Sentinel-6 Michael Freilich
Copernicus: Sentinel-6 Michael Freilich Mission — formerly Sentinel-6 / Jason-CS (Jason Continuity of Service) MissionSpacecraft Launch Mission Status Sensor Complement References
Jason-CS is the second component of the hybrid solution (Jason-3 + Jason-CS) agreed to in 2009. Jason-CS will ensure continuity with Jason-3 to guarantee adequate overlap with Jason-3. At least two satellites with a 7 years lifetime each (5 years + 2 years consumables) are planned to give time before new technologies such as swath interferometry (SWOT mission) can be considered as operational. 1) 2)
The Jason-CS satellite will carry a radar altimeter package to continue the high-precision, low-inclination altimetry missions of Jason-2 and -3. It will complement the high-inclination measurements on Sentinel-3 to obtain high-precision global sea-surface topography for the marine and climate user community.
The Jason-CS program constitutes EUMETSAT's contribution to the Copernicus Sentinel-6 mission to be developed and implemented through a partnership between the EU, ESA, EUMETSAT, NASA, and NOAA. From 2020 to beyond 2030, the Sentinel-6 mission will uniquely extend the climate record of sea-level measurements accumulated since 1992 by TOPEX/Poseidon, Jason-1 , Jason-2 , and Jason-3. A prime mission objective is to continue this long global sea-level time series with an error on the sea level trend of less than 1mm/year. The Sentinel-6 mission will also be an essential observing system for operational oceanography and seasonal forecasts in Europe and beyond. It will provide measurements of sea surface height, significant wave height, and wind speed without degradation in precision and accuracy compared to the currently flying Jason-2 mission. As such, like its predecessors, the proposed mission will provide key user measurement services for sea-level-rise monitoring, operational oceanography, and marine meteorology. These services will be aligned with those of the Sentinel-3 missions, which will be operational in the same era, see Figure 1. 9)
Figure 1: Overview of the current and future satellite altimeter missions (image credit: WMO, CEOS)
In addition to the altimeter data service, Sentinel-6 will also include a GNSS-RO (GNSS Radio Occultation) instrument as a secondary payload, taking advantage of the non-sun-synchronous orbit of Sentinel-6. The GNSS-RO measurements will provide information on atmospheric pressure, temperature and water vapor as well as ionospheric data. The radio occultation data service primarily addresses the needs of meteorological and climate users.
The Sentinel-6 mission program consists of two identical satellites (Jason-CS A and Jason-CS B) with each a nominal lifetime of 5.5 years and a planned overlap of at least 6 months. The satellites will be launched sequentially into the "Jason orbit" to take over the services of Jason-3 when this scheduled mission becomes of age. Currently, the launches of Jason-CS A and B are planned for 2020 and 2026, respectively.
Figure 2: Overview of the past, current and future satellite altimeter missions (image credit CNES)
Programmatic setup: 10)
Figure 3 outlines the multi-partner program and agreement setup underlying the Sentinel-6 missions. The European contribution will be implemented through the combination of the EU/ESA Copernicus program and the optional EUMETSAT Jason-CS program , for the joint benefits of the meteorological and Copernicus user communities in Europe. In addition, on behalf of the United States, NASA and NOAA are developing a dedicated Jason-CS program. The following high-level sharing of responsibilities is envisaged (which may still be subject to some changes):
• EUMETSAT is the system authority and is responsible for the Sentinel-6 ground segment development and operations preparation. EUMETSAT will also carry out the operations build-up and operations of the Sentinel-6 system including both satellites and delivery of data services to Copernicus service providers and users on behalf of the EU. Additionally EUMETSAT will fund S-6 B (together with the EU) and potentially part of S6 A as well.
• ESA is responsible for the development of the first Jason-CS satellite and the instruments prototype processors as well as for the procurement of the recurrent satellite on behalf of EUMETSAT, CNES and the EU. The industrial consortium strongly based on the CryoSat team. It will operate the satellite in the first few days after launch, until the basic check-out of the payload is complete. It is responsible also for the instruments prototype processors as well as for the procurement of the recurrent satellite on behalf of EUMETSAT and the EU.
• CNES (France) is providing expert support to the mission and system development. During operations will process data from the DORIS (Doppler-Orbitography-and-Radiopositioning-Integrated-by-Satellite) payload and provide precise orbits.
• The EU, through the EC (European Commission), will fund the procurement of S-6 B (together with EUMETSAT) and the operations for both A and B satellites.
• NASA will deliver the US payload instruments for both satellites and will provide ground segment development support, launch services, and contributions to operations.
• NOAA (National Oceanic and Atmospheric Administration) is providing ground stations to complement the EUMETSAT station and will process and distribute science data.
• NASA/JPL is developing the US payload instruments and procuring the launcher. NASA will also support the science team.
• The European Space Agency has selected Airbus Defence and Space as the prime contractor to develop and construct the two new satellites in Friedrichshafen, Germany.
Figure 3: The multi-partner program and agreement setup underlying the Sentinel-6 mission (image credit: Jason-CS collaboration)
The three space agencies will share the responsibility for the science team coordination and the calibration and/or validation activities, with EC being involved in the interactions with the science teams. In addition, agreements will be concluded between EUMETSAT and CNES and between NOAA and NASA for system and science expertise support.
Sentinel-6 will be a truly operational mission in all aspects of its main user services. Hence, full emphasis is put on reduction of downtime to a minimum, on timely distribution of data products, and on high quality and reliability of the measurement data. The mission will also include support to information service providers and major reprocessing activities.
The Sentinel-6 product suite is currently being detailed. The baseline is to provide a product suite that will enable an optimal combination with products from other altimeter missions. This is particularly pursued for combining Sentinel-6 with the Sentinel-3 Ku/C radar altimeter (SRAL) missions. Next to the conventional Level 2 products, known as GDRs (Geophysical Data Records) for the Jason missions, the Sentinel-6 product suite will include Level 1 products aimed at the further study of the intrinsic altimeter waveforms and development and innovative processing techniques. Also, the generation of higher-level single-mission products (Level 2P and Level 3) are supported in order to serve mainly the ocean modelling community.
Sentinel-6 products are to meet high standards, such that they will be of sufficient quality to serve as the high precision reference mission for other altimeter missions. It has been formally required that the mission performance shall not be worse than the known performance of Jason-2. With the current design, however, the expectation is that the Sentinel-6 mission will outperform Jason-2 on many aspects and will form a reliable state of the art reference for various other altimeter missions in the near future.
The Sentinel-6 products will also maintain their quality closer to the coastline than products from its predecessor Jason missions (e.g. Raney, 1998; Gommenginger et al., 2012; Halimi et al., 2014). 11) 12) 13) This, among other techniques, will be facilitated by replacing the conventional LRM (Low-Resolution Mode) altimeter by one that has along-track SAR (Synthetic Aperture Radar) capabilities.
The Sentinel-6 radio occultation products will contribute to operational weather forecasting and to assessments of atmospheric climate trends by providing information that allows to derive atmospheric temperature and water vapor profiles. In addition, ionospheric data are also provided up to 500 km altitude.
The Sentinel-6 Space Segment consists of two successive Jason-CS satellites (A and B), based on the CryoSat-2 heritage platform, with some tailoring to specific needs of the Sentinel-6 mission. The satellites will embark the following main payload:
• A radar altimeter (Poseidon-4), to measure the range between the satellite and the mean ocean surface, determine significant wave height and wind speed, and provide the correction for the altimeter range path delay in the ionosphere by using signals at two distinct frequencies (Ku-band and C-band).
• A microwave radiometer, called AMR-C (Advanced Microwave Radiometer-C) of JPL, to provide a correction for the wet tropospheric path delay for the altimeter range measurement.
• POD (Precise Orbit Determination) instruments – namely a GNSS (Global Navigation Satellite System) and precise orbit determination receiver (GNSS-POD), a DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) instrument, and a LRA (Laser Retroreflector Array) – to provide with high accuracy and precision a measurement of the orbital position as needed for the conversion of the measurement of altimeter range into a sea level.
• GNSS-RO (GNSS- Radio Occultation) instrument to provide (with high vertical resolution) all-weather atmospheric and ionospheric soundings by tracking GNSS satellites.
The GNSS-RO instrument is added to Sentinel-6 as a secondary mission to provide radio occultation observation services to meteorological users. However, the primary altimeter mission supported by the other instruments takes priority in all design and mission planning.
It is important to remark that the Poseidon-4 radar altimeter has evolved significantly from the Poseidon-3A and -3B instruments on board Jason-2 and -3, respectively. In addition to a conventional pulse-width limited processing, also known as low-resolution mode, the Poseidon-4 on board the Jason-CS satellites will also have the facility of simultaneous high-resolution (HR) processing, generally referred to as SAR (Synthetic Aperture Radar) mode processing. This HR processing will provide further service alignment with the SAR mode of the Sentinel-3 SRAL mission.
The Jason-CS satellites will fly in the same orbit as their predecessors, TOPEX/Poseidon and the Jason missions (Table 2). This is a non-sun-synchronous orbit with a nominal altitude of 1336 km and 66º inclination. The orbit period is 112 min and 26 s and the ground track cycle repeats approximately every 9 days and 22 hours. Because of the relatively large ground track spacing of 315 km at the equator, Jason-CS alone will not be able to satisfy the sampling requirements for mesoscale oceanography. Thus, the Sentinel-6 mission is coordinated with other altimeter missions, chiefly the Sentinel-3 mission, to provide together a complementary spatiotemporal sampling of the oceans and serve as a high-precision reference to sea-level-change studies.
A NASA/JPL presentation of ocean altimetry
• November 6, 2020: From a ship, a plane, or the beach, the oceans can look pretty flat and uniform. But in reality, the water in the ocean piles up in peaks and valleys. It stands higher on some shores than on others. It can slosh around in ocean basins like the water in a bathtub. The surface of the ocean rises and falls naturally, varying as much as 2 to 3 meters in places. 14)
- Scientists also know that the overall level of the sea has been rising around the world, and more in some places than others. They estimate that over the past 140 years, global mean sea level has risen 21 to 24 cm.
- There are many reasons why the ocean surface is lumpy. The friction between winds and water causes waves to build up. The tug of gravity from the Moon and Sun causes tides to rise and fall. The rotation of Earth (Coriolis effects) and the flow of currents also amass water in vast streams. Atmospheric pressure pushes and pulls on the water surface. Continents, islands, and even underwater seamounts exert a gravitational tug that draws water up around them.
- We also know that seawater of different temperatures and salinities (salt content) can be more or less dense, filling more or less volume. For instance, scientists have known for decades that sea level is generally higher in the Pacific than in the Atlantic—about 20 cm — because Pacific waters are usually warmer, fresher, and less dense.
Figure 4: New U.S.-European Satellite Tracking Sea Level Rise. The joint U.S.-European Sentinel-6 Michael Freilich is the next in a line of Earth-observing satellites that will collect the most accurate data yet on sea level and how it changes over time. With millimeter-scale precision, data from this mission will allow scientists to precisely measure sea surface height and gauge how quickly our oceans are rising (video credit: NASA/JPL/Caltech/NOAA)
- We know these things because we can measure them. For more than four decades, scientists have used satellite-based instruments known as radar altimeters to monitor ocean surface topography—the shape and height of the ocean's peaks and valleys. Radar altimeters continually send out pulses of radio waves (microwaves) that bounce off the surface of the ocean and reflect back toward the satellite. The instrument calculates the time it takes for the signal to return, while also tracking the precise location of the satellite in space. From this, scientists can derive the height of the sea surface directly underneath the satellite.
- Long before there were satellites, scientists measured the height of the sea with tide gauges mounted in coastal bays and harbors. Collected in some places since the early 19th century, these records have provided one way to detect changes in the coastal ocean. But since landmasses and islands are unevenly distributed among the world, and tide gauges tend to be clustered on the shores of wealthier countries, the view has been limited. Still, there is value in long-term records, and readings from more than 1500 tide gauges have been compiled and analyzed by research groups like the Permanent Service for Mean Sea Level. Their data help corroborate what satellites observe.
- In the Space Age, altimetry satellites have been building upon the tide gauge records. Since 1992, four missions have used very similar instruments and have repeated the same orbit every ten days: TOPEX/Poseidon (1992-2006), Jason-1 (2001-2013), Ocean Surface Topography Mission/Jason-2 (2008-2019), and Jason-3 (2016 to present). The missions were built through various partnerships between NASA, France's Centre National d'Etudes Spatiales (CNES), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), the European Space Agency, and the U.S. National Oceanic and Atmospheric Administration (NOAA).
- Known to the science community as the "reference missions," these altimetry satellites have been making standardized measurements of the fluctuations of sea level near and far. They provide a unified ocean topography record and the equivalent coverage of a half-million tide gauges. (Other altimetry missions employ different approaches and orbits to study ocean topography and further complement this record.) Two more successor satellites have been built to extend this reference record for another decade; the first of these, Sentinel-6 Michael Freilich, is scheduled to be launched in late 2020.
- Spotting a few millimeters of change amid the dynamic churning of the ocean is a challenge. The satellite has to look down through 1300 kilometers of atmosphere. While clouds are no trouble for radar—which penetrates cloud cover—the amount of moisture in the air slows down the radio signal and can make the ocean appear higher or lower than it actually is. To compensate for this, engineers have built instruments into the satellites to measure water vapor and account for its effects.
- Another challenge is knowing the exact height of the satellite—researchers call it "precise orbit determination." Each altimetry satellite has reflectors that can bounce laser signals from ground stations to measure altitude. The satellites also have Doppler and Global Positioning System receivers to further pinpoint location. The goal is to know exactly how far the satellite is from the center of the Earth at any moment. Finally, the orbital pattern takes the satellites directly over tide gauge stations on the French island of Corsica and an oil rig off of California to simultaneously measure sea level from above and at the surface every ten days.
Figure 5: Overview of PSMSL (Permanent Service for Mean Sea Level) tide gauge locations in 2020 (image credit: NASA Earth Observatory images by Joshua Stevens,using tide gauge data from PSMSL. Story by Michael Carlowicz, with science interpretation by Ben Hamlington/NASA JPL, Richard Ray/NASA Goddard, and Josh Willis, NASA/JPL)
Figure 6: Tide Gauges and Satellites agree: Global Mean Sea Level is Rising. The map shows the observed change in sea level from 1996-2016 in mm (image credit: NASA Earth Observatory using tide gauge data from PSMSL)
- Even when scientists account for all of the variables in measuring sea level, the planet offers more complications: sea surface patterns and rhythms that can span years and decades. Climate patterns such as El Niño and La Niña, the Pacific Decadal Oscillation, the North Atlantic Oscillation, and the Indian Ocean Dipole all cause water to warm or cool, rise and fall, and slosh around the ocean basins. Even major current systems can speed up or slow down.
- Scientists have accounted for that, too. By analyzing sea surface data over long periods and noting the occurrence of major events like El Niño, they can identify and remove the natural cycles to spot the comparatively small changes in overall sea level. This is why radar altimeters are now in their fifth generation: they have collectively accumulated a data record that is longer than the seasonal, yearly, and even decadal cycles.
- What scientists have found after all of that data gathering and cross-checking is that global mean sea level has risen a total of 95 mm since TOPEX-Poseidon first started flying in 1992. And the rate is accelerating. Over the course of the 20th Century, sea level rose at about 1.5 mm per year; in the early 1990s, the rate was about 2.5 mm per year. Over the past 30 years, the average rate has increased to 3.4 mm per year.
- That total rise in seal level is a global average, and the numbers can be significantly higher in some places (see the map of Figure 63). For instance, researchers have observed that sea level along much of the East Coast of North America has been rising faster than the global average.
- While a few mm of higher water may seem small, scientists estimate that every 25 mm of sea level rise translates into 2.5 m of lost beach along our coasts. It also means that high tides and storm surges can rise even higher, bringing more coastal flooding, even on sunny days. Some estimates suggest seas could rise another 650 mm by the year 2100 if Earth's ice sheets and glaciers keep melting and its waters keep warming.
- Ocean altimeters alone cannot tell us why seas are rising; other instruments and data sets are needed to tell us that. But together with tide gauges, these satellites tell us clearly that our planet is changing. And they help us see more clearly where that is happening.
ESA has selected Airbus DS as the prime contractor to develop and construct the two new satellites in Friedrichshafen, Germany. The development is well advanced and the project is going into the integration phase. Sentinel-6/Jason-CS satellites are designed to orbit for minimum 5.5 years each and will ensure measurements carried out on a continuous basis from 2020 onwards, with better performances in respect to earlier Jason series. The satellites will measure their distance to the ocean surface with an accuracy of a few centimeters, from an altitude of 1,336 km (Ref. 10). 15)
Sentinel-6 /Jason-CS will be an essential observing system for sea-level-rise monitoring, coastal zones altimetry, operational oceanography, seasonal forecast and marine meteorology. The two identically equipped A and B satellites are designed for a mission lifetime of 7.5 years and a planned overlap of at least 1.5 years. The S-6 satellites will give time before new technologies, such as the Interferometric Synthetic Aperture Radar (SWOT mission), will be consolidated (Ref. 9), which is currently expected to happen in the second half of the ‘20 decade.
Satellite System Design Overview:
Taking into account the Sentinel-6 mission objectives, satellite system requirements (SSRD), operational interface requirements (OIRD) and considering the following payload complement elements:
- Poseidon-4 SAR Radar Altimeter (POS4),
- Microwave Radiometer AMR-C,
- DORIS Receiver and Antenna,
- GNSS-POD Receiver and Antennas,
- LRA (Laser Retroreflector Array),
- REM (Radiation Monitoring Unit),
A set of major design drivers have been considered for the design of S6 satellites. These design drivers can be summarized as follows:
- Stringent center of mass knowledge and stability requirements until the end of the mission
- Accommodation of major payload elements with nadir pointing antennas and radiators
- Payload pointing and co-alignment accuracy
- End-of-life reentry and post-mission disposal
- Power / thermal / mechanical design adapted to the drifting orbit conditions
- Modular approach for assembly and testing
- Use of off-the-shelf equipments for the platform as far as possible for risk mitigation
- Harsh space radiation environment.
Mechanical Architecture and Configuration: As a result of these conditions a compact satellite body (Figure 7) has been selected based on the design principles from other missions designed for drifting orbits, like CryoSat-2. Since the majority of instruments requires nadir pointing of their antennas and thermal radiators, the principle dimensions of the satellite structure are vastly pre-determined by their size.
S-6 has a total length of 5085 mm (along Xsc), a height of 2349 mm (along Zsc) and a width of 2581 mm (along Ysc) in stowed configuration. The S/C dry mass with margin, is 1039 kg. The launch mass, including system margin and propellant mass, is 1362 kg, fully compatible also with the smaller among the proposed launchers (Antares).
Figure 7: S-6 deployed mechanical configuration (image credit: Airbus DS)
Two fixed Solar Arrays (SA) are located in the form of a tent. Two additional deployable solar panels are released by simple passive deployment mechanisms. The distribution of equipments has been determined mainly by the following constraints:
- Free fields of view for the instruments and short distance between the ones needing stable alignment.
- Short distance for RF path and reduction of RF interferences.
- Accommodation of the high dissipating equipments on a nadir panel and far from alignment critical payload elements.
- Accommodation of the monopropellant fuel tank close to the satellite's launcher interface.
- Distribution of units to control the overall center of mass.
Figure 8: S-6 mechanical configuration (nadir view), image credit: Airbus DS
The POS4 (Poseidon-4 Radar Altimeter) is the main instrument of the S-6/Jason-CS mission. Its redundant electronic units are mounted on the nadir pointing Main Payload Panel, with a large thermal radiator. The antenna itself is mounted almost isostatically to the Payload main panel that embeds heat pipes in order to comply with stringent temperature stability requirements of the Altimeter. The AMR-C Radiometer and the Star Trackers are mounted on the Payload front panel. The Payload Panel supporting the redundant RA (Radar Altimeter) is designed as a module to be assembled and tested independently.
Stability of alignment between Altimeter antenna, Star Trackers and Radiometer are guaranteed by the close distance resulting in similar temperatures and low relative thermal distortions.
The core elements of the satellite are installed in the bus section, the majority of the instruments instead are located in the payload section (Figure 8). These show significant thermal dissipation and unit masses, hence are accommodated on the dissipating nadir panels to achieve their operating temperatures and to balance the satellite center of mass. Data exchange is done with an X-band and an S-band systems located on the nadir panel. Nearby are located the DORIS receiver and antenna for precise position determination.
The MPPS (Mono-Propellant Propulsion System) items are mounted on a separate support structure. Therefore the MPPS can be assembled and tested separately from the satellite AIT sequence, then finally inserted into the launcher interface ring adapter. To cope with the stringent center of mass knowledge requirement, dedicated metal ring elements are installed inside the tank to control the gas bubble of the pressurant during the mission.
The redundant European GNSS-POD and its antennas are accommodated on the zenith panel. Regarding the US GNSS-RO, one antenna is mounted in zenith direction (GNSS-RO-PA), one in flight (GNSS-RO fore antenna) and one in anti-flight direction (GNSS-RO aft antenna).
Figure 9: Internal view of the S-6 mechanical configuration (image credit: Airbus DS)
The S-6 LRA is accommodated on the nadir plate of the satellite close to the center of gravity. The REM (Radiation Environment Measurement Unit) has been lately introduced as experimental payload and placed, outside, on the front panel. All structure panels are made of aluminum sandwich. The solar array panels are made of CFRP (Carbon Fiber Reinforced Polymer) facesheets and aluminum honeycomb.
TCS (Thermal Control Subsystem): The TCS design of the S-6 satellites incorporates passive and active elements. The passive elements are MLI (Multi Layer Insulation) blankets and dedicated radiators covered with SSM (Secondary Surface Mirrors) providing a rather homogeneous environment for heat rejection towards Earth. The main structure is partly painted black internally in order to minimize temperature gradients inside the structure. For active temperature control, heaters are implemented in dedicated areas.
Electrical and Functional Architecture: The "Electrical System" of the S-6 satellite comprises all the necessary hardware to operate the satellite, and to execute the software. This covers the following functional chains:
• EPS (Electrical Power System). Including:
- PCDU (Power Control and Distribution Unit, ESP)
- Batteries (UK)
- Solar Arrays (GER/NL/IT/USA)
- Harness (ESP).
• Data Handling System. Including:
- OBC (SWE) including: OBC Electronics (OBC-E) including TCAU (TC Authentication Unit). OBC Boot and Basic IO SW.
- RIU (Remote Interface Unit, FIN) including AOCS electronics.
• AOCS (Attitude and Orbit Control Subsystem) Including:
- Reaction Wheels (RW, GER)
- Magnetic Torquers (MTQ, GER)
- Magnetometers (MAG, GER)
- Coarse Earth Sun Sensors (CESS, GER)
- Rate Measurement Unit (RMU, FRA)
- Star Tracker (STR, GER) including electronics, optical head and baffles
- GNSS-POD (AT).
• Reaction Control System (RCS, UK). Including:
- Pressure Transducers (PT, NL), Flow Control Valves (FCV) including Catalyzer Bed Heaters (CBH), Latch Valves (LV), Thermocouples and Temperature Sensors.
• Payload Data-Handling and Transmission (PDHT). Including:
- MMFU (Mass Memory and Formatting Unit, IT)
- X-band System (XBS, GER/SWE).
• Tracking, Telemetry and Command System (TTC, ESP/SWE). Including S-band transponder and antennae.
• The instrument complement including: POS-4, DORIS, REM, AMR-C and GNSS-RO.
• Plus the instrument and system harness.
The electrical architecture chosen for S-6 applies the Electrical Interface Standardization for satellite architectures successfully implemented by Airbus in many recent programs, and in very close commonality with Sentinel-2 and the Airbus internal Astrobus concept. The architecture shows compliance at optimal cost and risk plus demonstrating reliable heritage.
EPS (Electrical Power Subsystem): The EPS generates electrical power in sunlight by operating the 17.5m2 body mounted solar array at its maximum power point. It can provide nearly 5.5 kW at BOL (Begin Of Life), about 1 kW average in flight. The EPS manages the charge and discharge of the Li-Ion battery based on 1152 cells, split into two modules, for a total of 147 Ah EOL (End Of Life).
The unregulated main-bus (29.5 - 33.6 V) is managed according the MPPT (Maximum Power Point Tracking) concept and the batteries are directly connected to it. Via LCLs (Latching Current Limiters), the EPS provides main-bus overvoltage and undervoltage protection and distributes protected unregulated primary power to all the satellite users. - The EPS provides also a hot redundant failure handling function, control of the heaters and passivation at EOL via leak path.
DHS (Data Handling Subsystem): The DHS is in charge of the overall satellite command and control including AOCS algorithms. It is running the on-board SW and FDIR (Fault detection, Isolation and Recovery). The DHS distributes ground and software issued commands to the satellite and collects the satellite housekeeping telemetry.
The platform and payload units are connected with the OBC each through dedicated MIL-buses and to the RIU (Remote Interface Unit) via discrete I/O interfaces. Direct telecommands and essential telemetry links are implemented to enable ground to directly command the various on-board subsystems and units.
The DHS comprises two internally redundant units, the OBC and the RIU. It includes a small mass memory, but the main one is a dedicated MMFU that is part of the PDHT system.
Each OBC side is composed by three main sub units:
• TTR (M) [Telemetry, Telecommand, Reconfiguration and mass memory] providing TM/TC handling, failure handlings, Timing and Synchronization and a small Mass Memory.
• Processor module based on SPARC ERC32, providing computation, Watch Dog Timer and communication via MIL and SpW buses.
• Power Converter Module, providing internal secondary power, High Power command, Relay Status reading and analogue signal management.
The OBC can send HPC-SHP (High Priority High Power Commands) to various equipments in order to allow their switching by direct commanding from ground without the need of software.
The RIU comprises several modules. While the "Core" part of the RIU is providing the standard I/O I/F, there are additional modules to control the non-standard functions.
AOCS (Attitude and Orbit Control Subsystem): The AOCS is responsible for the satellite's attitude and orbit control through the following functionalities: rate damping, vector sun acquisition, safe mode control, fine pointing of the payloads in nominal mode (with GNSS-POD support) and orbit control maneuvers.
Several individual sensors and actuators are necessary to carry out this task: RW, MTQ, CESS, MAG, RMU, STR and GNSS-POD. Some communicating via the MIL-bus, others via discrete TM/TC lines.
MPPS (Mono-Propellant Propulsion Subsystem): The MPPS uses hydrazine propellant. It is assembled with two independent, cold redundant branches each ending in four 8 N thrusters. For safety reasons, every thruster has two independent actuators in series. Each thruster is equipped with two CBH (Catalyzer Bed Heaters) and a PT 100 thermistor.
PDHT (Payload Data Handling and Transmission): The PDHT system consists of the internally redundant MMFU(Mass Memory and Formatting Unit) and XBS (X-band System). The MMFU is a standalone solid mass memory based on SDRAM (Synchronous Dynamic Random Access Memory) technology with 352 Gbit EOL capacity. It receives data from both the RA and the OBC (collecting from all the other data providers) via SpaceWire links. It manages and stores the incoming data in packet stores, on APID (Application Process ID) bases, and allows read and write accesses at the same time. The read data are formatted and routed on demand to either the XBS sides.
The XBS consists of the redundant X-band XDA (Downlink Assembly) and the X-band antenna. The XDA modulates the data onto the X-band carrier for transmission to the ground, transmitting them at 150 Mbit/s. The XBS is used only for scientific and telemetry data.
TT&C (Tracking, Telemetry & Command): The TT&C is a conventional S-band system for telecommand, telemetry and ranging consisting of two S-band RX/TX transponders (with a ranging channel), one hemispherical antenna (nadir) for nominal communications, one hemispherical antenna (zenith) and one hybrid coupler to simultaneously connect the antennas to both transponders. It is also used for telemetry data, during LEOP (Launch and Early Orbit Phase). -The data rates are 16 kbit/s in uplink and 32 kbit/s LR (Low data Rate) or 1 Mbit/s (high data rate, HR) in downlink.
Redundancy concept and implementation: The essential I/Fs (Interfaces) are double cross-strapped provided (with nominal and redundant driver and receiver functions, with 2 I/Fs each and external cross-strap). E.g. MIL and SpW buses. The standard I/Fs are cross-strapped inside RIU and OBC (with nominal and redundant driver and receiver functions, with one interface each and internal cross-strap on master side only). E.g. Discrete High Priority TM/TC. - A few special actuators are redundant but not cross-strapped.
Satellite SW Systems: The S-6 software system is distributed across the spacecraft. It consists of at least 7 different SW systems embedded in different units:
• OBC SW: it is embedded into the OBC. It is the master system data management and control unit. The SW performs the communication with the ground and comprises AOCS, thermal, system and data handling controls.
• MMFU Control SW: commands, controls and monitors the data flow and storage.
• Star Tracker SW: determines the 3-axes attitude.
• RA instrument Control SW: schedules the operational modes, executes the acquisition and tracking algorithms and manages the calibration mode.
• AMR-C instrument Control SW: measures the three bands signals, applies antenna pattern correction and performs the regular calibration.
• GNSS-POD Receiver Electronics SW: acquire the GNSS signals and computes the real-time navigation solutions.
• REM SW: performs the radiation measurement and periodic instrument calibration.
Figure 10: Sentinel-6 SW components diagram (image credit: Airbus DS)
Launch: The Sentinel-6 Michael Freilich satellite was launched on 21 November 2020 (17:17 UTC) on a Falcon-9 Block 5 vehicle of SpaceX from SLC-4A at Vandenberg Air Force Base, CA, USA. The Copernicus Sentinel-6 mission is a true example of international cooperation. While Sentinel-6 is one of the European Union's family of Copernicus missions, its implementation is the result of the unique collaboration between ESA, NASA, EUMETSAT and NOAA, with contribution from the French space agency CNES. 16) 17)
Figure 11: The Sentinel-6 Michael Freilich ocean observation satellite lifted off on a SpaceX Falcon 9 rocket from Space Launch Complex 4E at Vandenberg Air Force Base in California at 9:17 a.m. PST (12:17 p.m. EST) Saturday, Nov. 21, 2020 (image credit: NASA TV)
In October 2017, NASA selected SpaceX of Hawthorne, California, to provide launch services for the Sentinel-6A mission. The launch is currently targeted for November 2020, on a SpaceX Falcon 9 Full Thrust rocket from SLC-4E (Space Launch Complex 4E) at Vandenberg Air Force Base in California.18)
Orbit: The nominal orbit for S-6 is the same of the precedent missions (TOPEX/Poseidon, Jason-1 to -3) ensuring data consistency with the previously acquired time series. The Jason missions operate from a relatively high altitude (1336 km) prograde orbit with an inclination of 66º. The main orbit parameters are reported in Table 2.
Table 2: Parameters of the Sentinel-6 Michael Freilich orbit
Kiruna and Fairbanks (with Wallops as backup) are chosen as S- and X-band ground stations for sizing purposes but do not necessarily represent the final choice. Figures 12 and 13 show the intersections of reception cones of exemplary ground stations and the S-6 ground track. Considering the exemplary ground stations, the mean contact time will be 16 min with 76 min contact gap.
Figure 12: Reception cones of ground stations (image credit: Airbus DS)
Figure 13: Ground Track (red) and Kiruna (green) / Fairbanks (blue) Visibility Cones (image credit: Airbus DS)
• June 8, 2022: Satellite images of our planet have become essential to our survival, offering a new outlook of our world. With rising seas being one of the biggest threats to society, satellite altimeter missions such as Copernicus Sentinel-6 are essential in monitoring global and regional changes in sea level. 19)
Figure 14: Unbeknown to many, the island of Crete, Greece, plays an important role in the Copernicus satellite altimetry constellation and on an international stage. Satellite altimetry data have to be continuously monitored at the ESA's Permanent Facility for Altimetry Calibration where different techniques have pioneered the use of transponders to provide the best measurements to validate satellite altimeters in space soon after launch. - This documentary explains how measurements are taken from the top of the White Mountains to make sure users get the best data on sea height from satellite altimetry. - It features interviews with Craig Donlon, Head of ESA's Earth Surfaces and Interior Earth and Mission Science Division and Stelios Mertikas, Director of Laboratory of Geodesy & Geomatics Engineering at the Technical University of Crete (video credit: ESA/AORI Films)
• On March 22, 2022, the newest U.S.-European sea level satellite, named Sentinel-6 Michael Freilich, became the official reference satellite for global sea level measurements. This means that sea surface height data collected by other satellites will be compared to the information produced by Sentinel-6 Michael Freilich to ensure their accuracy. 20)
- Launched from Vandenberg Air Force Base in November 2020, the satellite is continuing a nearly 30-year legacy started by the TOPEX/Poseidon satellite, which began its mission to measure sea surface height in the early 1990s. A series of successor satellites have carried on the effort since then, with Sentinel-6 Michael Freilich being the most recent. Its twin, Sentinel-6B, is slated to launch in 2025.
- These missions, of which Sentinel-6 Michael Freilich is the latest, are the gold standard when it comes to sea level measurements, which are critical for understanding and monitoring climate change," said Josh Willis, Sentinel-6 Michael Freilich project scientist at NASA's Jet Propulsion Laboratory in Southern California.
- Long-term records of sea level height are key to monitoring how much, and how fast, the oceans are rising in a warming climate. "We can't lose track of how much sea level has gone up because if we do, it's hard to predict what's going to happen in the decades to come," Willis added.
Figure 15: Meltwater from Greenland glaciers like the one pictured can contribute significantly to sea level rise. Sentinel-6 Michael Freilich monitors the height of Earth's oceans so that researchers can better understand the amount and rate of sea level rise (image credit: NASA Earth Observatory using Landsat data from USGS)
- "The unprecedented accuracy of the sea level measurements provided by this mission ensures not only the continuity of a 30-year data record, but allows improving our understanding of climate change and the impact of rising seas on coastal areas and communities," said Julia Figa Saldana, ocean altimetry program manager at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT).
- After Sentinel-6 Michael Freilich launched, it settled into orbit flying 30 seconds behind its predecessor, Jason-3. Science and engineering teams have spent the time since launch making sure Sentinel-6 Michael Freilich was collecting the intended data and that the information was accurate. Some of the initial data was made available last year for use in tasks like weather forecasting. And after further validation, the scientists agreed that Sentinel-6 Michael Freilich should become the reference satellite for sea level measurements.
- Later this year, teams will move Jason-3 into what's called an interleaved orbit. From that new position, the ground track – or the strip of Earth that Jason-3's instruments see as the satellite travels around the planet – will run in between the ground tracks of successive orbits for Sentinel-6 Michael Freilich. Jason-3 will keep measuring sea level height from the interleaved orbit, although it will no longer serve as the official reference sea level satellite. But by continuing to collect sea level data, Jason-3 will essentially double the number of measurements seen by each pass of Sentinel-6 Michael Freilich, helping to greatly increase the spatial resolution of sea level measurements provided by both satellites.
• December 13, 2021: For the second year in a row, the cooler sister to El Niño showed up at the winter party in the Eastern Pacific. La Niña is expected to stick around until at least spring 2022 in the Northern Hemisphere. 21)
- Part of the El Niño-Southern Oscillation cycle, La Niña appears when energized easterly trade winds intensify the upwelling of cooler water from the depths of the eastern tropical Pacific, causing a large-scale cooling of the eastern and central Pacific ocean surface near the Equator. These stronger than usual trade winds also push the warm equatorial surface waters westward toward Asia and Australia. This dramatic cooling of the ocean's surface layers then affects the atmosphere by modifying the moisture content across the Pacific. This La Niña coupling of the atmosphere and ocean alters global atmospheric circulation and can cause shifts in the path of mid-latitude jet streams in ways that intensify rainfall in some regions and bring drought to others.
Figure 16: This image shows conditions across the central and eastern Pacific Ocean as observed from November 26 to December 5, 2021, by the Sentinel-6 Michael Freilich satellite and analyzed by scientists at NASA's Jet Propulsion Laboratory (JPL). The globe depicts sea surface height anomalies. Shades of blue indicate sea levels that were lower than average; normal sea-level conditions appear white; and reds indicate areas where the ocean stood higher than normal. The expansion and contraction of the ocean surface is a good proxy for temperatures because warmer water expands to fill more volume, while cooler water contracts (image credit: NASA Earth Observatory images by Joshua Stevens, using modified Copernicus Sentinel data (2021) processed by the European Space Agency courtesy of Josh Willis/NASA/JPL-Caltech, and information adapted from the Famine Early Warning Systems Network. Story by Michael Carlowicz)
- In the western Pacific, rainfall can increase dramatically over Indonesia and Australia during La Niña. Clouds and rainfall become more sporadic over the central and eastern Pacific Ocean, which can lead to dry conditions in Brazil, Argentina, and other parts of South America and wetter conditions over Central America. In North America, cooler and stormier conditions often set in across the Pacific Northwest, while weather typically becomes warmer and drier across the southern United States and northern Mexico. (These and other trends are reflected in the map lower in this story.)
- "This moderate strength La Niña can be seen in the Sentinel-6 data as an area of lower-than-normal sea level along and below the Equator in the central and eastern Pacific," said Josh Willis, a climate scientist and oceanographer at JPL. He noted that the deep trough (blue) above the Equator is not the La Niña water mass; it is a shift in the North Equatorial Counter Current, which tends to strengthen during La Niña events.
- "This La Niña probably means bad news for the American Southwest, which should see lower than normal rainfall this winter," Willis said. "This La Niña may not be a whopper, but it's still an unwelcome sign for an area already deep into a drought."
Figure 17: This La Niña is the first to be observed by Sentinel-6 Michael Freilich, which was launched in November 2020. "The new satellite is giving us a great picture of this La Niña," Willis said. "With the public release of the mission's climate-quality data, we are now in a position where Sentinel-6 Michael Freilich can soon take over the climate record of sea level rise, which goes all the way back to the early 1990s." (image credit: NASA Earth Observatory)
- The La Niña event that started in late 2020 fits into a larger climate pattern that has been going on for nearly two decades—a cool (negative) phase of the Pacific Decadal Oscillation (PDO). During most of the 1980s and 1990s, the Pacific was locked in a PDO warm phase, which coincided with several strong El Niño events. But since 1999, a cool phase has dominated. The long-term drought in the American Southwest coincides with this trend, Willis noted.
- In a report released on December 9, 2021, the NOAA Climate Prediction Center noted that sea surface temperatures in November in the eastern tropical Pacific ranged from 0.7 to 1.2 degrees Celsius below the long-term average and 0.9°C below average in the Niño 3.4 region of the tropical Pacific (from 170° to 120° West longitude). Forecasters predicted La Niña conditions would persist through Northern Hemisphere winter, with a 60 percent chance that the ocean would transition back to neutral conditions during the April through June period.
- This La Niña is the first to be observed by Sentinel-6 Michael Freilich, which was launched in November 2020. "The new satellite is giving us a great picture of this La Niña," Willis said. "With the public release of the mission's climate-quality data, we are now in a position where Sentinel-6 Michael Freilich can soon take over the climate record of sea level rise, which goes all the way back to the early 1990s."
- Engineers and scientists have spent the past year calibrating and analyzing data from the new satellite against the existing Jason-3 mission. The team is ensuring that the new, more advanced data correlate properly with long-term records. New, high-resolution Sentinel-6 Michael Freilich data sets were released at the end of November 2020.
• November 29, 2021: Sea-level rise is one of the most immediate consequences of climate change, as highlighted recently through urgent pleas from leaders of island nations at the COP26 summit. Global measures of sea-level rise are imperative to underpinning global policy and for strategies to protect coastlines and low-lying lands. Measuring tiny differences in the height of the sea surface from space is no easy task – but that's exactly what the Copernicus Sentinel-6 Michael Freilich satellite is doing. And, after a year of exhaustive testing, this new mission is now delivering the world's most accurate data on sea-level rise. 22)
- Launched on 21 November 2020, Copernicus Sentinel-6 Michael Freilich uses the latest radar altimetry technology, developed by ESA, to advance the long-term record of sea-surface height measurements that began in 1992 by the French–US Topex-Poseidon satellite and that were followed by the Jason series of satellite missions.
- With the importance of monitoring sea-level rise so high on the global agenda, numerous organisations have been involved in making Copernicus Sentinel-6 the gold standard reference mission to take the record of sea-surface height measurements into the future, and to do this with greater precision than ever before.
- So while Sentinel-6 is one of the European Union's family of Copernicus missions, its implementation is the result of an exceptional cooperation between the European Commission ESA, Eumetsat, NASA and NOAA, with support from the CNES French space agency.
Figure 18: The Copernicus Sentinel-2 mission takes us over the Tarawa Atoll in the Republic of Kiribati – a remote Pacific nation threatened by rising seas. The Republic of Kiribati is an independent island nation consisting of some 33 atolls near the equator in the central Pacific. The islands are spread over approximately 3.5 million sq km of ocean, but with a total land area of only 800 km2. - Tarawa Atoll, pictured here, lies approximately halfway between Hawaii and Australia. Tarawa consists of a large lagoon fringed by a V-shaped reef, around 35 km long, and is made up of more than 30 islets. Tarawa, the site of a brutal World War II battle, is divided into North and South Tarawa. - South Tarawa, is made up of a thin, string of islets joined by causeways and is home to more than half of Kiribati's 100,000 citizens. Bonriki International Airport, serves as the main gateway to the country, and can be seen in the bottom right of the image (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO) 23)
- Tarawa Kiribati Figure description continued: Kiribati is one of the lowest-lying nations in the world, with many of the country's atolls and coral islands rising no higher than 2 m above sea level – making them extremely vulnerable to sea level rise. Kiribati has already seen growing damage from storms and flooding. In 1999, two of the nation's unpopulated islets, Tebua Tarawa and Abanuea, disappeared underwater entirely.
- The Special Report on the Ocean and Cryosphere in a Changing Climate on sea level rise states that the global mean sea level is likely to rise between 0.29 m and 1.1 m by the end of this century. While this may not sound like a lot, small island nations, including Kiribati, will face particularly devastating consequences.
- Small changes in sea-level rise will not only cause flooding, erosion, soil contamination and coral degradation, but will ultimately shrink more of Kiribati's land area – displacing many of its inhabitants.
- It is vital that over the coming decades, the changing height of Earth's sea surface continues to be closely monitored. Set to launch in November, the Copernicus Sentinel-6 Michael Freilich satellite will accurately measure changes in global sea level. Mapping up to 95% of Earth's ice-free ocean every 10 days, it will provide key information on ocean currents, wind speed and wave height for maritime safety.
- This new satellite will assume the role as a reference mission, continuing the ‘gold standard' record for climate studies started in 1992 – extending the legacy of sea-surface height measurements until at least 2030. For more information, visit the Copernicus Sentinel-6 website.
- Back on Sentinel-6 page (Ref. 22): Eumetsat – the European Organisation for the Exploitation of Meteorological Satellites, is responsible for operating the satellite and for making the data available to users.
- The first data products, which were low-resolution products, were released in June. This was an important step in the transition to the higher-resolution products being released today. The data have been used for weather forecasting and seasonal forecasting models, and to forecast the development and track of hurricanes.
- Sentinel-6's Poseidon-4 altimeter was designed to bring new high-resolution Ku-band synthetic aperture radar measurements into the altimetry reference time series. It, therefore, delivers simultaneous low-resolution measurements and high-resolution measurements. The low-resolution measurements are matched with measurements from the mission's predecessor, Jason-3, and, therefore, critical to ensuring continuity, and the enhanced high-resolution data can then be offered with absolute confidence.
Figure 19: The Copernicus Sentinel-6 Poseidon-4 dual-frequency (C- and Ku-band) radar altimeter uses an innovative interleaved mode that has improved performance compared to previous satellite altimeter designs. The Sentinel-6 radar altimeter derives its height above Earth by measuring the time it takes for a transmitted radar pulse to reflect off Earth's surface. Over the ocean, the returned echo pulse provides a waveform. The shape of the waveform is used to determine the sea-surface height from the radar range, the significant wave height, from the slope of the waveform leading edge, and the surface wind speed from the ocean roughness expressed by the power of the radar echo. - Unfocussed synthetic aperture radar processing exploits the Doppler characteristics of the return echoes. Synthetic azimuth beams are formed and steered to a specific location on Earth's surface to build a stack of waveforms. By averaging the stack of waveforms, measurement noises are reduced and the along-track resolution is improved from several kilometers to about 300 m. -Radar pulses are transmitted and received using a new interleaved timing arrangement that allows conventional, low-resolution data to be acquired simultaneously with high-resolution delay-Doppler measurements. - The unique capability of the Copernicus Sentinel-6 Poseidon-4 altimeter is designed to ensure enhanced continuity with the long time series of measurements from the Topex Poseidon and Jason series of satellite altimeters. - Every 10 days, Sentinel-6 provides sufficient measurements to map the sea-surface height of the ocean from which sea-level rise can be computed. This gives the ‘big picture' allowing us to chart the sea level with confidence. - Sentinel-6 provides both continuity and enhanced measurements for sea level, helping Copernicus deliver the best quality climate change evidence to make more confident decisions — every day (video credit: ESA/ATG medialab; Voice of Craig Donlon - ESA's Copernicus Sentinel-6 Mission Scientist)
- To make sure that differences between the historical low-resolution time series and the new data from Copernicus Sentinel-6 Michael Freilich are fully understood, a 12-month tandem flight has been taking place. This involves Sentinel-6 flying 30 seconds behind its predecessor satellite, Jason-3, following the same ground track.
- Following months of careful testing, the stream of Copernicus Sentinel-6 high-resolution data are now available.
- Eumetsat's ocean altimetry program manager, Julia Figa Saldana, said, "Experts from around the globe have closely examined and validated the data, confirming that the Copernicus Sentinel-6 mission is robust, precise and highly reliable. By cross-calibrating Sentinel-6 against its predecessor Jason-3 to within 1 mm, we ensure that the 30-year record of mean sea level, as captured by satellite radar altimeters, continues uninterrupted.
- "The data released today is critical for monitoring the impact of climate change on Earth's oceans."
- ESA's Sentinel-6 mission scientist, Craig Donlon, added, "Measurements of sea-level rise provide a unique but integrated view of climate change since a warming ocean expands and increased melting of ice on land both lead to an increase in sea level.
- "Sentinel-6 Michael Freilich securely brings a new synthetic aperture radar measurement technique into the reference altimeter time series for the first time. This allows Sentinel-6 to provide improved sea state and wind speed measurements, enhanced capabilities for river and lake hydrology applications while maintaining the stability of sea-level rise estimates. These measurements provide evidence that is critical to develop and implement robust societal policy for our future."
Figure 20: On the occasion of the first anniversary of the launch of the Copernicus Sentinel-6 Micheal Freilich satellite, ESA Mission Scientist Craig Donlon reflects on the mission (video credit: ESA/NASA)
- ESA's Copernicus Sentinel-6 project manager, Pierrik Vuilleumier, noted, "The collaboration with NASA, EUMETSAT, NOAA and CNES has, today, come to fruition with the timely release of mission products to operational users, providing both continuity with previous reference missions and new high-resolution synthetic aperture radar products. The Poseidon-4 altimeter's digital technology brings unprecedented level of performance."
- "While Sentinel-6 Michael Freilich is exceeding expectations in orbit, we are busy with its follow on, Sentinel-6B. The satellite is now at IABG's facilities in Ottobrunn, Germany, undergoing an environmental test campaign that will run through until next March. In the middle of the year it will be put into storage until we prepare it for launch at the end of 2025."
• June 21, 2021: Following liftoff last November and more than six months spent carefully calibrating the most advanced mission dedicated to measuring sea-level rise, Copernicus Sentinel-6 Michael Freilich is now operational – meaning that its data are available to climate researchers, ocean-weather forecasts and other data users. 24) 25)
- Sentinel-6 is one of the European Union's family of Copernicus missions but its implementation is the result of an exceptional cooperation between the European Commission ESA, EUMETSAT, NASA and NOAA, with contribution from the CNES French space agency. The mission comprises two identical satellites launched five years apart: Copernicus Sentinel-6 Michael Freilich launched on 21 November 2020 and Copernicus Sentinel-6B, which will be launched in 2025.
- Sea-level rise is a key indicator of climate change so accurately monitoring the changing height of the sea surface over decades is essential for climate science, for policy-making and, ultimately, for protecting the lives of those in low-lying regions at risk.
- Using the latest radar altimetry technology, developed by ESA, this new mission will advance the long-term record of sea-surface height measurements that began in 1992 by the French–US Topex-Poseidon satellite and then the Jason series of satellite missions.
Figure 21: Improvement of Sentinel-6 significant wave height with respect to Jason-3 (image credit: CLS)
- Sentinel-6 Michael Freilich will soon pick up the baton and extend this dataset – a dataset that is the ‘gold standard' for climate studies.
- Julia Figa Saldana, ocean altimetry programme manager at EUMETSAT, said, "We have been flying Copernicus Sentinel-6 Michael Freilich on the same orbit as the current altimetry reference mission, Jason-3, for the past six months so that the satellites have the same ‘view' of the ocean.
- "Experts from around the world have collaborated intensively over the past six months, despite the workplace constraints caused by the coronavirus pandemic, to cross-calibrate their data to ensure accuracy. The data are now being processed at EUMETSAT in Darmstadt, Germany, from where the satellite is also being controlled, and from where the data are released to ocean and weather forecasting users."
Figure 22: Improvement of Sentinel-6 range noise with respect to Jason-3 (image credit: CLS)
- Craig Donlon, ESA's Sentinel-6 mission scientist, said, "Building on a long line of European heritage dual-frequency altimeter missions, Sentinel-6's Poseidon-4 altimeter was designed to bring new high-resolution Ku- band synthetic aperture radar measurements into the altimetry reference time series.
- "To manage the transition from Jason-3's lower resolution measurements to Sentinel-6's high-resolution products with confidence, Poseidon-4 acquires both conventional low-resolution measurements simultaneously with high-resolution synthetic aperture radar measurements. The high-resolution products will be available later this year. We are really pleased to see that the data from Sentinel-6 show great performance based on validation by independent measurements on the ground."
- The Permanent Facility for Altimetry Calibration on the island of Crete in Greece is home to a transponder (CND1) that is key to this validation, ensuring that Sentinel-6's low-resolution data are the same as that of Jason-3. The transponder receives the Sentinel-6 radar signal, which is amplified and transmitted back to the satellite providing a well-characterized external calibration source.
- Dr Donlon explained, "Measurements from the CDN1 transponder show that the absolute difference between measurements from Sentinel-6 and Jason-3 is less than 2 mm, which is remarkable for two independent satellites operating at an altitude of 1330 km.
- "Establishing the differences between Sentinel-6 and Jason-3 is important if stability in the sea-level rise time series from satellite altimetry is to be maintained with low uncertainties."
Figure 23: Sentinel-6 and Jason-3 tandem (video credit: ESA)
- Nadya Vinogradova-Shiffer, NASA program scientist for Sentinel-6 Michael Freilich, said, "The release of this data marks the beginning of a new Sentinel era of ocean satellite altimetry for the NASA science community and the international Ocean Surface Topography Science Team, who are excited, ready, and eager to expand nearly three decades of discoveries in ocean and climate science."
- As of 22 June 2021, two data streams of low-resolution sea-surface height are available to the public. The first is available within hours of Sentinel-6's Poseidon-4 altimeter collecting it and the second stream comes a couple of days after it is acquired. The difference in when the products become available balances accuracy with delivery timeliness for tasks like forecasting the weather or helping to monitor the formation of hurricanes.
- The third data stream, which is slated for distribution later this year or early next year, will be the most accurate.
- European Commission's Director for Space, Matthias Petschke, said, "Preparing our resilience and adapting to sea-level rise as an effect of climate change is a top priority in the decades to come, as part of the European Green Deal. From scenarios published in 2015 for Cop21, we can observe that the sea-level rise phenomenon is accelerating faster than expected. It will affect our EU coasts in decades to come, this is a certainty, and this is critical for some European countries.
- "Jason missions, in the past, and now Copernicus Sentinel-6, are unique solutions to give us accurate information on this trend, observing and monitoring it accurately, as well revealing this alarming acceleration of the rise."
• February 8, 2021: In November 2020, the Copernicus Sentinel-6 Michael Freilich satellite was launched into orbit from the Vandenberg Air Force Base in California, US. Now, months later, the satellite has successfully passed what is known as the ‘in-orbit verification phase', where its equipment is switched on and the instruments' performance is checked. 26)
- The Copernicus Sentinel-6 Michael Freilich satellite is the first of two identical satellites to provide critical measurements of sea-level change. The satellite carries a new digital altimeter, Poseidon-4, that uses dedicated onboard processing to return even more precise measurements of the height of the sea surface.
- In the satellite's early days post-launch, the dedicated flight control team at ESA's Operations Centre in Darmstadt, Germany, took meticulous care of the new Sentinel in what is known as the Launch and Early Orbit Phase (LEOP). Once completed, ESA's mission control team handed over command and control of the satellite to EUMETSAT – Europe's weather and climate satellite organization – who took over responsibility of commissioning, routine operations and distribution of the mission's vital data.
Figure 24: Mission Control Room waiting for the first telemetry from the Copernicus Sentinel-6 Michael Freilich spacecraft. This is used to check how well the satellite survived the harsh conditions of the launch (image credit: ESA)
- On 27 January, ESA along with Sentinel-6's key partners, including Airbus, Thales Alenia Space, EUMETSAT, NASA, French Space Agency CNES and NOAA, completed the satellite's ‘in-orbit verification phase'.
- One of the tests performed included cross-calibrating the satellite's altimeter data with measurements from the Copernicus Sentinel-3 and Jason missions. These tests are completed at ESA's Permanent Facility for Altimetry Calibration (PFAC) in Crete, Greece, where the use of transponders are used to receive and re-transmit radar pulses back to the satellite in space to verify its performance.
- These measurements have been used to demonstrate that the altimeter measurements are performing to expectation.
- Robert Cullen, Copernicus Sentinel-6 Payload and System Manager at ESA, said, "From our preliminary analyses, the altimeter significant wave height and range uncertainty are significantly better compared to the previous Sentinel-3 and Jason-3 missions."
- Luisella Giulicchi, Copernicus Sentinel-6 System Manager at ESA, responsible for coordinating the satellite's in-orbit verification phase, added, "We found all satellite subsystems to be working in perfect order. The satellite's newly-deployed GNSS Precise Orbit Determination receiver, which combines both GPS and Galileo constellations signals, shows an outstanding preliminary performance, along with the rest of the navigation systems on board the satellite."
- "Since 18 December, Sentinel-6 has been in its final orbit, trailing just 30 seconds behind Jason-3. This particular trailing formation is required for 12 months before Sentinel-6 Michael Freilich will take over from Jason-3 as the operational reference mission."
More about Copernicus Sentinel-6
- Rising seas are top of the list of major concerns linked to climate change. Monitoring sea-surface height is critical to understanding the changes taking place so that decision-makers have the evidence to implement policies to help curb climate change and so that authorities can take action to protect vulnerable communities.
- The first sea-surface height ‘reference' measurements were supplied by the French–US Topex-Poseidon satellite, which was followed by three successive Jason missions. They show that since 1993 the global sea level has risen, on average, by just over 3 mm every year. Even more worryingly, over the last few years the global ocean has risen, on average, by 4.8 mm a year. Copernicus Sentinel-6's role is to continue this legacy of critical measurements.
• December 15, 2020: Just like your mobile phone, satellites themselves rely on satellite navigation to find their way in space. Thanks to a new ESA-developed receiver, the recently-launched Sentinel-6 is making use of Europe's Galileo as well as the US GPS system, a fact set to sharpen the accuracy of its sea level rise measurements. 27)
- Copernicus Sentinel-6 Michael Freilich, launched on 21 November, is the world's next radar altimetry reference mission, set to extend the legacy of sea-surface height measurements until at least 2030.
- Developed by ESA with strong NASA support as part of Europe's Copernicus program, the satellite is now being commissioned for operation by EUMETSAT, Europe's weather and climate satellite organization.
- Sentinel-6 is also the first Sentinel satellite equipped with a dual-system satnav receiver, which can make use of both GPS and Galileo signals, to perform mission-critical Precise Orbit Determination (POD).
- The ESA-developed receiver's first results became available on 26-27 November and underwent initial analysis by the Navigation Support Office based at ESA's ESOC control center in Germany, immediately revealing a very good data quality.
- The receiver uses GPS and Galileo signals either separately or in combination. With Europe's satnav system the world's most precise, the Galileo POD measurements in particular were excellent, outperforming the GPS measurements by a factor of two in terms of accuracy.
- Werner Enderle, Head of the Navigation Support Office comments: "While validation activities are still ongoing, the initial results of our Sentinel-6 precise orbit determination based exclusively on Galileo data are very exciting."
- Craig Donlon, Sentinel-6's ESA Mission Scientist, explains that being able to more precisely fix the satellite's position in space is crucial to mission success: "Sentinel-6 is a radar altimeter, measuring sea-surface height by sending down radar pulses to be bounced back to space, deriving the distance to the ocean surface to a few centimeters.
- "But to know how far the signals have travelled we need to know the satellite's orbital height to a high level of certainty. Such a high-performance satnav receiver that includes high-quality Galileo signals is likely to give us this information very precisely. Combined with the very low onboard noise of the altimeter instrument and its onboard processing facility, these are promising signs for the working mission to come."
First flight of novel satnav receiver
- Flying in space for the first time, the multi-constellation PODRIX was designed through an ESA General Support Technology Program project, led by ESA navigation engineer Pietro Giordano: "We were driven by requirements from ESA's Earth Observation program: many future Sentinels will be employing these receivers as a common procurement. That put the onus on us to design and qualify a good product. The receivers are also designed for use all the way up to geostationary orbit.
- "We're not surprised to hear about the quality of its output, but also happy, because you can never be 100% sure something works until it is flying.
- "This project has turned out to be an excellent example of synergy between ESA domains, because we in the Directorate of Technology, Engineering and Quality worked closely with our Earth Observation counterparts, while also getting advice from the Directorate of Navigation, the ultimate source of knowledge on Galileo signals."
New generation of satnav circuits
- The receiver contains one essential ingredient in turn: the fourth generation Advanced GPS/GLONASS Application Specific Integrated Circuit, AGGA-4 for short, funded by the Earth Observation Directorate and built by Airbus with the support of ESA's Microelectronics section.
Figure 25: AGGA-4, the fourth generation Advanced GPS/GLONASS Application Specific Integrated Circuit, seen undergoing radiation testing at Astri Polska in Poland (image credit: Astri Polska)
- "Earlier versions of the AGGA relied solely on GPS and Russia's GLONASS, but with this generation we set out to make use of the other satnav constellations now available, including Europe's Galileo," says microelectronics specialist Roland Weigand, AGGA-4's design support engineer. "The more signals in space you can process then the better your accuracy becomes.
- "It basically works like any other GNSS receiver chip, except we have to make more effort in space, to overcome radiation, take account of signal dynamics and make use of distant signals that have been weakened by passage through Earth's atmosphere.
- "In fact, precise positioning was not the main design driver – the starting point was actually radio-occultation, where scientific and weather information is derived from GNSS signals' passage through the atmosphere – but based on the multiple AGGA-4s already in space, they also function well for positioning. It's motivating to know, after a long, development effort, that our customers are happy."
• December 10, 2020: Launched less than three weeks ago, the Copernicus Sentinel-6 Michael Freilich satellite has not only returned its first data, but results also show that it is functioning far better than expected. Thanks to its new, sophisticated, altimetry technology, Sentinel-6 is poised to deliver exceptionally precise data on sea-level height to monitor the worrying trend of sea-level rise. 28) 29)
- Sentinel-6 Michael Freilich was lofted into orbit on 21 November from VAFB in California. After it had sent back its first signal showing that it was alive and well in space, ESA's Operations Centre in Germany took care of the satellite's first few days in orbit before handing it over to EUMETSAT for commissioning, and eventual routine operations and distribution of data.
- The satellite carries Europe's latest radar altimetry technology to extend the long-term record of sea-surface height measurements that began in the early 1990s.
- On 30 November, flight operators switched on Sentinel-6's Poseidon-4 altimeter instrument, which was developed by ESA. Analyzing its initial data, specialists were astonished by the quality. These first data were presented today, by way of three main images, at the European Space Week.
Figure 26: Copernicus Sentinel-6 sea-level anomaly data, overlaid on a map showing similar products from all of the Copernicus altimetry missions: Jason-3, Sentinel-3A and Sentinel-3B. The background image is a map of sea-level anomalies from satellite altimeter data provided by the Copernicus Marine Environment Monitoring Service for 4 December 2020. The data for this image were taken from the Sentinel-6 'Short Time Critical Level 2 Low Resolution' products generated on 5 December (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by EUMETSAT)
- The image of Figure 26 shows some preliminary results of sea-surface height. The data are overlaid on a map showing similar products from all of the Copernicus altimetry missions: Jason-3, Sentinel-3A and Sentinel-3B. The background image is a map of sea-level anomalies from satellite altimeter data provided by the Copernicus Marine Environment Monitoring Service for 4 December 2020. The Sentinel-6 data products were generated on 5 December.
Figure 27: Copernicus Sentinel-6 first waveform results. Left: The image shows a comparison between normalized data processed on board Copernicus Sentinel-6 and downlinked (blue line), compared to full raw (SAR-RAW) data processed on the ground (red line). By removing the trailing edge of the data before being transmitted to Earth, the data rate is reduced by 50% (SAR-RMC) (Range Migration Compensation). High fidelity low-noise data are thanks to Sentinel-6's Poseidon-4 digital instrument architecture, which is a first. There are no significant differences in geophysical parameter retrieval performance, and the onboard processing demonstrates expected performance. Right: Example of sea-surface height measurements processed by the ESA Level-2 Ground Prototype Processor showing Low Resolution Mode, SAR-RAW and SAR-RMC data over a transect in the Southeast Atlantic Ocean. Significant sea-surface height structure is visible in the data revealed by a very low noise signal. The improvement of synthetic aperture processing is evident in the data (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA/isardSAT, CC BY-SA 3.0 IGO)
- ESA's mission scientist for Copernicus Sentinel-6, Craig Donlon, explained, "We can already see that the satellite is delivering incredible data, thanks to the digital architecture of Poseidon-4 and the inclusion of simultaneous high-resolution synthetic aperture radar processing and conventional low-resolution mode into altimetry for the first time. This gives us the opportunity to make measurements with much finer synthetic aperture radar techniques that can be compared to Jason-3 to understand the improvement of the climate record."
- "Importantly, we can also see that there is very little noise in the data, so we have extremely clean data to work with."
- The set of images in Figure 28 of Russia's Ozero Nayval Lagoon and surrounding rivers show multiple views from Copernicus satellites. The first is a ‘camera-like' image from Sentinel-2; the second is a radar image from Sentinel-1; and next is from Sentinel-6 in its conventional ‘low-resolution' mode, which does not reveal a lot of information. However, by processing the altimetry data using fully-focussed synthetic aperture techniques usually used for imaging radar data, the resulting image reveals exceptional detail, highlighting the power of the instrument (click on image for more information).
Figure 28: The images of Russia's Ozero Nayval Lagoon and surrounding rivers show multiple views from Copernicus satellites. The first is a 10-m resolution ‘camera-like' image captured on 29 October 2020 by Copernicus Sentinel-2. The peninsula lies on the eastern part of the Bearing Straits. The land-bound lagoon, various river and lake features are clearly visible. The image is marked with the ground track of Copernicus Sentinel-6 as it crosses the region. - The second is a radar image captured on 29 November 2020 by Copernicus Sentinel-1 in interferometric wide swath mode and processed to 10 m resolution. The radar look direction is from the right with layover effects seen on the mountainous region to the left of the image. The lagoon has frozen over and numerous cracks are visible in the ice. Ocean swell and wind sea roughness are also seen in the ocean with some wave reflection and refraction on the southern coastal areas. - The next image uses Copernicus Sentinel-6 pulse-limited low-resolution mode data for the same area. In this mode, similar to Jason-3, the strongest radar reflections appear as overlapping parabola features, but no discrimination of the ground can be made. - Overlying the third image, the Copernicus Sentinel-6 Poseidon-4 fully-focused synthetic aperture radar image reveals features of the Ozero Nayvak Peninsular in fine detail. The high performance and low noise of Poseidon-4 when processed using these ESA-developed techniques reveals exceptional results. In this example, the altimeter data were first processed at a resolution of 1.1 m in the azimuth direction (left to right) and <0.4 m in the range direction (vertical). These data are then further multi-looked in azimuth to reduce speckle noise providing an image at a resolution of ~30 m. The radar backscatter power is coded by color as a function of across-track range and clearly reveals the vertical elevation of sea ice in the lagoon and low-lying river and lake features. Unlike the Sentinel-1 image, the Sentinel-6 Poseidon-4 radar is illuminating the scene from the north and in this case, ocean wave structure and refraction at the coastline can be clearly seen (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA/Aresys, CC BY-SA 3.0 IGO)
- Josef Aschbacher, director of ESA's Earth Observation Programs said, "We are delighted with these first results and proud to see our ESA-developed radar altimeter is working so well. Nevertheless, Copernicus Sentinel-6 is a mission that has been built in cooperation with the European Commission, EUMETSAT, NASA, NOAA and CNES – with all parties playing essential roles that make this mission the success we are seeing today."
- Another surprising result suggests that the satellites position in space can be better understood than previously thought. A radar altimeter derives the height of the satellite above Earth by measuring how long a transmitted radar pulse takes to reflect from Earth's surface. Sentinel-6 therefore carries a package of positioning instruments, including a system that can make use of both GPS and Galileo signals. Remarkably, the addition of Galileo measurements brings an improvement in orbit determination quality – which adds to the overall performance of the mission.
More about Copernicus Sentinel-6
- Rising seas are at the top of the list of major concerns linked to climate change. Monitoring sea-surface height is critical to understanding the changes taking place so that decision-makers have the evidence to implement appropriate policies to help curb climate change and so that authorities can take action to protect vulnerable communities.
- The first sea-surface height ‘reference' measurements were supplied by the French–US Topex-Poseidon satellite, which was followed by three successive Jason missions. They show that since 1993 the global sea level has risen, on average, by just over 3 mm every year. Even more worryingly, over the last few years the global ocean has risen, on average, by 4.8 mm a year.
- While the Copernicus Sentinel-6's role is to continue this legacy of critical measurements, the satellite carries new digital altimeter technology with dedicated onboard processing that will return even more precise measurements of the height of the sea surface.
- Sentinel-6 brings, for the first time, synthetic aperture radar into the altimetry reference mission time series. To ensure that the multi-satellite data time series remains stable, Sentinel-6 delivers simultaneous conventional low-resolution mode measurements, that are similar to measurements from Jason-3, as well as the improved performance of the synthetic aperture radar processing that yields high-resolution along-track measurements. A 12-month tandem flight, where Sentinel-6 flies just 30 seconds behind Jason-3, will be used to compare measurements from the two independent satellites in order to extend the sea-level climate record with confidence.
- A video of Copernicus Sentinel-6 in action is provided in Figure 58.
• November 2020: EUMETSAT took over flight operations at its mission control center in Darmstadt on 24 November, as Sentinel-6 Michael Freilich was drifting towards Jason-3, the current operational high-precision ocean altimetry mission, EUMETSAT's Sentinel-6 Flight Operations Manager Gareth Williams said. 30)
- With Sentinel-3A and Sentinel-3B, this is the third Copernicus ocean-monitoring satellite operated by the organization on behalf of the European Union.
- "We will stop the drift of Sentinel-6 Michael Freilich and keep it flying 30 seconds behind Jason-3 on the same orbit to allow for cross calibration of the data from their instruments," Williams said. "This will ensure the seamless continuity of a unique sea level record.
- "We will also switch on all instruments, acquire and pre-process first mission data and share them with ESA and NASA for evaluation, leading to the completion of satellite in-orbit commissioning in January."
- EUMETSAT's Sentinel-6 System Commissioning Manager Conrad Jackson said that, in parallel, the organization would work with ESA, NASA, NOAA, CNES and scientists from Europe and the United States to calibrate the products and validate the end-to-end Sentinel-6 system.
- "This will be achieved in June, with release to all users of near-real-time products equivalent to those of those of Jason-3."
- Another six months will be necessary to validate and release the highest accuracy sea level products used for climate monitoring. Then, Copernicus Sentinel-6 Michael Freilich will replace Jason-3 as the reference high-precision ocean altimetry mission, Jackson said.
• November 25, 2020: It was a spectacular launch on 21 November, as the Copernicus Sentinel-6 Michael Freilich satellite was lifted into space on a SpaceX Falcon 9 rocket. After taking care of the Earth observation spacecraft during the critical early days and making it at home in its new environment, ESA is ready to hand over control to EUMETSAT. 31)
- About an hour after a flawless launch, Sentinel-6 Michael Freilich Earth separated from the SpaceX Falcon 9 rocket, and for the first time it was flying on its own.
- Soon after, ESA's mission control in Germany received the very first signals from the fledgling mission. This vital moment, the ‘acquisition of signal', is what teams had been waiting for, because it meant they could lock on to the satellite with ground stations across the globe and receive its ‘telemetry' - data providing information on the mission's health.
- Of course, controlling missions is a two way conversation. We receive information about the spacecraft and all the observational data it has gathered in the signals it beams down to Earth, but we also speak to it, sending commands. Once teams at ESA's ESOC Operations Centre had sent their first commands, it was time to declare "we have a mission!".
- "It's always tense in the moments before we capture the first signal, until then, it's too soon to celebrate as we haven't yet taken control," explains Jose Morales, Head of Earth observation missions at ESA.
- "Once our screens lit up green, we knew that Sentinel-6 was in our hands and it is then that the real work began getting the spacecraft ready for an important life in space."
- In the days since launch, known as the Launch and Early Orbit Phase (LEOP), the flight control team at the Agency's Operations Centre took meticulous care of the new Sentinel. These early days came with many challenges, as the new spacecraft began using its solar arrays for power, woke up to test its core functioning and performed two maneuvers to initiate its drift towards its final, operational orbit, all the while at its most vulnerable to the hazards of space.
- Now the LEOP is complete, ESA's mission control team is handing over command and control of the satellite to EUMETSAT – Europe's weather and climate satellite organization – who will complete the final ‘orbit acquisition' and take on responsibility for commissioning, routine operations and distribution of the mission's vital data.
- "The critical Launch and Early Orbit Phase went smoothly and is now complete, and we are thrilled to pass on this mission to our friends at EUMETSAT, who will distribute its data on Earth's changing oceans," explains Simon Plum, ESA's Head of Mission Operations. "I am particularly proud of the dedication shown by everyone involved at all stages of this important mission. Their commitment has gone above and beyond expectation and truly demonstrates how seriously they take their roles."
- "It's testament to the hard work and expertise of our teams at ESOC that in the midst of a global pandemic they continue to safely carry out some of the hardest jobs in space."
Sensor complement: [Poseidon-4, AMR-C, Navigation Instruments (DORIS, GNSS-POD, LRA), GNSS-RO receiver]
The Sentinel-3 SRAL (SAR Radar Altimeter) derived RA (Radar Altimeter) is the principle payload instrument; its scope is measuring geophysical parameters (SSH, wind speed and SWH). To retrieve these data, additional information is required from a number of supporting instruments: a DORIS receiver (recurrent from CryoSat-2) to enable precise orbit determination and a Microwave Radiometer to provide the measurement of water vapor necessary to correct the altimeter data. Orbit tracking data are also provided by a GPS receiver (partially recurrent from Sentinel-3b and that in its own right can is capable of POD), and a LRA (Laser Retro-Reflector), that supports POD. Star trackers are used to meet the science objectives needed from the altimeter SAR data. An additional GPS receiver, GNSSRO, provided by NOAA and developed by NASA/JPL, will be dedicated to radio-occultation measurements.
Figure 29: S-6/Jason-CS payload accommodation (image credit: ESA, Airbus DS)
POS4 (Poseidon-4 SAR Radar Altimeter)
The Poseidon 4 is a fully redundant normal incidence Ku- and C- band pulse-width limited SAR Radar Altimeter. It has the capability of acquiring phase coherent measurements of a surface allowing synthetic-aperture processing to improve along-track sampling and reducing range noise and SWH (Significant Wave Height) noise.
The POS4 (Poseidon-4) architecture is composed of two cold redundant DPUs (Digital Processing Units), two cold redundant RFUs (Radio Frequency Units), a dual-frequency antenna and three RF Switches. With the improvements of the SAR method over pulse-width limited processing demonstrated, the agencies requested industry to investigate the possibility of operating both SAR and pulse-width limited modes at the same time over all open ocean improving science return. The pulse/burst characteristics of this new mode of operation, named ILM (InterLeaved Mode, open burst scenario), allows the reference LRM (Low Resolution Mode) data series to be continued , whilst providing science users with a unique global data set with reduced uncertainties (SAR).
Pulse-width limited LRM data are obtained from single pulse/burst allowing retrieval of geophysical parameters (elevation, wind speed and SWH) over single shot scales between about 1 km and 5 km (Figure 30 left). It has to be noted that the footprint is not only proportional to the satellite altitude and pulse length, but also to the SWH, due to non-nadir reflections (e.g. without waves the Jason-1/2 footprint is 2 km, with 15 m waves it becomes about 12 km.
Filtering the data acquired along the track wrt the rates of change of phase (SAR), are obtained slices of range rings coherent in phase between them (Fig. 30 right). This allows improvement of the along-track resolution to about 300 m, independently of the SWH.
Figure 30: Pulse-width limited (left) vs SAR (right) illuminated surface (image credit ESA)
When all available beams are collected, the range is corrected for Doppler Shift effects and range migration, they can be output as a stack file and multi-looked to form the Level 1b echo waveform. Artificially focusing the echoes (Figure 31) improves the overall SNR (rejecting all reflections from non-nadir sources) and thus improves performances.
Figure 31: SAR echoes focusing along the track (image credit ESA)
The SNR can be even more improved by averaging pulses since the noise on the signal is independent in each gate until a limit defined by the Walsh PRF (Pulse Repetition Frequency) bound. In order to increase the number of echoes per unit time the transmit bursts are interleaved with receive bursts in what is known as Open Burst transmission, Figure 32, third chronogram.
The cons of the Open Burst transmission vs the Closed Burst are: that the data volume and the power demand is increased and it is needed to vary the PRF (Pulse Repetition Frequency) around the orbit.
Figure 32: Top: LRM chronogram, low PRF (1-4 kHz), continuous Tx(red)/Rx(green) (Jason-3). Middle: SAR Closed Burst, high PRF (about 18 kHz) Tx/Rx in bursts (Sentinel-3). Bottom: SAR Interleaved Mode, Open Burst, Moderate PRF (about 9 kHz), continuous Tx/Rx (Sentinel-6). Time is in ms (image credit: adapted from ESA)
The RA PRF is fixed during an Rx tracking cycle but adjusted along the orbit (around 9.1 kHz) to cope with the altitude changes. Therefore, the PRF is constant in reception to avoid a Tx/Rx pulse overlap. To assure continuity, the PRF is a close multiple of the Jason-3 one: LRM is an embedded subset (decimation). The fact that the S-6 PRF is lower than the S-3 one (Figure 32) will allow a better sensibility determining the range delay of the leading edge of the echoes. This is so because the impulse widening (Figure 33) will be less severe therefore almost all off-nadir Doppler beams will be useable (e.g. not too broad to resolve low SWH due to "toe effect").
Figure 33: The impulse response broadens quadratically as the beams move off-nadir (image credit: R. Keith Raney)
To reduce the large SAR data volume produced on-board, an on-ground-reversible RMC (Range Migration Correction) is implemented, whose effects can be seen in Figure 34. The useful range is more or less reduced to about half. As a consequence, the data rate is reduced by a factor of 2. The process is reversed on-ground.
Figure 34: 2D FFT (Fast Fourier Transform) of a raw burst power, before (left) and after (right) RMC processing. High power is red and low power (in practice thermal noise) is dark blue (image credit ESA)
The DPU handles several measurements modes. One is the Acquisition Mode, operating only in Ku-band to look for echo in a defined altitude range. The other modes are combinations of Open and Closed Loop mode with generation of LRM or SAR or raw SAR I&Q data or combination of these, see Table 3.
The Closed Loop tracking mode makes use of an automatic echo recognition and tracking. Instead the Open Loop tracking mode exploits an on-board DEM (Digital Elevation Map) to adapt the PRI (PRI=1/PRF) for the elevation of inland waters.
Nominally, the following modes are used, according to Table 3 indications:
• LRM: Interleaved in Closed/Open Loop tracking mode with only LRM data
• LX: Interleaved in Closed/Open Loop tracking mode with LRM + SAR I&Q data.
• LRMC: Interleaved in Closed/Open Loop tracking mode with LRM + SAR RMC data.
Table 3: RA (Radar Altimeter) data rates per mode
In the measurement modes, each of the red lines in Figure 32 is made by combination of Ku & C-band pulses which pattern varies depending on the mode. One echo is received every pulse transmission at fixed PRI (~1/9.1 kHz). The majority of these are measurement pulses but there are also C-band pulses transmitted in order to retrieve a correction for ionospheric path delay, CAL pulses to trace the instrument behavior.
A blanking capability is part of the baseline design of POS-4 and AMR-C as any RFI (Radio Frequency Interference) between both would lead to performance impacts.
In addition, the instrument design relies on state of the art digital hardware improving on-board calibration strategy whilst reducing the manufacturing time.
The current POS-4 design theoretical performances have been assessed and are provided in Figure 35 that demonstrates S-6 will improve on its required performances for both LRM and SAR processing.
Figure 35: Theoretical performance of S-6/Jason-CS versus Jason-1, 2 and 3 (image credit: TAS-F)
AMR-C (Advanced Microwave Radiometer-Climate Quality)
AMR-C is a 3-frequency radiometer provided by NASA/JPL (funded by NOAA) enhanced to minimize the effects of instrument drift which is a key design driver for overall mission success. 32) It will be a direct successor to AMR instrument on Jason-3. 33)
AMR-C comprises a nadir-viewing offset Gregorian telescope with a one meter primary aperture feeding a single broad-band corrugated horn, followed by an Orthomode Transducer and two identical three-band radiometers observing the H and V linear polarizations of emission from the ocean surface and the atmosphere. One radiometer is treated as a cold spare. The radiometer is calibrated every second via a Dicke switch and a calibration noise source employing noise diodes.
Figure 36: AMR-C – Perspective view with calibration target and cold space reflector (image credit: NASA/JPL)
Once per month, a cold sky calibration is expected to be executed rotating the satellite with a pitch maneuver (nose up). It is also possible to execute a supplementary calibration by rotating the secondary reflector about the axis of the feed horn to focus on sequentially a reflector pointing to cold space or a warm calibration target. The calibrations are done when the satellite is over land. - The electronics and feed horn are thermally stabilized via a circuit controlled by the spacecraft and a radiator that dissipates heat excesses.
AMR-C incorporates AMR (Advanced Microwave Radiometer) and the experimental HRMR (High Resolution Microwave Radiometer). AMR measures the radiation reflected by the oceans at three different frequencies (e.g. 18.7 / 23.8 / 34.0 GHz) to calculate the water vapor path delay corrections. HRMR is an experimental radiometer receiver measuring high frequency channels (e.g. 90 / 130 / 168 GHz) for better resolution in coastal regions.
The signals measured are noise power expressed as a noise temperature in units of Kelvin. The measured noise temperature referenced to the AMR / HRMR feeds are referred to as the TA (Antenna Temperature).
An Antenna Pattern Correction is applied to the TA measurements to subtract noise temperature contributions from outside the main beam, yielding the Level 1 data product, main beam brightness temperature. A retrieval algorithm using empirically derived coefficients yields the Level 2 data product, the wet path delay estimate (cm) used in the RA range correction.
The AMR-C brightness temperatures are not only used for the RA water vapor path delay correction but are also fundamental climate data record from which are derived ocean measurements of wind speed, water vapor, cloud liquid water, rain rate, and sea surface temperature.
The AMR-C receiver is based on heritage from the previous missions with addition of a HRMR (High Resolution Microwave Radiometer) and a SCS (Supplemental Calibration System). The radiometer channels at 18.7 GHz, 23.8 GHz, and 34.0 GHz are inherited from previous AMRs and constitute the radio frequency subassembly (RFA). The 18.7 GHz channel estimates ocean surface components in observed brightness temperature, the 23.8 GHz channel estimates water vapor, and the 34.0 GHz channel estimates cloud liquid. HRMR consists of bands at 90 GHz, 130 GHz, and 168 GHz. The SCS is an additional calibration system in order to meet the level 3 payload requirement of long term radiometric stability. In addition to the RFA, HRMR, and SCS subassemblies, the AMR-C instrument also contains a parabolic mirror in the Reflector Subassembly (RSA), and the Electronics Unit (EU) in the Electronics Subassembly (ESA). 34)
A block diagram of the AMR-C instrument is shown in Figure 37. HRMR sits at the focus of the primary reflector and the lower frequency channels in the RFA are offset. There are two identical lower frequency radiometer units in the AMR-C system, a nominal unit (H-polarization) and a redundant unit (V-polarization) shown in green. All three of these receivers have a separate EU containing the Power Converter Unit (PCU), a Data Acquisition and Control Unit (DAC), and a Housekeeping Unit (HKU). The DACs of the AMR-H and AMR-V units are crossed-strapped to the SCS shown in purple, which has fully redundant Control Mechanism Interface Electronics (CMIE) units, both of which can control either or both motors in the Standard Dual Drive Actuator (SDDA). Please note that crossstrapping in Figure 37 is only shown for the EU-H unit to reduce clutter in the figure. HRMR is in turquoise.
Figure 37: AMR-C block diagram. Each subsystem is color-coded (with its EU unit), image credit: NASA/JPL
AMR-H and AMR-V Receiver Design: Signal is relayed to the receiver through a circular feed horn. The signal is split by the Ortho-mode Transducer (OMT) into H and V units, nominal and redundant, respectively, although the polarization is arbitrary. The redundant unit will be used as a cold spare. From the OMT a diplexer divides the signal into 18/24 GHz and 34 GHz channels and the 18/24 GHz channel is then spilt into separate 18 and 24 GHz channels. A detector diode along with an ADC converts the signal to a digital signal, which is then relayed to the spacecraft and transmitted to the ground. A model of the receivers is shown in Figure 38 and an internal block diagram is shown in Figure 39. In operation, a Dicke switch at the receiver waveguide output toggles between the antenna signal and 50 W load for a differential measurement.
Figure 38: Top and bottom views of the AMR receivers for 18/24 and 34 GHz (image credit: NASA/JPL)
Table 4: Level 6 AMR instrument requirements
Figure 39: AMR receiver block diagram (image credit: NASA/JPL)
A fully characterized noise source at the input of each receiver is used for internal gain stability calibration. Each noise source contains 3 sets of redundant diodes that can be used separately or together. A block diagram for the noise source is shown in Figure 40. The noise signals are coupled at the receiver input using a directional coupler. The level 6 receiver requirements flow from the level 4 instrument requirements. These requirements are summarized in Table 4.
Figure 40: The AMR noise source block diagram (image credit: NASA/JPL)
HRMR Receiver Design: Previous AMRs were limited to a 25 km diameter footprint on the ocean. In order to provide higher spatial resolution to improve the coastal zone measurement accuracy to a 3-5 km diameter footprint, a THz radiometer, HRMR, has been added to the AMR-C instrument. HRMR includes receiver bands at 90 GHz, 130 GHz, and 168 GHz and is based on radiometers designed for airborne and cubesat missions, the HAMMER (High-frequency Airborne Microwave and Millimeter-wave Radiometer), 35) and the TEMPEST (Temporal Experiment for Storms and Tropical Systems), respectively. 36) HRMR has been designed to attach to three mounting points at the focus of the RSA to minimize AMR beam blockage. The feedhorn and millimeter wave modules will be assembled and delivered on a radiatively-cooled plate, which will be enclosed for better thermal shielding.
HRMR will interface with EU hardware identical to the AMR units through its digitizer driver unit (DDU). This receiver utilizes low noise, high gain Indium Phosphide (InP) MMICs to amplify incoming signal in order to detect it. 37) Like the AMRs, HRMR signal is relayed through a feedhorn into diode detectors for each frequency. The calibration noise source is integrated in the multi-chip module (MCM). It has two noise diodes and directional couplers to provide stable calibration references. Additional calibration and stability is provided by the integrated Dicke switch that toggles between the antenna and reference load at 2 kHz rate to reduce NEDT, see Figure 47. A model of the HRMR receivers is shown in Figure 41 and design parameters are shown in Table 2.
Table 5: HRMR receiver design parameters
Figure 41: Top and bottom views of the HRMR receiver (image credit: NASA/JPL)
The SCS (Supplemental Calibration System): Due to long term fluctuations seen in the noise source from the Jason-3 mission 38) a SCS has been included on AMR-C. This subsystem is designed to turn the secondary mirror every 5-10 days so that the AMR receivers look at a warm load at ambient temperature (~200 K) and a cold load (cold sky, ~3 K), shown in Figure 42. As shown in Figure 37, the SCS only calibrates the AMR receivers, not HRMR, whose signal path is instead at the focus of the primary. These calibrations will be done over land in order to maximize observation times over the ocean.
The SCS is driven by an SDDA motor, which is a block redundant, single fault tolerant mechanical/electronic assembly that provides a rotary output with fully characterized torque, speed, and current relationships. The gearbox couples dual spur gears for the first stage with dual harmonic gears in the final stage. The redundancy in the SDDA means that no single mechanism failure within the assembly will prevent the output from rotating. The SDDA power is supplied separately from the rest of the instrument. The mechanism control is cross-strapped to both the H and V flight computers. During launch the secondary mirror is held in place by the Launch Lock Mechanism (LLM).
Figure 42: The SCS, which rotates a secondary mirror to look at ambient and cold calibration targets (image credit: NASA/JPL)
Thermal Modeling: The AMR-C instrument will have a PID-controlled thermal loop run by the spacecraft. The preliminary thermal design was simulated using a P-regulator and modeling shows that the receiver will meet its thermal requirements detailed below. The thermal analysis was done for three different cases: a hot winter, a hot summer, and a cold summer. Results are shown for several simulations lasting the duration of one orbit, which is 112 minutes long. Figure 43 models the AMRH receiver thermal stability over one orbit showing that it can be kept to within ~0.04 °C/min. Similarly, Figure 44 shows the modeled thermal stability for HRMR. HRMR has no requirement, but the goal for this receiver is ≤ 0.1 °C of variation over an orbit. Peaks and minimums in these models are a result of the satellite's orbit as it transitions in and out of the sun. In Figure 45, models show the thermal variation within the AMR-H receiver will be ± 2.5 °C. Figure 46 shows the thermal variations between the feed horn assembly (FHA) and the AMR-H receiver. The requirement is these thermal variations not exceed 10 °C and models show that this difference is well within the model's margin.
Figure 43: AMR-H receiver thermal stability can be kept to less than 0.04ºC/min during an orbit (image credit: NASA/JPL)
Figure 44: HRMR thermal stability models. The goal is ≤ 0.1ºC (image credit: NASA/JPL)
Figure 45: The AMR-H temperature range is ± 2.5ºC within the receiver (image credit: NASA/JPL)
Figure 46: The thermal variations between the feed horn and the receiver over one orbit (image credit: NASA/JPL)
HRMR Prototype: The HRMR 90 GHz prototype's measured noise temperature is ~500 K. The noise equivalent differential measurement (NEDT) was measured for both 90 and 160 GHz. The NEDT is a measure of sensitivity that determines the threshold for the minimum differential temperature that the system can detect. This measurement is taken by looking at the difference between the receiver looking at a blackbody radiator and a 50 W reference load using a Dicke switch. The results of the NEDT measurements for 90 and 160 GHz prototype receivers are presented in Figure 47. The NEDT at 90 GHz is in green and the NEDT at 160 GHz is in blue. At the Dicke switch frequency of 2 kHz, the NEDTs ~0.1 K, which provides a 50% margin on the sensitivity requirement.