 1.3 m x 1.3 m. The Sentinel-1 spacecraft has a launch mass of ~2,200 kg, the design life is 7.25 years (consumables for up to 12 years). 22) 23)
Since the B2 Phase of Sentinel-1, a commonality approach with Sentinel-2 and Sentinel-3 was introduced and deeply investigated, to optimize and minimize as much as possible new developments, HW procurement and operations costs. Besides the differences among payload instruments and their relative required performances, each of these three satellites have its own orbital parameters, as well its own specific requirements. 24) 25)
Table 2: List of some Sentinel-1, -2, -3 characteristics and key requirements impacting on end-to-end performance 26)
The Sentinel-1 spacecraft design is characterized by a single C-band SAR (Synthetic Aperture Radar) instrument with selectable dual polarization, a deployable solar array, large on-board science data storage, a very high X-band downlink rate, and stringent requirements on attitude accuracy and data-take timing. In addition, the spacecraft will embark the LCT (Laser Communication Terminal) unit allowing downlink of recorded data via the EDRS (European Data Relay Satellite). 27) 28)
Table 3: Main parameters of the Sentinel-1 spacecraft
AVS (Avionics Subsystem): The AVS performs both Data Handling & Attitude/Orbit Control functions. This is realized through the concept of an integrated control system that performs the control of the platform and payload. The AVS performs all data management & storage functions for the satellite, including TM/TC reception and generation, subsystem & unit monitoring, autonomous switching actions and synchronization. The AVS includes the AOCS processing and the interfaces to the AOCS sensors Star trackers, fine sun sensors, and fine gyroscope and actuators, 4 reaction wheels, 3 torque rods, 14 thrusters, 2 solar array drive mechanism. 29)
The AOCS comprises all means to perform transfer- and on-orbit control maneuvers and to control all necessary satellite attitude and antenna pointing states during all mission phases, starting at separation from the launcher until de-orbiting of the satellite at end of life. This includes the attitude steering of the LEO satellite to provide both yaw and roll steering capability. At present, a dedicated precise orbit predictor is implemented within the AOCS, in addition to making use of the data uploaded to the payload by the GPS constellation. The AOCS (Attitude and Orbit Control Subsystem) is able to perform some functions autonomously and it is supported by a very reliable FDIR scheme (Ref. 15). Telecommand data will be received from the TT&C subsystem and will be decoded and deformatted in the AVS.
AOCS consists of the following sensors and actuators: fine sun sensors, magnetometers, gyroscopes, star trackers, GPS receivers, magnetic torquers, a reaction wheels assembly and a monopropellant (hydrazine) propulsion system. The propulsion system has 3 pairs of 1 N orbit control thrusters and 4 pairs of reaction control thrusters for attitude correction. Every pair is made up of a prime and a redundant component. The attitude control thrusters are fired when the spacecraft enters RDM (Rate Damping Mode) after separation, damping any residual rotation left by the launcher upper stage and achieving a spacecraft pitch rotation of -8 times the orbital period. In the subsequent AOCS mode, called SHM (Safe Hold Mode), magnetotorquers and reaction wheels maintain the attitude and reduce the pitch rotation rate to twice the orbital period.
The periodic behavior of the Earth’s magnetic field in a polar orbit and the polarization of the angular momentum with the loading of the reaction wheels allow the magnetotorquers to maintain this pitch rate while aligning the spacecraft –Y axis with the orbit normal, which in a dusk-dawn orbit coincides with the direction to the Sun (Figure 3). When the appendages deployment commences, the effect of the gravity gradient torque dominates over the magnetic torque, resulting in the alignment of the S/C X axis (appendages axis) with the nadir direction, maintaining thus a pitch rate equal to the orbital period. Upon ground telecommand, a transition into the NPM (Normal Pointing Mode) occurs, where the spacecraft performs a fine attitude control based on the use of reaction wheels in close loop with star trackers, gyroscopes and GPS, and magnetotorquers for wheel unloading (Ref. 20).
Figure 3: Sentinel-1A stowed representation (in RDM and SHM). +X S/C axis points towards the flight direction. S/C Y axis is aligned with the Sun direction. Solar Array –Y illuminated when stowed (image credit: ESA, Ref. 20)
Figure 4: Architecture of the avionics subsystem (image credit: TAS-I)
Table 4: Sentinel-1 attitude steering modes (Ref. 80)
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. 102).
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)
Development status of Sentinel-1C
• August 7, 2020: In two years’ time, the next Copernicus Sentinel-1 satellite will be launched to join its two siblings in orbit around Earth. With engineers busy building Sentinel-1C, they have recently tested the mechanism that opens its 12 m-long radar antenna. 55)
- Copernicus Sentinel-1C is the third Sentinel-1 satellite, following Sentinel-1A and Sentinel-1B, which were launched in April 2014 and April 2016, respectively. The three satellites are identical, each carrying an advanced radar instrument to provide an all-weather, day-and-night supply of imagery of Earth’s surface. The mission has been used to monitor the movement of icebergs, ice sheets and glaciers, ground deformation because of subsidence and earthquakes, floods after severe storms, and much more.
- Sentinel-1C is set to ensure the continuity of critical radar images that so many Copernicus environmental services and scientists now rely on.
- The mission’s technical success is thanks to its radar instrument – which when open spans a whopping 12 m. Because the radar is folded to fit into the rocket fairing for liftoff, the deployment mechanism must be thoroughly tested to ensure that all will be well once it is in space.
- This important milestone test has recently been passed at Airbus’ facilities in Germany.
- To simulate this operation in as near realistic environment as is possible on Earth, engineers hang the radar from a structure that helps to mimic weightlessness. The deployment test not only enables the hardware needed for the deployment to be tested, but also allows for the antenna planarity and flatness to be measured when fully deployed.
- Following these deployment test and planarity checks, the instrument will now undergo radio frequency measurements to measure its radiation patterns and radiometric performance.
- A second and last deployment test will be carried out in France, once the radar instrument has been connected to the satellite platform.
- “While, a lot of attention is, quite rightly, devoted to the further expansion of the measurement capabilities of the Copernicus system, we are also focused on ensuring the long-term availability of data produced by the current suite Sentinels to which Europe is fully committed,” says Guido Levrini, ESA’s Copernicus Space Segment Program Manager.
- “This impressive milestone involving the deployment of the huge Sentinel-1C radar antenna - a technological jewel – has, remarkably, been achieved amid the COVID pandemic.”
- Copernicus is the biggest provider of Earth observation data in the world – and while the EU is at the helm of this environmental monitoring program, ESA develops, builds and launches the dedicated satellites. It also operates some of the missions and ensures the availability of data from third party missions.
Figure 17: Copernicus Sentinel-1C is the third Sentinel-1 satellite. The three satellites are identical, each carrying an advanced radar instrument to provide an all-weather, day-and-night supply of imagery of Earth’s surface. When deployed in space, the radar measures a whopping 12 meters. Because the radar is folded to fit into the rocket fairing for liftoff, the deployment mechanism must be thoroughly tested to ensure that all will be well once it is in space. To simulate this operation in as near realistic environment as is possible on Earth, the radar is hung from a structure that helps to mimic weightlessness. The deployment test not only enables the hardware needed for the deployment to be tested, but also allows for the antenna planarity and flatness to be measured when fully deployed. The tests were carried out at Airbus in Germany (image credit: Airbus)
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 some of its imagery for the period 2020
• September 11, 2020: This image, captured by the Copernicus Sentinel-1 mission, shows the Amazon River meandering through one of the most vital ecosystems in the world – the Amazon rainforest in South America. 56)
- The Amazon is considered the widest river in the world with a width of between 1.6 and 10 km, but expands during the wet season to around 50 km. With more than 1000 tributaries, the Amazon River is the largest drainage system in the world in terms of the volume of its flow and the area of its basin. As a consequence of its ever-changing flow, older riverbeds can be seen as thin lines around the main river at the top of the image.
- One of its tributaries, the Javari River, or Yavari River, is visible as a thinner blue line weaving through the tropical rainforest. The river flows for 870 km, forming the border between Brazil and Peru, before joining the Amazon River.
- In the image, cities and built-up areas are visible in cyan, for example the cities of Tabatinga and Leticia with two airports are easily identifiable in the far-right. The yellow and orange colors in the image show the surrounding Amazon forest.
- 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 image.
- As radar images provide data in a different way than a normal optical camera, the images are usually black and white when they are received. By using a technology that aligns the radar beams sent and received by the instrument in one orientation – either vertically or horizontally – the resulting data can be processed in a way that produces colored images such as the one featured here. This technique allows for a better distinction of features on the ground.
Figure 18: This Sentinel-1 image has been processed in a way that shows water bodies, such as the Amazon River, in blue. The Amazon river begins its journey in the Andes and makes its way east through six South American countries before emptying into the Atlantic Ocean on the northeast coast of Brazil. The river has a length of around 6400 km – the equivalent of the distance from New York City to Rome. This image, acquired on 3 March 2019, 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)
• August 11, 2020: On August 4, 2020, a devastating explosion rocked the port area around Beirut, Lebanon. After the event, scientists used satellite radar imagery to map the extent of the damage and help identify areas where people may need assistance. 57)
- According to the Associated Press, a fire near the port ignited a large nearby store of ammonium nitrate, a highly explosive chemical often used in fertilizer. At least 135 people died, about 5,000 were wounded, and at least 300,000 people were left homeless. Losses from the blast are estimated to be at least $10 to $15 billion, according to news reports.
- The team at ARIA (NASA Jet Propulsion Laboratory) and EOS examined synthetic aperture radar (SAR) data collected before and after the explosion, mapping changes in the land surface and built structures. SAR instruments send pulses of microwaves toward Earth’s surface and listen for the reflections of those waves. The radar waves can penetrate cloud cover, vegetation, and the dark of night to detect changes that might not show up in visible light imagery. When Earth’ crust moves due to an earthquake, when dry land is suddenly covered by flood water, or when buildings have been damaged or toppled, the amplitude and phase of radar wave reflections changes in those areas and indicates to the satellite that something on the ground has changed.
- Though other U.S. and international agencies play more immediate roles in response to disasters, NASA plays a role in providing observations and analysis. Over the past decade, NASA has actively built its capacity to share Earth observations that can improve the prediction of, preparation for, response to, and recovery from natural and technological disasters. For instance, NASA often responds to calls for data and imagery from the International Charter for Space and Major Disasters.
- “We look at areas of likely exposed populations and fragile infrastructure, as well as areas subject to social stresses and crises. We model and map risks, while also tracking emissions, debris, infrastructure damage, and other effects from things like volcanic eruptions, fires, industrial accidents, earthquakes, and floods,” said David Green, head of NASA’s Disasters Applications team. “Our observations and analyses can help our partners increase their situational awareness of systemic risks and of real-time events, leading to better-informed decisions and early action.”
Figure 19: This image is a damage proxy map created by scientists affiliated with NASA’s Advanced Rapid Imaging and Analysis (ARIA) team and the Earth Observatory of Singapore (EOS). Dark red pixels represent the most severe damage, while orange and yellow areas are moderately or partially damaged. Each colored pixel represents an area of 30 m2 (about the size of a baseball diamond), [image credit: NASA Earth Observatory image by Joshua Stevens, using modified Copernicus Sentinel data (2020) processed by ESA and analyzed by Earth Observatory of Singapore (EOS) in collaboration with NASA-JPL and Caltech, Landsat data from the U.S. Geological Survey, and data from OpenStreetMap. Story by Esprit Smith, NASA's Earth Science News Team, and Michael Carlowicz]
• August 8, 2020: NASA's Advanced Rapid Imaging and Analysis (ARIA) team, in collaboration with the Earth Observatory of Singapore, used satellite-derived synthetic aperture radar data to map the likely extent of damage from a massive August 4 explosion in Beirut. Synthetic aperture radar data from space shows ground surface changes from before and after a major event like an earthquake. In this case, it is being used to show the devastating result of an explosion. 58)
- Maps like the one of Figure 20 can help identify badly damaged areas where people may need assistance. The explosion occurred near the city's port. It claimed more than 150 lives and is estimated to have caused billions of dollars' worth of damage.
Figure 20: The map contains modified Copernicus Sentinel-1 data processed by ESA (European Space Agency) and analyzed by ARIA team scientists at NASA's Jet Propulsion Laboratory, Caltech, and Earth Observatory of Singapore. Based in Pasadena, California, Caltech manages JPL for NASA. The ARIA team used satellite data to map the extent of likely damage following a massive explosion in Beirut. Dark red pixels represent the most severe damage. Areas in orange are moderately damaged, and areas in yellow are likely to have sustained somewhat less damage. Each colored pixel represents an area of 30 meters (image credit: NASA/JPL-Caltech/Earth Observatory of Singapore/ESA)
• July 24, 2020: The Falkland Islands are featured in this radar image captured by the Copernicus Sentinel-1 mission. The Falkland Islands lie in the South Atlantic Ocean, around 500 km northeast of the southern tip of South America. The Falklands comprise two main islands, West Falkland and East Falkland, as well as hundreds of other smaller islands and islets, which form a total land area approximately five times the size of Luxembourg. The two main islands are separated by the Falkland Sound, a channel that averages around 20 km in width. 59)
- The Copernicus Sentinel-1 mission provides a continuous sampling of the seas, offering information on wind and waves. This is useful for understanding interactions between waves and currents and to improve efficiency for shipping and wave-energy applications, potentially producing economic benefits.
- The landscape of the Falkland Islands comprises mountain ranges, flat plains, rugged coastline and cliffs. Hills run east-west across the northern parts of the two main islands, with the highest point being Mount Usborne on East Falkland (around 700 m). Two inlets, Berkeley Sound and Port William, visible in the far right of the image, run far into the land and provide anchorage for shipping. The majority of the population of the islands live in Stanley, on East Falkland.
- The islands are covered with grasslands, but not trees, which are widely used as pastureland for sheep and cattle. The islands are also an important habitat and breeding grounds for birds, penguins and seals.
Figure 21: This multi-temporal image combines two radar acquisitions from the Copernicus Sentinel-1 mission taken one month apart to show changes over time. The first image was captured on 29 December 2019, while the second was taken on 22 January 2020. Here, the main changes between acquisitions occurred in the open ocean, with the bright red colors showing wavy waters in December 2019. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2019-20), processed by ESA, CC BY-SA 3.0 IGO)
• July 23, 2020: On the morning of 23 June 2020, a strong earthquake struck the southern state of Oaxaca, Mexico. The 7.4 magnitude earthquake prompted evacuations in the region, triggered a tsunami warning and damaged thousands of houses. Satellite radar data, from the Copernicus Sentinel-1 mission, are being used to analyze the effects of the earthquake on land. 60)
- Mexico is one of the world’s most seismically active regions, sitting on top of three of Earth’s largest tectonic plates – the North American, Cocos and Pacific. Near Mexico’s southern region, the North American plate collides with the Cocos plate, which is forced underground in a subduction zone. This geological process is associated with many of the damaging earthquakes on the Pacific coast of Mexico – including the most recent on 23 June.
- The earthquake reported in the Oaxaca region occurred at 10:29 local time – with its epicenter located around 12 km southwest of Santa María Zapotitlán. Several powerful aftershocks were registered the same day, with five more recorded in the following 24 hours.
- While there is currently no way to predict when earthquakes will occur, radar imagery from satellites allow for the effects of earthquakes to be observed. Since its launch, the Copernicus Sentinel-1 mission has proven a magnificent system to measure the surface deformation caused by tectonics, volcanic eruptions and land subsidence.
- In the maps of Figure 22, data from the Sentinel-1A and Sentinel-1B satellite, acquired shortly before and after the earthquake, have been combined to measure the coseismic surface displacement, or changes on the ground, that occurred between the two acquisition dates. This leads to the colorful interference (or fringe) pattern known as an ‘interferogram’, which enables scientists to quantify the surface displacement.
Figure 22: Interferogram showing the coseismic surface displacement in the area of Oaxaca, Mexico, generated from multiple Sentinel-1 scans – before and after the 23 June earthquake. By combining data from the Copernicus Sentinel-1 mission, acquired before and after the earthquake, changes on the ground that occurred between the two acquisition dates lead to the colorful interference patterns in the images, known as an 'interferogram', enabling scientists to quantify the ground movement (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
- Ramón Torres, Copernicus Sentinel-1 Project Manager, explains, “The interferogram represents surface displacement in the radar line of sight, i.e. half of the radar wavelength. The distance between the interference cycle, from yellow to yellow, corresponds to 28 mm deformation in the radar line of sight. For example, a blue-green-red color cycle represents a relative movement towards the radar, while a red-green-blue color cycle means a deformation away from the radar.”
- “The fringes can be unwrapped to allow the conversion into meters. The result, referred to as the surface displacement map (Figure 23), shows the relative deformation caused by the earthquake.”
- In the Oaxaca images, ground deformation of up to 0.45 m was observed in the coastal city of La Crucecita – where the epicenter was located.
- With its 250 km-wide swath over land surfaces, the Copernicus Sentinel-1 mission gives scientists a broad view of the displacement, allowing them to examine the ground displacement and further develop the scientific knowledge of quakes.
- By benefitting from the availability of both Sentinel-1A and Sentinel-1B imagery, scientists are able to quantify the ground movement in both vertical and east-west directions by combing the radar scans obtained as the satellites flew both south to north and north to south.
- While current radar missions are limited in measuring the east-west component of surface displacement, the proposed Earth Explorer candidate mission, Harmony, will augment the capabilities by adding additional ‘lines of sight’ to the Sentinel-1 mission.
- In areas where the displacement is predominantly in the north-south direction, Harmony will have the ability to systematically and accurately measure an additional dimension of displacement. This will help resolve ambiguities in the underlying geophysical processes that lead to earthquakes, landslides and volcanism.
- Looking to the future, the upcoming six high-priority candidate missions will expand the current capabilities of the Sentinel missions, one of them being the L-band Synthetic Aperture Radar, ROSE-L, mission, which will also augment the current capabilities of Sentinel-1. The mission will allow scientists to further improve the mapping of earthquakes over the next decade.
- Ramón Torres says, “The Sentinel-1 services are very well guaranteed for decades to come. The upcoming Sentinel-1C and Sentinel-1D are in the process of being completed, and the design of the next generation of satellites will begin later this year.”
Figure 23: Surface deformation. Displacement measured in the radar line of sight from the descending (top) and ascending (bottom) passes (image credit: ESA, the maps contain modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• 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. 61)
- 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 24 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 24: 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 25) 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 25: 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 26 from the Copernicus Sentinel-3 mission shows A-68A’s position in February 2020.
Figure 26: 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. 62)
- 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 27: 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. 63)
Figure 28: 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 29: 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. 64)
- 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 30: 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 31: 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. 65)
- 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 32: 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. 66)
- 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 33). 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 33: 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. 67)
Figure 34: 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. 68)
- 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 35: 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. 69)
- 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 36: 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. 70)
- 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 37), 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 37: 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)