Copernicus: Sentinel-1

Quick facts

Overview

Related Resources

Summary

Mission Capabilities

Both Sentinel-1 satellites are identical and have the C-SAR (C-band Synthetic Aperture Radar) onboard. This instrument is composed of the SAR Electronics Subsystem (SES) in charge of radar command and data handling, and the SAR Antenna Subsystem (SAS) in charge of signal radiation and reception. The C-SAR has four observation modes each with a different swath and resolution to suit their purpose: Stripmap (SM), Interferometric Wide swath (IW), Extra Wide swath (EW), and Wave (WV) mode.

The satellites are used for the monitoring of sea and land ice, surveillance of oil spills and ships, monitoring of marine wind and waves, monitoring of land surface motion risks, and the mapping of forest, water, and soil management. SM mode is designed to support ERS (European Remote Sensing) and Envisat missions; IW mode is the default mode over land; EW mode is designed for maritime, ice, and polar zone observation services where wide coverage and short revisit times are demanded; and WV mode is the default mode over the open ocean.

Performance Specifications

SM mode has an 80 km swath and 5 m spatial resolution; IW mode has a 250 km swath and a 5 m x 20 m resolution; EW mode has a 400 km swath and a 25 m x 100 m resolution, and WV mode takes 20 m x 20 m images at 100 km intervals with a resolution of 5 m x 20 m. The C-SAR has a centre frequency of 5.405 GHz.

The two satellites follow the same sun-synchronous orbit with an inclination of 98.2° but are 180° out of phase. The satellites are situated at an altitude of 693 km and have a period of 98.7 minutes.
 

Space and Hardware Components

The PRIMA (Piattaforma Italiana Multi Applicativa) bus consists of the propulsion module (PPM) which carries all propulsion equipment, the service module (SVM) which carries all other necessary equipment to stay operational, and the payload module (PLM) which contains the C-SAR and other secondary payloads. SVM contains a variety of subsystems that control the thermal environment, power generation and storage, orbit path, and payload data handling and transmission (PDHT). The PDHT subsystem has a storage capacity of 1410 Gbit and a 520 Mbit/s X-band downlink capacity.

On 23 December 2021, Sentinel-1B experienced an anomaly related to the instrument electronics power supply provided by the satellite platform, leaving it unable to deliver radar data. As a consequence, ESA and EC announced the end of the Sentinel-1B mission on 3 August 2022, noting that plans are in force to launch Sentinel-1C as soon as possible.

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 ongoing 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 the 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 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

• Operations:

- Consistent, reliable and conflict-free mission operations

- Near real-time delivery of data within 3 hours (worst case) with 1 hour as the goal

- Data delivery from the archive within 24 hours

• Sensitivity: NESZ (Noise Equivalent Sigma Zero), σ^o = -25 dB

• Radiometry:

- 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 reliably 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).

Copernicus: The New Name for the GMES Programme

Copernicus is the new name of the European Commission's Earth Observation Programme, previously known as GMES (Global Monitoring for Environment and Security). The new name was announced on December 11, 2012, by EC (European Commission) Vice-President Antonio Tajani during the Competitiveness Council.

In the words of Antonio Tajani: “By changing the name from GMES to Copernicus, we are paying homage to a great European scientist and observer: Nicolaus Copernicus (1473-1543). As he was the catalyst in the 16^th century to better understand our world, so the European Earth Observation Programme gives us a thorough understanding of our changing planet, enabling concrete actions to improve the quality of life of the citizens. Copernicus has now reached maturity as a programme and all its services will enter soon into the operational phase. Thanks to greater data availability user take-up will increase, thus contributing to that growth that we so dearly need today.” Copernicus is the new name of the former GMES program ¹⁾

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.

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 using 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 because of the large benefits offered in terms of mass and energy efficiency.

The spacecraft dimensions in stowed configuration are: 3.4 m 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 orbital parameters, as well its specific requirements. ^{24) 25)}

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)}

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) can 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).

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 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 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)

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).

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.

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 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.

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 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.

• Before 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⁾

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.

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. 119).

Development status of Sentinel-1C

• April 7, 2022: ESA on behalf of the European Commission, and Arianespace have signed a launch contract for the third radar satellite in the Sentinel-1 mission, Sentinel-1C. ^{55) 56)}

- The launch is scheduled in the first half of 2023 from the Guiana Space Center, Europe’s Spaceport in French Guiana. Sentinel-1C is part of Copernicus, a joint European Union and European Space Agency (ESA) Earth Observation program. The satellite with a mass of 2.3 tons will be placed in a Sun-synchronous orbit with an altitude around 690 km.

- “We are very proud of this new launch contract for the European Commission and the European Space Agency; it underlines our long-standing partnership for the success of Copernicus”, said Stéphane Israël, CEO of Arianespace. “For Arianespace, this contract is a sign of the confidence in the Vega-C system and a strong sign of the commitment of European institutions for an autonomous access to space”.

- Before Sentinel-1C satellite, both Sentinel-1A and -1B were previously launched with Arianespace in 2014 and 2016. Sentinel-1C will round out the initial capacity offered by the two preceding satellites to offer a comprehensive response to the needs for environmental and security monitoring via spaceborne radar systems. Sentinel satellites are part of the Copernicus program designed to give Europe continuous, independent and reliable access to Earth observation data.

- ESA’s new Vega-C launcher, built by Avio (Colleferro, Italy) as prime contractor, has been specifically upgraded to launch satellites of the class of Sentinel-1C and it is perfectly suited to serve the Earth Observation market because of its performance and versatility.

- According to Ref. 56), Sentinel-1B is currently unavailable due to a technical anomaly, so it is important to get Sentinel-1C into orbit and operational as soon as possible.

• August 7, 2020: In two years, 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. ⁵⁷⁾

- 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.

As of 9 February 2022, the Sentinel-1 article has been split into four parts.

• Sentinel-1 mission and its imagery in the period 2020

• Sentinel-1 mission and its imagery in the period 2019

• Sentinel-1 imagery in the period 2018-2014

Mission Status

• July 26, 2022: Any movement beneath our feet – from barely perceptible subsidence to the sudden appearance of a sinkhole or a crashing landslide – spells big trouble. Even relatively modest subsidence can weaken buildings and infrastructure and lead to issues such as flooding, and at worst the abrupt disappearance of sections of land brings immediate threat to life. Monitoring and predicting our shifting land is essential for adopting mitigating strategies. ⁵⁸⁾

- And now, thanks to Europe’s environmental Copernicus programme and the Sentinel-1 radar satellite mission, the first Europe-wide subsidence and soil movement analysis service is available to the public.

- The new European Ground Motion Service, the latest offering of the Copernicus Land Monitoring Service and implemented by the European Environment Agency, provides free and accessible ground motion data to anyone who wishes to use it.

- Based on radar data from Copernicus Sentinel-1 satellite mission, the service provides information on the motion of land, structures and infrastructure in Copernicus Participating States. It aims to give users reliable information on ground motion at a local, regional or national scale.

- Radar images – as provided by the Sentinel-1 mission’s advanced C-band synthetic aperture radar – are the best way of tracking land subsidence and structural damage.

- The ‘radar interferometry’ remote sensing technique combines two or more radar images from the same orbital geometry over the same area to detect changes occurring between acquisitions. Interferometry allows for the monitoring of even slight ground movement – down to a few millimetres – across wide areas.

- Using these millimetre-scale measurements of ground motion, the European Ground Motion Service enables the mapping of subsidence and landslides, as well ground affected by seismic and tectonic activity.

- Subsidence, for instance, is a serious concern, often caused by the removal of groundwater, oil, natural gas or mineral resources. Even modest subsidence can weaken buildings, bridges, railway lines, sewer pipes, dams and sea defences, the list goes on.

- Needless to say that damage to the infrastructure on which we rely can pose danger. Repairing structures is expensive, but so too is adopting mitigating strategies. It is therefore paramount that the authorities, urban planners and construction companies, for example, have accurate information so that they plan and invest as effectively as possible.

- Help is now at hand through the European Ground Motion Service, which provides data on a continental scale, in the form of maps colour-coded by the velocity of ground motion, in terms of millimetres per year. The first baseline product, covering 2015–2020, is made from Sentinel-1 satellite data acquired every six days during that period.

- The measurements provided by the European Ground Motion Service are valuable for many user communities. They are, for example, critical for studying, monitoring, and understanding a variety of deformation phenomena of anthropogenic origin, but also deformation resulting from seismic and tectonic activity.

- The images here are examples of data that can be accessed through the service. They depict ground motion around Mount Etna and Bologna in Italy, as well as an area around Larissa in Greece. While subsidence and uplift around Mount Etna is linked to volcanic activity, the subsidence we see in Bologna and villages near Larissa is associated with the extraction of groundwater.

- In fact, among the most important and valuable applications of this new service is for risk assessments related to geohazards. Risk assessment through ground motion analyses is essential for the development of mitigation strategies and more effective exploitation of resources, as well as for better management and planning of urban areas and infrastructures.

- This new service, which was initiated by the European Union as part of the Copernicus programme, is also closely linked with other aspects of ESA’s Earth Observation Programmes such as the Geohazards Exploitation Platform. This is a research and development processing environment dedicated to terrain motion mapping that has been upgraded to allow for the visualisation of products delivered by the European Ground Motion Service. It offers further interferometric processing with complementary on-demand processing such as the SNAPPING service and the P-SBAS service that use Sentinel-1 time series.

- Accessing the European Ground Motion Service on the Geohazards Exploitation Platform and combining it with complementary processing chains is intended to maximise the impact and benefit of the technology with user communities.

- Beyond this, the availability of the European Ground Motion Service is intended to help increase awareness and acceptance of radar interferometry as a solution in various industry sectors on a global basis where service providers from Europe have commercial Earth observation-based capabilities using both Sentinel-1 synthetic aperture radar and very high-resolution synthetic aperture radar missions.

- The European Ground Motion Service is available to the public, everyone can register on the platform and access the data.

• June 30, 2022: Bangladesh is no stranger to heavy rain, but this year the northeast region is being subjected to the highest rainfall it has seen in more than a century. Days of heavy rain at the beginning of monsoon season have caused widespread flooding, leaving millions stranded and vast areas affected. This flooding has come shortly after a pre-monsoon flood that badly affected the same region a month ago. ⁵⁹⁾

- The country's Flood Forecasting and Warning Center has reported that major rivers in the districts of Sylhet and Sunamganj have swollen to dangerously high levels because of heavy rain. The government has deployed armed forces across the region to speed up flood rescue and relief efforts.

- Space is on hand to help too. The International Charter Space and Major Disasters has been triggered to respond to the disaster. The services take advantage of observations from several satellites and provide on-demand mapping to help civil protection authorities and the international humanitarian community in the face of major emergencies.

• June 10, 2022: The Republic of Singapore is located just off the southern tip of the Malayan Peninsula, between Malaysia and Indonesia, around 135 km north of the equator. It consists of the 710 km² Singapore Island, visible in the top-centre of the image, as well as some 60 small islets.

- Nearly two-thirds of the Singapore Island is less than 15 m above sea level. The highest summit, Timah Hill, has an elevation of only 160 m. Changi Airport, one of the largest transportation hubs in Asia, can be seen at the eastern end of the island.

- Singapore Island is separated from the Peninsular Malaysia to the north by the Johore Strait, a narrow channel crossed by a road and train causeway, while the southern end faces the Singapore Strait, where the Riau-Lingga Archipelago (part of Indonesia) extends.

- Singapore is home to the largest port in Southeast Asia and one of the busiest in the world. The port offers connectivity to more than 600 ports in 123 countries. It owes its growth and prosperity to its position at the southern extremity of the Malay Peninsula, where it dominates the Strait of Malacca, which connects the Indian Ocean to the South China Sea.

- This week’s image contains satellite data stitched together from three separate radar scans, to detect changes occurring between acquisitions. The sea surface reflects the radar signal away from the satellite, making water appear dark in the image and contrasts with metal objects, in this case ships and vessels, which appear as bright, sparkly dots in the dark water.

- In this image, boats from 28 December 2021 appear in red, those from 9 January 2022 appear in green, and those from 21 January 2022 appear in blue. The various colours in the ocean are due to the changing surface currents and sediments from river deltas, while major cities and towns are visible in white owing to the strong reflection of the radar signal.

- The advantage of radar as a remote sensing tool is that it can image Earth’s surface through rain and cloud, and regardless of whether it is day or night. This is particularly useful for monitoring areas prone to long periods of darkness – such as the Arctic – or providing imagery for emergency response during extreme weather conditions.

• May 26, 2022: The effects of our warming climate are seen across a multitude of measures, usually as incremental changes: more frequent extreme weather, heatwaves, droughts and wildfires. The cumulative impact of these changes, however, can cause fundamental parts of the Earth system to change more quickly and drastically. These ‘tipping points’ are thresholds where a tiny change pushes the system into an entirely new state. ⁶⁰⁾

- This week, at ESA’s Living Planet Symposium, scientists came together to discuss the latest research evidence for climate tipping points and identify the opportunities and challenges of using remote sensing data to understand them.

- Tipping points are typically self-propelling, so that, once triggered, they drive deeper change. Examples include strongly increased ice sheet melt, permafrost thaw, ocean circulation changes and forest dieback. The threat to society from climate tipping points – both individually and due to their interactions – is not yet well understood. It is an area needing urgent research to develop predictions of when and where these abrupt changes can occur and the risk that they pose to communities and ecosystems worldwide.

- The global view and high spatial resolution afforded by satellites pose a particularly useful opportunity, said researchers at ESA’s Living Planet Symposium in Bonn, Germany. Remote sensing has already been used to provide critical evidence of the proximity of several tipping elements, ranging from ice sheets to boreal and tropical forests.

- “Successive IPCC (Intergovernmental Panel on Climate Chang) Assessment Reports have revised upwards the risks posed by climate tipping points,” says Tim Lenton, from the University of Exeter.

- “Tipping points have already informed climate mitigation targets, including the Paris Agreement’s goal to limit warming to well below two degrees Celsius. If the risk of tipping is still underestimated, it would provide a reason to strengthen such pledges and action to meet them. That’s why it’s so important to understand the proximity of tipping points and their impact on society.”

- The recently published IPCC sixth climate Assessment Report warns that high impact climate tipping points at regional scales cannot be ruled out. The risk of crossing thresholds into more abrupt changes increases at higher levels of global warming.

- Human activities can also be a factor: for example, continued deforestation of the Amazon along with a warming climate are together making it more likely that the forest will tip into a savannah state before 2100.

- “Climate tipping points are a new and growing area of research that is critical for developing early warning systems to inform policy and action going forward,” says Wendy Broadgate, Director of Future Earth’s Sweden hub who chaired the discussion. Future Earth hosts the Earth Commission, which uses a tipping points analysis to quantify the parameters to maintain a safe and resilient Earth system, which feeds into work with cities, businesses and nations for target-setting.

- “Researchers have been making progress on resilience indicators in conceptual models as well as models of Arctic sea ice, ice sheets, the Atlantic Meridional Overturning Circulation and other potential Tipping Elements,” said keynote speaker Sebastian Bathiany from Technical University of Munich. But models can only go so far in improving our understanding of how and when abrupt change will occur.

- “In order to understand what is going on in the real world, we need to work toward a meaningful interpretation of resilience indicators in Earth observation records. This is why discussions across scientific communities and disciplines are so important."

- “In order to understand what is going on in the real world, we need to work toward a meaningful interpretation of resilience indicators in Earth observation records. This is why discussions across scientific communities and disciplines are so important."

- Didier Swingedouw, who is based at the University of Bordeaux, explains, “We have moderate confidence that ocean circulation in the North Atlantic will not collapse during this century because climate models still miss key processes.”

- To gain a better understanding of ocean dynamics and therefore make more reliable predictions of any potential rapid changes, the models should be better coupled to Earth observation datasets. He says, “A rapid weakening of the Atlantic Meridional Overturning Circulation would have worldwide impacts: it is therefore urgent to work on the risk of such an event occurring.”

- Today’s discussion follows on from an ESA workshop held in 2021 that considered how Earth observations can be used for monitoring and early warning of tipping points, hosted by the International Space Science Institute with the Future Earth ‘AIMES’ project and the World Climate Research Programme.

- Key areas under discussion included how to develop systems for early warning of tipping onset and using remote sensing to examine the interactions between tipping elements.

- The Living Planet Symposium has attracted thousands of scientists and data users, and is amongst the biggest Earth observation conferences in the world. The event not only sees scientists present their latest findings on Earth’s environment and climate derived from satellite data, but also focuses on Earth observation’s role in building a sustainable future and a resilient society.

• March 11, 2022: The Copernicus Sentinel-1 mission takes us over the archipelago of Lofoten in northern Norway. ⁶³⁾

- Extending around 175 km from north to south, the archipelago comprises five main islands (Austvågøya, Gimsøya, Vestvågøya, Flakstadøya, and Moskenesøya), as well as many small islands and skerries (rocky islets and reefs). Lofoten is known for its distinctive scenery, with dramatic mountains and peaks, sweeping beaches, deep blue fjords and sheltered bays.

- Svolvær, the chief town and port of the Lofoten island group, is located on the southern coast of Austvågøya, the easternmost island of the archipelago. The economy largely depends on cod fisheries, with the town’s population swelling during the spawning season as fishermen flock in. The fjord of Vestfjorden lies between the archipelago and the mainland.

- What appears in yellow indicates what has fewer differences between polarisations, such as forests and other vegetated land, as well as built-up areas.

- Sentinel-1 is a radar mission and unlike optical cameras, 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 coloured images such as the one featured here. This technique allows scientists to better analyse Earth’s surface.

• February 8, 2022: The Copernicus Sentinel satellite missions measure and image our planet in different ways to return a wealth of complementary information so that we can understand and track how our world is changing, and how to better manage our environment and resources. Thanks to the benefits of different types of data from two particular Copernicus Sentinel missions and an ingenious new dataset tool, people working in the agriculture sector, but who are not satellite data experts, can monitor the health and development of crops, right down to each crop in individual fields. ⁶⁴⁾

- It goes without saying that we all depend on agriculture for food. However, the growing global population and the climate crisis is putting untold pressure on food production and water resources. While rich developed nations may be able to weather a failed crop, the loss of a precious harvest can be disastrous for many countries facing the serious issue of food security.

- Taking measurements from hundreds of kilometres above, satellites are key to monitoring crop health, forecasting yields, assessing vulnerability to drought and even to estimating carbon uptake by the soil so that agricultural carbon footprints can be reduced.

- And, the benefits are even greater if completely different types of satellite data can be used in synergy.

- However, very few of us are satellite data experts, so it is critical that datasets are made available in easy to understand and easy to use ways.

- Addressing this need, ESA worked with the Delft University of Technology in the Netherlands to develop the Agricultural Sandbox NL. As its name suggests, this toolset has been used to hone in on the Netherlands where much land is given over to agriculture.

- The first milestone in the project, marked the team succeeding in creating nationwide maps, over an entire year, of land showing crop type, indicators of crop health and growth at parcel level, which is a continuous area of land declared by one farmer and includes no more than one crop type.

- Remarkably, the toolset manages to reduce terabytes of satellite data to just 10 gigabytes per year.

- Agricultural Sandbox NL makes use of radar data from Copernicus Sentinel-1 and optical, or camera-like, data from Copernicus Sentinel-2.

- ESA’s Björn Rommen explains, “Both missions give us frequent coverage, which is critical to monitoring crop health in the growing season, and both missions give us detailed information in different ways. Sentinel-2 tells us about the greenness of the plants by measuring how much light they reflect and their colour. These images are the closest to what we see with our own eyes. But the radar on Sentinel-1 can unveil information about the plant’s structure and how much water is in the plant.

- “And of course, Sentinel-1’s radar that images regardless of clouds and rain, which is pretty important in the Netherlands.

- “Exploiting both types of measurement together gives us a really good indication of crop health, but the Agricultural Sandbox NL is the key thing that makes these data perfect for non-expert data users.”

- The team produced maps, which are freely available to the public, for every agricultural parcel in the Netherlands from 2017 to 2020.

- This experimental period shows that the toolset has real promise. Thanks to parcel and crop information, the toolset is already being used to train neural networks, allowing knowledge to be carried to other regions where such information is not readily available.

- Delft University of Technology’s Susan Steele-Dunne adds, “One of the nice things about working in the Netherlands is that there are freely available data on parcel boundaries and crop types. By combining those with the Sentinel data, we created a database where new users can see how data from Sentinel-1 and Sentinel-2 look for different crops.

- “They can search the data by area, crop type and time period to see the effects of drought, flooding and storm damage, for example. And the data are already processed, so they are much easier to work with. We hope that by making the data more accessible to new users, we can stimulate new applications of Sentinel data in agriculture.”

• January 14, 2022: Following the previous news on the Sentinel-1B anomaly that occurred on 23 December 2021, further attempts to reactivate the proper functioning of the satellite power system’s affected unit were executed, but were not successful. ⁶⁵⁾

- All necessary investigations to identify the root cause and possibly fix the issue are on-going.

- The satellite remains under control, the thermal control system works properly and the regular orbit control maneuvers are routinely performed.

• January 14, 2022: The Kangerlussuaq Glacier, one of Greenland’s largest tidewater outlet glaciers, is pictured in this false-color image captured by the Copernicus Sentinel-1 mission. Meaning ‘large fjord’ in Greenlandic, the Kangerlussuaq Glacier flows into the head of the Kangerlussuaq Fjord, the second largest fjord in east Greenland. ⁶⁶⁾

- At the top of the image, stable ice can be seen in white and is present in all three radar acquisitions. Ice and snow visible only in the early-summer acquisitions can be seen in bright yellow and are not present in the last acquisition as they have melted by this time. The different shades of red highlights ice and snow detected only in the first acquisition captured on 4 June. Colors on the sea surface vary owing to surface currents and sea ice dynamics.

- Research using satellite imagery suggests that since 2017, Kangerlussuaq has entered a new phase of rapid retreat and acceleration, and its ice front is now at its most retreated position since the early 20th century. ⁶⁷⁾

- As global temperatures increase, the melting of the massive ice sheets that blanket Greenland has significantly accelerated, contributing to sea-level rise. Over the past decade alone, findings have revealed that 3.5 trillion tons of ice have melted from the Greenland ice sheet and spilled into the ocean – enough to cover the UK with meltwater 15 m deep.

- Using data from ESA’s CryoSat mission, the research shows that extreme ice melting events in Greenland have become more frequent and more intense over the past 40 years, raising sea levels and the risk of flooding worldwide.

- Raised sea levels heighten the risk of flooding for coastal communities worldwide and disrupt Arctic Ocean marine ecosystems, as well as altering patterns of ocean and atmospheric circulation – which affect weather conditions around the planet.

- Observations of Greenland runoff from space can be used to verify how climate models simulate ice sheet melting which will allow improved predictions of how much Greenland will raise the global sea level in the future.

• January 07, 2022: Following the previous news on the Sentinel-1B anomaly that occurred on 23 December 2021, the resuming of the operations was carefully prepared including the on-board configuration changes preventing the anomaly to occur again. ⁶⁸⁾

- However, during the preparation of the recovery operations, it became clear that the initial anomaly was a consequence of a potential serious problem related to a unit of the power system of the Sentinel-1B satellite. The operations performed over the last days did not allow to reactivate so far a power supply function required for the radar operations.

- Further investigations to identify and remedy the root cause will be performed over the next days.

• December 25, 2021: Copernicus Sentinel-1B is unavailable since 23 December 2021 at 06:53 UTC, no data are being generated. ⁶⁹⁾

- Following the related news of 23 December 2021, detailed investigations have taken place. Specific actions will be performed over the next days to implement an onboard configuration change that will prevent the re-occurrence of the anomaly (that could result in satellite safety risks). This requires simulations and system validation activities on ground, before upload to the satellite.

- This satellite unavailability period may potentially last up to 2 weeks, however all efforts to shorten this unavailability are being deployed.

• October 4, 2021: This week marks seven years since the very first satellite that ESA built for the European Union’s Copernicus program started delivering data to monitor the environment. The Sentinel-1A satellite has shed new light on our changing world and has been key to supplying a wealth of radar imagery to aid disaster response. While this remarkable satellite may have been designed for an operational life of seven years, it is still going strong and fully expected to be in service for several years to come. ⁷⁰⁾

- Launched on 3 April 2014 and delivering a stream of operational data by the beginning October 2014, Copernicus Sentinel-1A marked a new era in global environmental monitoring. Carrying the latest radar technology to provide an all-weather, day-and-night supply of imagery of Earth’s surface, this new mission not only raised the bar for spaceborne radar, but also set the stage for Europe’s Copernicus program.

- Copernicus has been the largest provider of Earth observation data in the world for some years now. The suite of Sentinel missions in orbit delivering complementary data and the range of services offered through Copernicus help address some of today’s toughest environmental challenges such as food security, rising sea levels, diminishing ice, natural disasters, and the overarching issue of the climate crisis.

- “It is with great pride that we see the first satellite ESA built for Copernicus pass its all-important seven-year operational life expectancy,” said ESA’s Director General, Josef Aschbacher.

- “We have another seven Copernicus Sentinel satellites currently in operation, all of which are surpassing expectations. With more missions in the pipeline and an ever-growing community using the Sentinel missions’ free and open data, the approach of building a long-term reliable observing system is clearly paying off.”

- ESA’s Acting Head of Earth Observation Programs, Toni Tolker-Nielsen, added, “The Copernicus program as a whole is going to be even more relevant as the climate crisis takes a tighter hold. Information from satellites is indispensable in measuring progress towards climate goals set by the UN and the EC’s Green Deal.”

- Mauro Facchini, Head of the Earth Observation Unit (DEFIS.C.3) at the European Commission, said, “The launch of Sentinel-1A has been historical for Copernicus – the start of the successful story of the family of Sentinel satellites serving Copernicus services and a huge number of users around the world with their data. The emphasis of the Copernicus program has always been on its operational nature, going far beyond the time frame of research activities. The fact that Sentinel-1A is exceeding its design lifetime in best health underpins that both, policy-makers and businesses can really rely on Copernicus data and information being provided continuously and in long term.”

- The Copernicus Sentinel-1 mission comprises two identical satellites orbiting 180° apart to image the planet with a repeat frequency of six days, down to a daily coverage at high latitudes to support operational sea-ice monitoring. Sentinel-1B was launched in April 2016.

- The mission benefits numerous services and applications, such as those that relate to Arctic sea-ice monitoring, iceberg tracking, routine sea-ice mapping, glacier-velocity monitoring, surveillance of the marine environment including oil-spill monitoring and ship detection for maritime security as well as illegal fisheries monitoring. It is also used for monitoring ground deformation resulting from subsidence, earthquakes and volcanoes, mapping for forest, water and soil management, and mapping to support humanitarian aid and crisis situations.

- Over the last seven years, the mission has, for example, tracked the huge A-68 iceberg that calved from Antarctica and had a near-collision with South Georgia, has been used in synergy with the Copernicus Sentinel-2 optical mission to map crop types and with ESA’s CryoSat to map ice loss from ice sheets and diminishing sea ice as well as ice lost from the world’s glaciers.

- The mission has also been used to map subsidence and shifts in the ground following earthquakes, track surface wind speeds below tropical storms and hurricanes and been called upon through the Copernicus Emergency Mapping Services and the Disaster Charter to map floods at times of disaster.

- Sentinel-1 data have also formed the basis for countless scientific papers that shed new light on how our planet functions. The list goes on.

- With the mission designed to work as a pair of satellites, when the time does come for Sentinel-1A to retire, Sentinel-1C will take its place in orbit. The same goes for Sentinel-1B, which will eventually be replaced by Sentinel-1D. The latter two Sentinel-1 satellites will further improve performance and services with new instruments dedicated to marine applications.

- To ensure the provision of data over next decades, the same approach is taken for the other Sentinel missions.

- Looking even further ahead, it’s all systems go as ESA and the European Commission are developing the next generation of Sentinels building on the newest technology developments. Not only will this ensure continuity of data that many users have come to rely on, but it will also lead to new users and applications.

• October 1, 2021: The Copernicus Sentinel-1 mission takes us over the Mackenzie River, a major river system in the Canadian boreal forest. Its basin is the largest in Canada and is the second largest drainage basin of any North American river, after the Mississippi. ⁷¹⁾

- The Mackenzie River flows through a vast region of forest and tundra through the Northwest Territories from the Great Slave Lake to the Beaufort Sea in the Arctic Ocean. Its delta covers an area around 12 000 km², measuring more than 190 km from north to south and is around 80 km wide along the Arctic shore. The maze of branching and intertwining channels is dotted with numerous lakes and ponds.

- The landscape pictured here is very typical for these latitudes, with the whole region subject to a harsh winter climate. Many of the lakes are frozen during the winter months, except for some of the lakes visible in black in the center of the image, which are ice-free. One of the lakes appears red most likely due to new ice which has formed between image acquisitions.

- The town of Inuvik lies along the east channel of the Mackenzie River delta, around 100 km from the Arctic Ocean and approximately 200 km north of the Arctic Circle. The hamlet of Tuktoyaktuk lies on the shores of the Arctic Ocean and is the only community in Canada on the Arctic Ocean that is connected to the rest of Canada by road.

- Around 75% of the Mackenzie basin sits within a permafrost area. Permafrost, ground which remains completely frozen for at least two consecutive years, is common in high latitude regions. With increasing temperatures causing permafrost to thaw, it not only releases methane and carbon dioxide into the atmosphere, but it can cause erosion, flooding and landslides.

- Satellite data can be used to map permafrost, even in remote and inaccessible areas such as the Mackenzie River delta. The maps, using data from ESA’s Climate Change Initiative, are the longest, satellite-derived permafrost record currently available.

• August 20, 2021: Iceberg A-74, approximately 1.5 times the size of Greater Paris, calved from Antarctica’s Brunt Ice Shelf earlier this year. Over the last six months, it has remained close to the shelf it broke away from owing largely to ocean currents. In early August, strong easterly winds have spun the iceberg around the western tip of Brunt, brushing slightly against the ice shelf before continuing southwards. ⁷²⁾

- For years, glaciologists have been monitoring the formation and extension of the fractures, known as rifts, and the opening of large chasms in the 150 m thick Brunt Ice Shelf. Chasm 1, the large crack running northwards from the southernmost part of Brunt, is narrowly separated from the more recent Halloween crack.

- Had the drifting iceberg hit the unstable ice shelf with severe force, it may have triggered the release of a new 1700 km²-sized iceberg. Despite reports of a minor impact, the prospective berg remains tenuously attached in the vicinity of McDonald Ice Rumples, where the ice shelf is locally grounded on the seabed.

- ESA’s Mark Drinkwater comments, “The nose-shaped piece of the ice shelf, which is even larger than A-74 remains connected to the Brunt Ice Shelf, but barely. If the berg had collided more violently with this piece, it could have accelerated the fracture of the remaining ice bridge, causing it to break away. We will continue to routinely monitor the situation using Sentinel satellite imagery.”

- During the dark winter months in Antarctica, radar images are indispensable because, apart from the region being in a remote region, radar continues to deliver images regardless of the weather or seasonal darkness. The Copernicus Sentinel-1 mission returns images regardless of whether it is day or night, also allowing for continuous imaging during what is now Antarctic mid-winter.

- With the ice shelf deemed unsafe due to the encroaching cracks in 2017, the British Antarctic Survey closed their Halley VI Research Station and re-positioned it to a more secure location, around 20 km away from Chasm 1. Halley is made up of eight interlinked pods built on skis which allows the pods to be easily moved in case of unstable ice or new chasms forming on the ice shelf.

• July 16, 2021: Record rainfall has caused swollen rivers to burst their banks and wash away homes and other buildings in western Europe – leading to more than 90 casualties and over 1000 people missing. Data from the Copernicus Sentinel-1 mission are being used to map flooded areas to help relief efforts. ⁷³⁾

- The German states of Rhineland-Palatinate and North Rhine-Westphalia were among the worst hit by the torrential rainfall, with water levels rising in the Rhine River, as well as the Walloon Region in Belgium. The storms and high waters have also battered neighboring Switzerland, the Netherlands and Luxembourg.

- The service uses observations from multiple satellites to provide on-demand mapping to help civil protection authorities and the international humanitarian community in the face of major emergencies.

• June 25, 2021: The Copernicus Sentinel-1 mission takes us over Lake Mar Chiquita – an endorheic salt lake in the northeast province of Córdoba, Argentina. ⁷⁴⁾

- Lake Mar Chiquita, around 70 km long and 24 km wide, is fed primarily by the Primero and Segundo rivers from the southwest and the Dulce river from the north. While these rivers flow into the lake, there isn’t a natural outflow of water so it only loses water by evaporation, hence Lake Mar Chiquita being described as an endorheic lake. The lake’s surface area, as well as its salinity, varies considerably (ranging between 2000 and 6000 km²), although it is slowly diminishing in size owing to evaporation.

- Several small islands lie in the lake, the most important of which is El Médano. Vast expanses of saline marshes can be seen on the lake’s northern shore. The lake has been designated as a Ramsar Site of International Importance, and is considered one of the most important wetlands in Argentina owing to its rich biodiversity. Over 25 species of fish are known to breed in Lake Mar Chiquita, with fishing and livestock being the principal land uses.

- The colors of this week’s image come from the combination of two polarizations from the Sentinel-1 radar mission, which have been converted into a single image.

- As radar images provide data differently 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 scientists to better analyze Earth’s surface.

- Shades of blue in the image show us where the differences between the two polarizations are higher, for example the saline marshes in the lake’s north, whereas the crops and agricultural fields in the surrounding area appear yellow, indicating fewer differences between polarizations. Fields, such as those visible in the bottom-left corner of the image, appear blue most likely because they are wetter. Several villages, including San Francisco and Rafaela, are identifiable in white in the bottom-right of the image.

• May 19, 2021: An enormous iceberg has calved from the western side of the Ronne Ice Shelf, lying in the Weddell Sea, in Antarctica. The iceberg, dubbed A-76, measures around 4320 km² in size – currently making it the largest berg in the world. ⁷⁵⁾

- The enormity of the berg makes it the largest in the world, snatching first place from the A-32A iceberg (approximately 3880 km² in size) which is also located in the Weddell Sea. In comparison, the A-74 iceberg that broke off the Brunt Ice Shelf in February earlier this year, was only 1270 km².

- The iceberg was spotted by the British Antarctic Survey and confirmed from the US National Ice Center using Copernicus Sentinel-1 imagery. The Sentinel-1 mission consists of two polar-orbiting satellites that rely on C-band synthetic aperture radar imaging, returning data regardless of whether it is day or night, allowing us year-round viewing of remote regions like Antarctica.

- Icebergs are traditionally named from the Antarctic quadrant in which they were originally sighted, then a sequential number, then, if the iceberg breaks, a sequential letter.

• March 26, 2021: The giant container ship 'Ever Given' ran aground in the Suez canal on 23 March on its journey from China to the Netherlands. The image on the left, captured on 21 March, shows routine maritime traffic in the canal with vessels visible every 2 to 3 km. The image on the right, captured on 25 March, shows the 400 m-ship blocking the canal. ⁷⁶⁾

- The canal connects Port Said on the Mediterranean Sea to the Indian Ocean via the Egyptian city of Suez on the Red Sea. The blockage has delayed hundreds of tankers and vessels in reaching their destination, and more maritime traffic is still heading to the crucial waterway. Ships can be seen accumulating in the Gulf of Suez.

- Tug boats are working hard to dislodge the 200,000 ton ship, however Egyptian authorities say it is unclear when the route will reopen.

- 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, making it ideal to monitor ship traffic.

- 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.

• March 24, 2021: Stretches of land across New South Wales, Australia, have been hit with torrential rain leading to record-breaking floods. The heavy rainfall has caused dams to spill over, rives to burst their banks and thousands of people forced to evacuate their homes. Data from the Copernicus Sentinel-1 mission are being used to map flooded areas to help relief efforts. ⁷⁷⁾

- Images acquired before and after flooding offer immediate information on the extent of inundation and support assessments of property and environmental damage. Copernicus Sentinel-1’s radar ability to ‘see’ through clouds and rain, and in darkness, makes it particularly useful for monitoring floods.

- Data from the Copernicus Sentinel-1 mission have been used by the Copernicus Emergency Mapping Service, activated on 20 March, to map the flooded areas. The service provides information for emergency response to different types of disasters, including meteorological hazards, geophysical hazards, deliberate and accidental man-made disasters and other humanitarian disasters, as well as prevention, preparedness, response and recovery activities.

• March 17, 2021: Fluctuations in the carbon-rich biomass held within the world’s forests can contribute to, or slow, climate change. A series of new maps of above ground biomass, generated using space observations, is set to help our understanding of global carbon cycling and support forest management, emissions reduction and sustainable development policy goals. ⁷⁸⁾

- Above ground biomass refers to the stem, bark, branches and twigs of woody components of vegetation. As photosynthesis withdraws carbon dioxide from the atmosphere, it stores carbon in vegetation in an amount comparable to that of atmospheric carbon. The vegetation has the potential to sequester more carbon in the future or to contribute as an even larger source.

- Vegetation biomass is a crucial ecological variable for understanding the evolution and potential future changes of the climate system, on a local, regional and even global scale. For this reason, it is recognized by the Global Climate Observing System (GCOS) as one of the 54 Essential Climate Variables used to characterize climate.

- New maps, generated by a research team working as part of ESA’s Climate Change Initiative, provide a global view of above ground biomass distribution and spatial density over three separate years – 2010, 2017 and 2018. The maps are derived from a combination of data, depending on the year, from the Copernicus Sentinel-1 mission, Envisat’s ASAR instrument and JAXA’s Advanced Land Observing Satellite (ALOS-1 and ALOS-2), along with additional information from Earth observation sources.

- Earth observation data are routinely used to validate the accuracy, or identify biases, in climate models. The new maps, provided at 100 m resolution, have trimmed uncertainty estimates and will help to further constrain models.

- Crucially, and according to the team’s science leader, Shaun Quegan, the new maps capture the higher biomass levels in high density forest areas, such as in the tropics, due to major improvements to the algorithm.

- By using a globally consistent retrieval methodology, the advent of multi-year biomass maps brings the prospect of monitoring change a step closer to reality. However, users are currently discouraged from quantifying biomass changes just by subtracting the current maps, since the retrieval procedure is still being tuned to account for the different mission and sensor observations used in their generation.

- The team is currently developing a map for 2020 while also addressing temporal consistency between the different years, with the integration of additional low geometric resolution data streams under consideration, namely L-band vegetation optical depth from ESA’s Soil Moisture and Ocean Salinity (SMOS) satellite and scatterometer data from the ASCAT on board Eumetsat’s MetOp satellites.

- Shaun Quegan explains, “Combining these new data is anticipated to increase the consistency of these high-resolution maps, and move a step closer towards tracking changes and direct estimation of gross gains and losses of above ground biomass at scale.” Alternative approaches to correct for bias are also under investigation.

- With a decade of global biomass estimates on the horizon, the maps are set to allow scientists to undertake trend analyses, allowing, for example, the impact of regional climate phenomena such as El Niño on biomass dynamics to be better understood.

- Significantly, an ability to track global biomass change is set to support global and national policy aimed at meeting emission reduction commitments to limit global warming. Biomass estimates provide critical support for both reporting of national greenhouse emissions under the Paris Agreement and forest management through the United Nations’ Reducing Emissions from Deforestation and Degradation-plus (REDD+) initiative.

- Tracking biomass change is also becoming increasingly important as national governments work towards reporting for the Global Stocktake – an aspect of the global Paris climate deal – that will periodically check international progress towards meeting emissions reduction commitments to limit global warming.

• March 2, 2021: A giant iceberg, approximately 1.5 times the size of Greater Paris, broke off from the northern section of Antarctica’s Brunt Ice Shelf on Friday 26th February. New radar images, captured by the Copernicus Sentinel-1 mission, show the 1270 km² iceberg breaking free and moving away rapidly from the floating ice shelf. ⁷⁹⁾

- Glaciologists have been closely monitoring the many cracks and chasms that have formed in the 150 m thick Brunt Ice Shelf over the past years. In late-2019, a new crack was spotted in the portion of the ice shelf north of the McDonald Ice Rumples, heading towards another large crack near the Stancomb-Wills Glacier Tongue.

- ESA’s Mark Drinkwater said, “Although the calving of the new berg was expected and forecasted some weeks ago, watching such remote events unfold is still captivating. Over the following weeks and months, the iceberg could be entrained in the swift south-westerly flowing coastal current, run aground or cause further damage by bumping into the southern Brunt Ice Shelf. So we will be carefully monitoring the situation using data provided by the Copernicus Sentinel-1 mission.”

- Although currently unnamed, the iceberg has been informally dubbed ‘A-74’. 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.

- The calving does not pose a threat to the presently unmanned British Antarctic Survey’s Halley VI Research Station, which was re-positioned in 2017 to a more secure location after the ice shelf was deemed unsafe.

- Routine monitoring by satellites offer unprecedented views of events happening in remote regions like Antarctica, and how ice shelves manage to retain their structural integrity in response to changes in ice dynamics, air and ocean temperatures. The Copernicus Sentinel-1 mission carries radar, which can return images regardless of day or night and this allows us year-round viewing, which is especially important through the long, dark, austral winter months.

• February 23, 2021: Using a 25-year record of satellite observations over the Getz region in West Antarctica, scientists have discovered that the pace at which glaciers flow towards the ocean is accelerating. This new research, which includes data from the Copernicus Sentinel-1 mission and ESA’s CryoSat mission, will help determine if these glaciers could collapse in the next few decades and how this would affect future global sea-level rise. ⁸⁰⁾

- Ice lost from Antarctica frequently hits the headlines, but this is the first time that scientists have studied this particular area in depth.

- Led by scientists at the University of Leeds in the UK, the new research shows that between 1994 and 2018, all 14 glaciers in Getz accelerated, on average, by almost 25%, with three glaciers accelerating by over 44%.

- The results, published today in Nature Communications, also reported that the glaciers lost a total of 315 gigatons of ice, adding 0.9 mm to global mean sea level – equivalent to 126 million Olympic swimming pools of water. ⁸¹⁾

- Heather Selley, lead author of the study and a glaciologist at the Centre for Polar Observation and Modelling at the University of Leeds, said, “The Getz region of Antarctica is so remote that humans have never set foot on the majority of it.

- “However, satellites can tell us what is going on and the high rates of increased glacier speed, coupled with ice thinning, now confirms the Getz basin is in ‘dynamic imbalance’, meaning that it is losing more ice than it gains through snowfall.”

- The scientists used two different types of satellite measurements.

- Radar data from the Copernicus Sentinel-1 mission, legacy data from the ERS mission through ESA’s Climate Change Initiative and NASA’s MEaSUREs data record allowed them to calculate how fast the glaciers have been moving over the 25-year study period.

- To measure how much the ice has been thinning, they used altimetry data from ESA’s ERS, Envisat and CryoSat missions through the IMBIE assessment.

- “Using a combination of observations and modelling, we show highly localized patterns of acceleration. For instance, we observe the greatest change in the central region of Getz, with one glacier flowing 391 meters a year faster in 2018 than in 1994. This is a substantial change as it is now flowing at a rate of 669 meters a year, a 59% increase in just two and a half decades,” continued Heather.

- The research, funded by the Natural Environment Research Council and ESA’s Science for Society programme, reports how the widely reported thinning and acceleration observed in the neighboring Amundsen Sea glaciers, now extends over 1000 km along the West Antarctic coastline into Getz.

- Anna Hogg, study co-author, said, “The pattern of glacier acceleration shows the highly localized response to ocean dynamics.

- “High-resolution satellite observations from satellites such as Sentinel-1, which collects a repeat image every six-days, means we can measure localized speed changes with ever greater detail.

• February 19, 2021: Traditionally, optical, or ‘camera-like’, satellite images are used to map different crops from space, but a recent study shows that Copernicus Sentinel-1 radar data along with interferometric processing can make crop-type mapping even better. This, in turn, will help improve crop-yield forecasts, production statistics, drought and storm damage assessments, and more. ⁸²⁾

- The Sentinel-1 mission comprises two identical satellites, each carrying an advanced radar instrument to provide a day-and-night, all-weather supply of images of Earth’s surface. These images are used for numerous applications such as monitoring sea ice and floods, as well as shifts in the land surface or ice surface through the process of interferometry (InSAR) – which is where images of the same place from consecutive satellite passes are compared to reveal differences that occurred between image acquisitions.

- Going further, a paper, published in the IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, describes how Sentinel-1 InSAR can go beyond the measurement of terrain displacement. ⁸³⁾

- Part of this research was carried out through ESA’s SInCohMap project, which is dedicated to exploring innovative methodologies for land-cover and vegetation mapping using Sentinel-1 multitemporal InSAR coherence data.

- The constellation of the two identical Sentinel-1 satellites orbiting Earth 180° apart allows most parts of the world to be imaged every six days. The satellites’ radar instruments can transmit a signal in either horizontal (H) or vertical (V) polarisation, and then receive in both H and V polarisations.

- The year-long time series of data from 2017 combined pairs of Sentinel-1 images of an agricultural area in Seville, Spain. The images were used to classify 17 different crop types cultivated that year. Coherence was measured by using the pairs of consecutive images, acquired with the separation of six days and at the two polarizations.

- The plot (Figure 52) shows the temporal patterns of coherence, in both VV and VH, measured for four crop classes: alfalfa, maize, sugar beet and wheat. Alfalfa exhibits a long growth season, translated into low coherence, from January to September, whereas maize presents a much shorter cycle concentrated in summer.

- “The analysis of the time series shows that coherence drops in presence of crop growth, whereas it increases out of season,” describes Alejandro Mestre-Quereda from the University of Alicante and lead author of the paper. “Since the cultivation dates and the duration of the campaign is typical for each crop type, this characteristic response has enabled the generation of crop-type maps with an overall accuracy of 80%.”

- Juan M. Lopez-Sanchez, also from the University of Alicante, added, “Some crop types which were poorly classified by either intensity or coherence are better distinguished by the combination of both types of polarization data types. In other words, both sources of information are indeed complementary.”

- Crop-type mapping using satellite imagery plays a key role in Europe’s Common Agricultural Policy, in which ‘checks by monitoring’ are being established routinely to reduce field work and the associated bureaucracy. Moreover, it also helps monitor potential threats to ecosystems if new crops are introduced close to protected areas.

- During the development of the SInCohMap project, all the possible combinations of images acquired in one year were computed and analyzed.

- “One of the big challenges was the amount of data that needed to be handled, but the uniqueness of the six-day coherence in this case greatly reduces the data storage requirements,” explains Alexander Jacob of EURAC Research, in charge of the data infrastructure.

- In conclusion, the study shows that Sentinel-1 interferometry constitutes a solid source of information for performing crop-type classification, hence going beyond the well-known application of terrain displacement monitoring.

- ESA’s Marcus Engdahl remarks, “The Copernicus Sentinel-1 constellation has been a game-changer for radar remote sensing globally. This study demonstrates that it is possible to derive still more useful information about vegetation and land-cover by using InSAR techniques. Radar remote sensing is profoundly changing vegetation monitoring, and the planned Copernicus Expansion mission ROSE-L with its L-band radar system is going to enable another large increase in global monitoring capabilities.”

- The SInCohMap project consortium includes DARES Technology (ES), EURAC Research (IT), University of Alicante (ES), Technical University of Catalonia (ES), IGIK (PL), and University of Rennes 1 (FR).

• February 12, 2021: In early 2019, all eyes were fixed on the Brunt Ice Shelf in Antarctica, where a massive iceberg, around the size of Greater London, appeared poised to break off. Almost two years later, the berg is desperately clinging on, although current data indicate calving is imminent. A new crack, spotted in images captured by the Copernicus Sentinel missions, now suggests the potential for calving of multiple bergs. ⁸⁴⁾

- For years, glaciologists have been tracking a number of cracks in the Brunt Ice Shelf, which borders the Coats Land coast in the Weddell Sea sector of Antarctica. The lengthening of two main cracks in the ice shelf, separated only by a few kilometers, have been closely monitored by satellite imagery. Chasm 1, the large crack running northwards from the southernmost part of Brunt, has been set in place for more than 25 years, while the Halloween crack was first spotted on 31 October 2016.

- A more recent, unnamed crack was first noticed in observations from the Copernicus Sentinel-1 mission in late-2019, recently extending by more than 20 km in length. Satellite data has also been used to track the movement and measure the resulting strain in the ice shelf. The map below shows the ice surface velocity on the Brunt and Stancomb-Wills Ice Shelf complex, derived by comparing two Sentinel-1 acquisitions captured on 5 January and 17 January 2021.

- The data indicate the region of the floating ice shelf, to the north of the new crack, to be the most unstable, with an approximate movement of almost 5 m per day. The central portion has an average velocity ranging from 2 to 2.5 m per day, while the lower area (visible in blue) suggests a more stable zone of the ice shelf.

- “Though appearing poised to calve in 2019, the south westernmost region of the Brunt Ice Shelf tenaciously resisted separation,” noted ESA’s Mark Drinkwater. “Since then, Sentinel-1 data indicate the nose of the ice shelf to be pivoting clockwise around the McDonald Ice Rumples region in which point the shelf ice is grounded on shallow underwater topography.”

- “Meanwhile, the strong gradient in ice velocity towards the faster moving Stancomb-Wills ice stream, and ice shelf in the north, has activated a new rift which now threatens the release of a second large iceberg.”

- Routine monitoring from satellites offer unprecedented views of events happening in remote regions, and show how ice shelves are responding to changes in ice dynamics, air and ocean temperatures. Since Antarctica is in the dark winter months, radar images are indispensable because, apart from the region being remote, radar continues to deliver images regardless of the dark weather.

- Mark Drinkwater continued, “With today’s Copernicus monitoring system, we are far better equipped not only to observe events in remote places like Antarctica in near real time, but more importantly, to turn this scientific data into theoretical understanding of complex ice fracture processes.”

- History shows that the last major event took on the Brunt Ice Shelf took place in 1971, when a portion of ice calved north of the area known as the McDonald Ice Rumples in what appears to be replicated by today’s Halloween Crack.

- With the ice shelf deemed unsafe due to the encroaching cracks in 2017, the British Antarctic Survey closed up their Halley VI research station, and re-positioned south of Halloween Crack to a more secure location. Operational since 2012, Halley VI is made up of eight interlinked pods built on skis. This allows the pods to be easily moved in case of unstable ice and cracks on the ice shelf.

• February 3, 2021: Satellite images have revealed that the once colossal A-68A iceberg has had yet another shattering experience. Several large cracks were spotted in the berg last week and it has since broken into multiple pieces. These little icebergs could indicate the end of A-68A’s environmental threat to South Georgia. ⁸⁵⁾

- One of the largest icebergs of all time, A-68A broke off from the Larsen-C ice shelf in 2017 and has been closely monitored over recent months as it veered dangerously close to South Georgia in the South Atlantic.

- The iceberg’s close position to the remote island prompted fears that it would anchor itself to the coast and impact the fragile ecosystem that thrives around the island, through the scraping of the seabed or the release of cold freshwater into the surrounding ocean.

- In December 2020, the iceberg changed direction, as ocean surface currents steered by sea floor bathymetry, diverted it in a southeast direction away from the island, losing a huge chunk of ice in the process.

- Images, captured by the Copernicus fleet of satellites have charted the process of A-68A on its journey over the past three years. Latest data coming from the Copernicus Sentinel-1 radar mission shows the iceberg suffered further damage in 2021 as a new iceberg calved from A-68A just last week. The smaller slab, promptly named A-68G by the US National Ice Center, measures approximately 53 km in length and around 18 km at its widest point.

- Soon after, a large crack developed where A-68G broke free, resulting in the almost-immediate calving of an additional two icebergs: A-68H (around 20 km in length and 9 km width) and A-68I (around 30 km long and 5 km width at its widest point). 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.

- The main A-68A iceberg, once the world’s largest, now measures only around 60 km in length with a maximum width of 22 km. The collective group of icebergs appear to be drifting apart, with the A-68H moving northwards, approximately 130 km from South Georgia. As of today, the main A-68A iceberg appears to be moving south and is currently around 225 km from South Georgia. This latest calving event could indicate that the bergs will most likely travel away from the island, no longer threatening the island’s wildlife.

- Optical imagery from the Copernicus Sentinel-3 mission, while revealing great details of A-68A, is only available in cloud-free conditions. Sentinel-3 and soon, Sentinel-6, radar altimeter measurements can monitor the trajectory of icebergs and are also used to calculate estimates of geostrophic ocean currents that carry A-68A and its children on their journey. Sentinel-1 radar imagery is not affected by clouds, and has been vital in tracking the break-up of A-68A.

• January 28, 2021: After almost three months at sea, competitors of the Vendée Globe sailing race are now nearing the finishing point back in France, but while they were near the treacherous iceberg-infested waters of the Southern Ocean they remained relatively safe thanks to satellite observations. Read full story: Copernicus satellites keep eyes on icebergs for Vendée Globe. ⁸⁶⁾

- To ensure their safety, CLS, a subsidiary of the French CNES space agency, and CNP used information from satellites to detect and monitor icebergs. This information comes from satellites carrying altimeters, as well as those carrying synthetic aperture radar (SAR) such as Copernicus Sentinel-1. Satellites are the only way of detecting and monitoring icebergs effectively in the remote Southern Oceans.

- Before the race started, this satellite information allowed CLS to establish an initial exclusion zone around Antarctica to keep sailors away from icebergs. Remarkably, they updated the zone no less than five times as the icy waters changed. These updates were provided to the Vendée Globe race organizers, who then communicated it to the skippers.

• January 15, 2021: The Copernicus Sentinel-2 mission takes us over the Tanezrouft Basin – one of the most desolate parts of the Sahara Desert. ⁸⁷⁾

- The barren plain extends to the west of the Hoggar mountains and southeast of the sandy Erg Chech. The terrain shows evidence of water erosion that occurred many years ago, when the Sahara Desert’s climate was much wetter, as well as wind erosion caused by frequent sandstorms – exposing ancient folds in the Paleozoic rocks.

- The region is characterized by dark sandstone hills, steep canyon walls, salt flats (visible in white in the image), stone plateaus and seas of multi-storey sand dunes known as ‘ergs’. Concentric rings of exposed sandstone strata create a stunning pattern predominantly visible in the left of the image.

- White lines in the right of the image are roads that lead to In Salah – the capital of the In Salah Province and In Salah District. Just above the center-left of the image, an airstrip can be seen. An interesting, grid-like pattern can be seen in the bottom of the image and mostly consists of human-made clearings and roads.

Sentinel-1B Status

March 21, 2022: European Space Agency officials said prospects are dimming for the recovery of a radar imaging satellite that malfunctioned nearly three months ago, but that efforts to save the spacecraft continue. ⁶¹⁾

- The Sentinel-1B spacecraft malfunctioned in December, keeping the spacecraft from collecting C-band synthetic aperture radar (SAR) imagery. ESA said in January that they were investigating a problem with the power system for the SAR payload on the satellite, launched in April 2016.

- In a Feb. 25 update, ESA said work was continuing to investigate problems with both the main and backup power system for the payload but that effort had yet to identify a root cause of the anomaly. The problem doesn’t affect operations of the spacecraft itself, which has remained under control.

- ESA leaders were not optimistic about the prospects of recovering Sentinel-1B. “The situation doesn’t look very good, but we have not given up hope yet,” Josef Aschbacher, director general of ESA, said in response to a question about the status of the satellite. “We are still looking into technical options of what the root cause could be.”

- Simonetta Cheli, director of Earth observation at ESA, said engineers have reviewed 18 potential causes for the power failure. “We are not yet at the end of the story,” she said, saying that investigations will continue for at least the next few weeks.

- But, like Aschbacher, she appeared pessimistic that Sentinel-1B could be recovered. “It doesn’t look very good, but for the moment it’s not the final word on 1B.”

- In January, ESA floated the possibility of moving up the launch of a new SAR satellite, Sentinel-1C, to compensate for the potential loss of Sentinel-1B. The mission currently is scheduled for launch in the middle of 2023 although the spacecraft would be ready for launch after a flight acceptance review scheduled for October.

- Cheli confirmed that Sentinel-1C would be ready for launch in October but said that ESA had not yet decided if the launch could be moved up. “We are assessing, in the current situation, options with Arianespace for launch. We are looking at the earliest options because we want to support the users.”

- A problem for any effort to move up the Sentinel-1C launch is the Russian decision to halt Soyuz launches from French Guiana, requiring five European missions that planned to fly on that rocket there to look for alternative vehicles. That would include the Vega C, the rocket currently planned to launch Sentinel-1C. The Vega C also faces concerns about access to a Ukrainian-built engine for its upper stage, leading ESA to examine options to replace it with an alternative that could affect future launches.

Mission Updates from ESA  ⁶²⁾

• December 23, 2021: Copernicus Sentinel-1B is unavailable since 23 December 2021 at 06:53 UTC, no data are being generated.

- An anomaly occurred onboard, that will require some time for detailed analyses.

- The satellite is in a nominal mode, the SAR has been temporarily switched off. Further news will be issued on the recovery operations. 

• December 25, 2021: Copernicus Sentinel-1B is unavailable since 23 December 2021 at 06:53 UTC, no data are being generated.

• January 7, 2022: Following the previous news on the Sentinel-1B anomaly that occurred on 23 December 2021, the resuming of the operations was carefully prepared including the on-board configuration changes preventing the anomaly to occur again.

- However, during the preparation of the recovery operations, it became clear that the initial anomaly was a consequence of a potential serious problem related to a unit of the power system of the Sentinel-1B satellite. The operations performed over the last days did not allow to reactivate so far a power supply function required for the radar operations.

- Further investigations to identify and remedy the root cause will be performed over the next days.

• January 14, 2022: Following the previous news on the Sentinel-1B anomaly that occurred on 23 December 2021, further attempts to reactivate the proper functioning of the satellite power system’s affected unit were executed, but were not successful.

- All necessary investigations to identify the root cause and possibly fix the issue are on-going.

- The satellite remains under control, the thermal control system works properly and the regular orbit control manoeuvres are routinely performed.

• February 25, 2022: Following the previous news on the Sentinel-1B anomaly that occurred on 23 December 2021, very detailed investigations related to the satellite power system’s affected unit are still on-going. The anomaly is related to a 28V power regulated bus that supplies power to the SAR electronics subsystem. The reactivation of both the main and redundant power regulators of this 28V bus have not been successful so far. 62)

- Whereas good progress has been made on the investigation of 18 identified possible failure scenarios linked to the affected power unit, at this stage the root cause of the anomaly has not been clearly identified. Satellite system level analyses are conducted in parallel, to perform new recovery attempts of the unit.

- The satellite remains under control, the thermal control system works properly, regular orbit control manoeuvres are routinely performed.

- Sentinel-1 users should assume a long-term unavailability of data provision (several months). It is however too early to consider a permanent unavailability of Sentinel-1B.

• August 03, 2022: The European Space Agency and the European Commission announce the end of the mission for the Copernicus Sentinel-1B satellite and regret the inconvenience caused to the Copernicus Services, Copernicus Participating States, and the many users worldwide affected by the lack of Sentinel-1B data.

- The conclusions drawn by the Anomaly Review Board resulted in the impossibility to recover the 28V regulated bus of the C-SAR Antenna Power Supply unit (CAPS), which provides power to the radar electronics. The capability of Sentinel-1B to support the mission is therefore considered lost.

- Copernicus Sentinel-1A remains fully operational. Strong efforts have been deployed to launch Sentinel-1C as early as possible, the current plans target a launch in the second quarter of 2023. The successful VEGA-C maiden flight on 13 July 2022 marks a major milestone towards the launch of Sentinel-1C.

- The start of the Sentinel-1B disposal is planned to take place after the Sentinel-1C In-Orbit Commissioning Review (IOCR), indicatively during the third quarter of 2023. The de-orbiting activities will last several months (current estimate in the order of 9 months).

- The de-orbiting of Sentinel-1B is an example of ESA’s and EU’s commitments to clean and responsible space. It will be performed according to the safety and clean space environment requirements in place for ESA projects at the time of design of Sentinel-1A and Sentinel-1B (re-entry in the atmosphere within maximum 25 years).

Sensor Complement

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. ^{88) 89) 90) 91) 92) 93) 94) 95) 96) 97) 98) 99) 100) 101)}

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.

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.

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 11).

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). ^{102) 103) 104)}

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. ^{105) 106) 107) 108) 109) 110)}

TOPS is employing a rotation of the antenna in the azimuth direction as is shown in Figure 66. 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. ¹¹¹⁾

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. 119).

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.

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. 111).

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., before the launch of Sentinel-1A. ^{112) 113)}

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.

Instrument Description

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).

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. ^{114) 115) 116)}

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: ¹¹⁷⁾

- 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.

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.

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 72). 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. ¹¹⁸⁾

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.

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 before 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 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.

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. 118).

Roll steering mode (Ref: 93): 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).

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. ^{119) 120)}

Figure 75 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. ^{121) 122)}

The SAS consists of 14 identical tiles (12.3 m x 0.84 m) in 5 deployable panels as shown in Figure 76.

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 several 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 77: 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 79 and 80.

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 78). Each network (Tx, RxH, RxV) of the EPDN consists of a 1:10 divider to supply the 10 EFEs. Short cables connect the outputs of the 1:10 dividers to the EFEs.

The EFE comprises several TRMs (Transmit/Receive Modules) and associated electronics. It represents the active part of the RF equipment of the antenna. The basic function of each EFE is:

• to transmit Tx pulses in one of two polarizations (either H or V) to the corresponding two radiator elements

• to receive the echoes from the two H and the two V radiator elements independently and simultaneously.

The EFE provides the capability to perform antenna beam steering and forming by electronic means:

• control phase setting in transmit

• control phase and gain settings in receive.

C-SAR instrument calibration

In contrast to SAR systems already existing in C-band like ASAR/ENVISAT or RADARSAT-2, high demands on the radiometric accuracy are made for C-SAR on Sentinel-1. Thus, product quality is of paramount importance and the success or failure of the mission depends essentially on the method of calibrating the entire Sentinel-1 system efficiently. ^{124) 125) 126) 127) 128) 129)}

The most important point for 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.

- Concerning 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. ¹³⁰⁾

Internal Calibration

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 concerning 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 several 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.

External Calibration

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 before the in-flight measurements.

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 (3&#963;) 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. ¹³¹⁾

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 83 and the novel transponders as depicted in Figure 84, 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 85 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).

Hence, the test site shown in Figure 85, 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.

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. ¹³²⁾

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. ¹³³⁾

The EC has also defined an overall Copernicus data policy, declaring the Sentinel mission data free and open. ¹³⁴⁾ 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 12. 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. 133). ¹³⁵⁾

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 87 displays the structure of the PDGS and the outcome of the selection of the providers.

Within this competitive selection process, ESA has awarded the DLR/DFD (German Remote Sensing Data Center) with the set-up and operations of the PACs for Sentinel-1 and Sentinel-3 (OLCI part). T-Systems, Germany, is assisting DLR in the network parts of the PACs. In addition and under separate procurement, DFD designs, builds and operates the payload data ground segment) for the Sentinel-5 Precursor mission.

High-level tasks of the DLR Sentinel PAC are:

- receive Sentinel data from CGSs via the GMES WAN

- ingest these data into the STA (Short-Term Archive) and MTA (Mid-Term Archive) of the Sentinel PGDS

- ingest these data in a LTA (Long-Term Archive) for a period of more than 7 years

- perform consolidation and re-assembly of level-0 data received from CGS facilities

- perform systematic and request-driven processing of Sentinel data to higher-level products

- host Sentinel data products within a layered architecture of on-line dissemination elements that will facilitate the direct access of end-users via public networks

- share and exchange any locally processed data with a 2nd partner PAC for the purpose of redundancy.

According to the GSC (GMES Space Component) operations concept, the Sentinel PDGS will become operationally embedded in the GSCDA (GMES Space Component Data Access) System that ESA implements in support of data access to GMES/Copernicus Service Projects and their users.

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. 133).

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. 135): ¹³⁶⁾

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.

The selected geographical locations and operators for the Sentinel-1 PDGS centers are (Ref. 135):

• 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).

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. ¹³⁸⁾

Figure 92 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.

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 assessed by several external validation institutions, which are a part of the Copernicus POD QWG (Quality Working Group). The purpose of the Copernicus POD QWG is to monitor the performance of the operational POD products (both the orbit products and 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 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 several external validation centers.

The Copernicus POD Service has been developed and it is operated by GMV. However, it interacts with different entities (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 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 to be re-processed by the external validation institutions (Ref. 138).

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 93 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. 139).

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 94 and 95) show the differences concerning 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 96 shows the results of the last comparison campaign of January 2016; it shows the 3D RMS per day, during 10 days in January 2016, between the operational Sentinel-2 NTC solution (CPOD) and the daily solutions provided by each institution. Additionally there is a combined solution (COMB) computed as a weighted average of all individual solution. It can be seen that the differences are systematically below 5 cm, like in the case of Sentinel-1A.

Copernicus / Sentinels EDRS system operations:

EDRS (European Data Relay Satellite) will provide a data relay service to Sentinel-1 and -2 and initially is required to support 4 Sentinels simultaneously. Each Sentinel will communicate with a geostationary EDRS satellite via an optical laser link. The EDRS GEO satellite will relay the data to the ground via a Ka-band link. Optionally, the Ka-band downlink is planned to be encrypted, e.g. in support to security relevant applications. Two EDRS geo-stationary satellites are currently planned, providing in-orbit redundancy to the Sentinels. ¹⁴⁰⁾

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.

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 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. ¹⁴¹⁾

• 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.

ESA Member States approved these principles in September 2009. ^{142) 143) 144) 145) 146)}

Sentinel-1 operational products:

• Level-0 products: Compressed, unprocessed instrument source packets, with additional annotations and auxiliary information to support the processing. ¹⁴⁷⁾

• 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.

• 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.

Preparatory campaigns:

In the context of the Copernicus program, ESA is conducting several 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.

AgriSAR 2009:

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 several 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. ¹⁴⁹⁾

IceSAR 2007:

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. ^{150) 151)}

AgriSAR 2006:

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. ^{152) 153) 154)}

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 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.

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17) Aniceto Panetti , Friedhelm Rostan, Michelangelo L’Abbate, Claudio Bruno, Antonio Bauleo, Toni Catalano, Marco Cotogni, Luigi Galvagni, Andrea Pietropaolo, Giacomo Taini, Paolo Venditti, Markus Huchler, Ramon Torres, Svein Lokas, David Bibby, “Copernicus Sentinel-1 Satellite and C-SAR Instrument,” Proceedings of the ESA Living Planet Symposium, Edinburgh, UK, Sept. 9-13, 2013 (ESA SP-722, Dec. 2013)

18) Aniceto Panetti, Friedhelm Rostan, Michelangelo L’Abbate, Claudio Bruno, Roberto Baldassarri, Antonio Bauleo, Toni Catalano, Marco Cotogni, Luigi Galvagni, Fabio Ippoliti, Pierluigi Petrini, Sergio Pica, Andrea Pietropaolo, Francesca Spataro, Giacomo Taini, Paolo Venditti, Mathias von Alberti, Ramon Torres, Svein Lokas, David Bibby, “Copernicus Sentinel-1: Satellite outline and early results,” Proceedings of the 65^th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14,B1,1.1

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57) ”Europe’s next radar Sentinel unfolds,” ESA Applications, 07 August 2020, URL: https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-1/Europe_s_next_radar_Sentinel_unfolds

58) ”Tap into Europe in motion,” ESA Application, 26 July 2022, URL: https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-1/Tap_into_Europe_in_motion

59) ”Copernicus Sentinel-1 maps Bangladesh flood,” ESA Applications, 30 June 2022, URL: https://www.esa.int/ESA_Multimedia/Images/2022/06/Copernicus_Sentinel-1_maps_Bangladesh_flood

60) ”What can satellites reveal about climate tipping points?,” ESA Applications, 26 May 2022, URL: https://www.esa.int/Applications/Observing_the_Earth/Space_for_our_climate/What_can_satellites_reveal_about_climate_tipping_points

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64) ”Monitoring crop health across the Netherlands,” ESA Applications, 8 February 2022, URL: https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel-1/Monitoring_crop_health_across_the_Netherlands

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76) ”Suez Canal traffic jam seen from space,” ESA Applications, 26 March 2021, URL: https://www.esa.int/ESA_Multimedia/Images/2021/03/Suez_Canal_traffic_jam_seen_from_space

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105) Adriano Meta, Pau Prats, Ulrich Steinbrecher, Josef Mittermayer, Rolf Scheiber, “TerraSAR-X TOPSAR and ScanSAR comparison,” Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany

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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 (eoportal@symbios.space).