Copernicus: Sentinel-6/Jason-CS (Jason Continuity of Service) Mission
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.
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). 14)
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 4) 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 4: 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 5: 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 5). 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 6: 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 7: Sentinel-6 SW components diagram (image credit: Airbus DS)
• September 3, 2019: Airbus DS has completed the ocean satellite ‘Copernicus Sentinel-6A', and is now sending it on its first journey. Its destination: Ottobrunn near Munich in Germany, where over the next six months the satellite will undergo an extensive series of tests at Industrieanlagen Betriebsgesellschaft mbH (IABG) to prove its readiness for space. 15)
Figure 8: Airbus has completed the ocean satellite ‘Copernicus Sentinel-6A' (image credit: Airbus / Lorenz Engelhardt)
- ‘Copernicus Sentinel-6' will carry out high-precision measurements of ocean surface topography. The satellite will measure its distance to the ocean surface with an accuracy of a few centimeters and, over a mission lasting up to seven years, use this data to map it, repeating the cycle every 10 days. It will document changes in sea-surface height, record and analyze variations in sea levels and observe ocean currents. Exact observations of changes in sea-surface height provide insights into global sea levels, the speed and direction of ocean currents, and ocean heat storage. These measurements are vital for modelling the oceans and predicting rises in sea levels.
- The findings will enable governments and institutions to establish effective protection for coastal regions. The data will be invaluable not only for disaster relief organizations, but also for authorities involved in urban planning, securing buildings or commissioning dykes.
- Global sea levels are currently rising by an average of 3.3 mm/year as a result of global warming; this could potentially have dramatic consequences for countries with densely populated coastal areas.
- Two Sentinel-6 satellites for the European Copernicus Program for environment and security are currently being developed under Airbus's industrial leadership. While it is one of the European Union's family of Copernicus satellite missions, Sentinel-6 is also being realized thanks to an international cooperation between ESA, NASA, NOAA and EUMETSAT.
- Each satellite has a mass of approximately 1.5 tons. From November 2020, Sentinel-6A will be the first of the two Sentinel-6 satellites to continue collecting satellite-based measurements of the oceans' surfaces, a task that began in 1992. Sentinel-6B is then expected to follow in 2025.
• April 12, 2019: Records show that, on average, global sea level rose by 3.2 mm a year between 1993 and 2018, but hidden within this average is the fact that the rate of rise has been accelerating over the last few years. Taking measurements of the height of the sea surface is essential to monitoring this worrying trend – and the Copernicus Sentinel-6 mission is on the way to being ready to do just this. 16)
- The mission will be a constellation of two identical satellites that are launched sequentially.
- Over the next decade, the Copernicus Sentinel-6A and then Sentinel-6B satellites will, importantly, take the role as reference missions, picking up the task of continuing the long-term record of sea-surface height measurements that have so far been supplied by the French–US Topex-Poseidon and Jason missions.
Figure 9: Copernicus Sentinel-6 radiometer integration. The AMR-C (Advanced Microwave Radiometer for Climate monitoring) is being integrated on to the Copernicus Sentinel-6A satellite. The photo shows teams at Airbus in Friedrichshafen, Germany, lowering the instrument on to the satellite prior to mechanical mounting and alignment checks. As part of the international cooperation for this mission, the radiometer has been supplied by NASA/JPL. The satellite's main instrument is a radar altimeter to measure sea-surface height. The radiometer accounts for the amount of water vapor in atmosphere, which affects the speed of the altimeter's radar pulses (image credit: Airbus)
- The Copernicus Sentinel-6 satellites will each carry a radar altimeter, which works by measuring the time it takes for radar pulses to travel to Earth's surface and back again to the satellite. Combined with precise satellite location data, altimetry measurements yield the height of the sea surface.
- Over the next decade, the Copernicus Sentinel-6A and then Sentinel-6B satellites will, importantly, take the role as a reference mission, picking up the task of continuing the long-term record of sea-surface height measurements that have so far been supplied by the French–US Topex-Poseidon and Jason missions.
- The Copernicus Sentinel-6 satellites will each carry a radar altimeter, which works by measuring the time it takes for radar pulses to travel to Earth's surface and back again to the satellite. Combined with precise satellite location data, altimetry measurements yield the height of the sea surface (Figure 12).
- With Copernicus Sentinel-6A scheduled for liftoff at the end of next year, the satellite is currently being equipped with its measuring instruments, which also include an advanced microwave radiometer at Airbus' facilities in Friedrichshafen in Germany.
- The radiometer accounts for the amount of water vapor in atmosphere, which affects the speed of the altimeter's radar pulses. While it is one of the European Union's family of Copernicus satellite missions, which all deliver a wealth of information for a number of environmental services, Copernicus Sentinel-6 is also being realized thanks to cooperation between ESA, NASA, NOAA and EUMETSAT.
- As part of this international cooperation, the Copernicus Sentinel-6 radiometer has been supplied by NASA.
- ESA's Copernicus Sentinel-6 mission scientist, Craig Donlon, said, "The advanced microwave radiometer has been designed to make sure that the measurements from Copernicus Sentinel-6 will be of the highest quality to monitor changes in global sea level and ensure a complete record of sea level for the coming decades."
- Pierrik Vuilleumeir, ESA's Copernicus Sentinel-6 project manager, added, "We are very happy with progress so far and, in fact, both satellites are being built in parallel. We are now looking forward to the next step, which will be to complete the satellite with the altimeter and the precise orbit determination instruments. The satellite will then be put through testing, which includes simulating the vibrations and temperature during liftoff and also the environment of space for its life in orbit around Earth."
Figure 10: Copernicus Sentinel-6 with radiometer. The photo shows the instrument after the integration process (image credit: Airbus)
• August 30, 2018: The integration of Sentinel-6A, the first of two satellites to continue measuring sea levels from 2020, has reached a new milestone and its critical phase: the propulsion module has been "mated" with the main structure of the satellite at Airbus. 17)
- In a complex operation, the Airbus satellite specialists hoisted the approximately 5 m high satellite platform with pin-point precision over the drive module, which had already been positioned (Figure 11). The two components were then fixed in place and assembled. Before this could happen, the propulsion module, which includes the engines, control devices and a 240 liter tank with an innovative fuel management system, had to undergo technical acceptance, since this subsystem can no longer be accessed once it has been integrated. The propulsion module now needs to be ‘hooked up', which will then be followed by the system tests.
Figure 11: Sentinel-6, built by Airbus will provide high accuracy altimetry for measuring global sea-surface height, primarily for operational oceanography and for climate studies (image credit: Airbus DS, Friedrichshafen)
- Two Sentinel-6 satellites for the European Copernicus Program for environment and security, headed by the European Commission and ESA, are currently being developed under Airbus' industrial leadership, each weighing roughly 1.5 tons. From November 2020, Sentinel-6A will be the first to continue collecting satellite-based measurements of the oceans' surfaces, a task that began in 1992. Sentinel-6B is then expected to follow in 2025.
- Sentinel-6 is a mission to carry out high-precision measurements of ocean surface topography. The satellite will measure its distance to the ocean surface with an accuracy of a few centimeters and, over a mission lasting up to seven years, use this data to map it, repeating the cycle every 10 days. It will document changes in sea-surface height, record and analyze variations in sea levels and observe ocean currents. Exact observations of changes in sea-surface height provide insights into global sea levels, the speed and direction of ocean currents, and ocean heat storage. The measurements made are vital for modelling the oceans and predicting rises in sea levels.
Figure 12: Artist's rendition of the deployed Sentinel-6/Jason-CS satellite in orbit (image credit: ESA)
- These findings enable governments and institutions to establish effective protection for coastal regions. The data is invaluable not only for disaster relief organizations, but also for authorities involved in urban planning, securing buildings or commissioning dykes. - Global sea levels are currently rising by an average of 3 mm/ year as a result of global warming; this could potentially have dramatic consequences for countries with densely populated coastal areas.
• September 2017: The satellite CDR (Critical Design Review) took place, enabling the project to move into the production Phase-D. Most flight hardware is being manufactured and satellite integration will start in September 2017. Joint activities with the NASA, NOAA and Eumetsat partners are proceeding. Working groups have been formed to address the system engineering and mission performance aspects. The independent Mission Advisory Group advising the project partners on scientific issues specific to the Sentinel-6/Jason-CS mission had its first meeting in June. 18)
Launch: A launch of Sentinel-6 / Jason-CS mission is planned for November 2020. NASA will provide the payload and launch the Sentinel-6A and -6B satellites.
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.19)
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/Jason-CS 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 13 and 14 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 13: Reception cones of ground stations (image credit: Airbus DS)
Figure 14: Ground Track (red) and Kiruna (green) / Fairbanks (blue) Visibility Cones (image credit: Airbus DS)
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 15: 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 16 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. 16 right). This allows improvement of the along-track resolution to about 300 m, independently of the SWH.
Figure 16: 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 17) improves the overall SNR (rejecting all reflections from non-nadir sources) and thus improves performances.
Figure 17: 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 18, 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 18: 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 18) will allow a better sensibility determining the range delay of the leading edge of the echoes. This is so because the impulse widening (Figure 19) 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 19: 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 20. 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 20: 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 18 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 21 that demonstrates S-6 will improve on its required performances for both LRM and SAR processing.
Figure 21: 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. 20) It will be a direct successor to AMR instrument on Jason-3. 21)
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 22: 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). 22)
A block diagram of the AMR-C instrument is shown in Figure 23. 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 23 is only shown for the EU-H unit to reduce clutter in the figure. HRMR is in turquoise.
Figure 23: 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 24 and an internal block diagram is shown in Figure 25. In operation, a Dicke switch at the receiver waveguide output toggles between the antenna signal and 50 W load for a differential measurement.
Figure 24: 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 25: 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 26. 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 26: 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), 23) and the TEMPEST (Temporal Experiment for Storms and Tropical Systems), respectively. 24) 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. 25) 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 33. A model of the HRMR receivers is shown in Figure 27 and design parameters are shown in Table 2.
Table 5: HRMR receiver design parameters
Figure 27: 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 26) 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 28. As shown in Figure 23, 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 28: 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 29 models the AMRH receiver thermal stability over one orbit showing that it can be kept to within ~0.04 °C/min. Similarly, Figure 30 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 31, models show the thermal variation within the AMR-H receiver will be ± 2.5 °C. Figure 32 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 29: AMR-H receiver thermal stability can be kept to less than 0.04ºC/min during an orbit (image credit: NASA/JPL)
Figure 30: HRMR thermal stability models. The goal is ≤ 0.1ºC (image credit: NASA/JPL)
Figure 31: The AMR-H temperature range is ± 2.5ºC within the receiver (image credit: NASA/JPL)
Figure 32: 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 33. 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.
Further measurements made on the prototype indicate that the power and mass are within the margins of their allotted budgets. These results are presented in Table 6.
Table 6: HRMR prototype specifications
Figure 33: HRMR NEDT measurements for prototype HRMR receivers at 90 and 160 GHz (image credit: NASA/JPL)
Current status and future work
The AMR-C team plans to deliver two flight instruments, one for each mission ~5 years apart. The instrument has passed the preliminary design review (PDR) and Phase C has begun. Hardware testing will begin in the summer of 2017 and the critical design review (CDR) will be in the fall of 2017. Instrument I&T for the first flight module will start in the Spring of 2018 for delivery to payload I&T in early 2019. Instrument I&T for the second flight module will begin in early 2019 after the delivery of the first flight module, and begin payload I&T in fall 2019. Sentinel-6 is expected to launch in 2020.
The DORIS DGXX-SEV receiver, carried on-board S-6, is a direct evolution of the DGXX-S of Jason-3. It is part of an overall system which is able to provide tracking measurements for precise orbit determination, and time-transfer. The DORIS system comprises a network of 55 ground beacons, a number of receivers on several satellites in orbit and in development, and ground-segment facilities. It is part of the IDS (International DORIS Service), which also offers the possibility of precise localization of user-beacons.
DORIS is an up-link radio frequency tracking system based on the Doppler principle. Each beacon in the ground network broadcasts stable two frequencies, at S-band and VHF (2036.25 MHz and 401.25 MHz respectively). Every 10 seconds the receiver delivers the Doppler shift data calculated using the on-board ultra-stable oscillator (USO with a stability of 5 x 10-13 over 10 to 100 seconds) as a reference; essentially this enables the line- of-sight velocity to be determined. The use of two frequencies allows the ionospheric effects to be compensated and also enables the ionospheric total electron content to be estimated. The set of radial velocities from the dense network of precisely located beacons is rich set of tracking data.
The DORIS's USO sync signal is used also to drive the GNSS-POD and RA pinpointing on the on-board DEM used in Open Loop.
The DORIS instrument consists of a receiver and processing unit (BDR), which is composed of 2 identical functional chains in cold redundancy. Both share a common RF signals distribution unit (DRF) which also contains a (cold) redundant USO (Ultra Stable Oscillator). And a dual-frequency antenna.
Figure 34: Geographical coverage of the DORIS network beacons (image credit: CNES)
The DORIS system includes the possibility of encoding information on the uplinked signals, and three privileged master beacons, at Toulouse, Kourou and Papeete, provide such uplink services. Data uplinked from these stations (which is updated weekly and used by all DORIS instruments in orbit) include the coordinates of the stations, earth rotation parameters, etc.
The DORIS is not only used for POD, but also for geodesy and geophysics applications: measuring the continental drift, fitting the local geodesic network, monitoring the geophysical deformations, determining the rotation and the gravity parameters of the Earth and contributing to the international reference system. 27)
GNSS-POD (Global Navigation Satellite System-Precise Orbit Determination)
The GNSS-POD receiver is a recurring PODRIX model in common procurement of the Sentinel program (S-1, S-2, S-3 and S-6). A PODRIX unit is a multi-constellation (GPS & Galileo) multi-frequency (L1/E1, L2 and L5/E5a) GNSS receiver. The GNSS on-board system is composed of two cold-redundant receivers, each including one tri-frequency (L1/E1, L2 & L5/E5a) receiver with 16 dual frequency channels. Two Extended Patch Excited Cup POD antennas are provided, one per single electronic box.
GNSS-RO (GNSS-Radio Occultation)
GNSS-RO is a CFI from NASA/JPL. As a secondary mission instrument, it is used to measure physical properties of the atmosphere such as temperature, pressure and water vapor, via detecting the occultation of GNSS signals as they pass through the limb of the atmosphere.
It is composed of: one non-redundant EU (Electronics Unit) and three antennas (POD antenna, RO fore antenna (3 x 2 array), RO aft antenna (3 x 4 array).
To measure radio occultations, three antennas are necessary. One antenna is used for POD, while two other antennas are directed at the Earth's limb to collect RO data. One of these antennas faces the fore of the spacecraft while the other faces the aft. These antennas enable tracking of the highly defocused rapidly shifting in frequency GNSS signal passing through the lower regions of Earth's atmosphere. The GNSS-RO uses the several high gain antennas, with digital beam forming to enable the occultation measurement of signals with lower level.
The GNSS-RO receiver has a configurable digital processing section enabling processing of multiple combinations of GNSS signals. It is able to track not only GPS but also GLONASS and can be configured to track additional GNSS signals. Most of the low-level signal processing will be done inside multiple reconfigurable FPGAs, which can be updated postlaunch to track new in-band GNSS signals as they become available.
The ability to track multiple GNSS satellite signals allows the capability to operate during the transition to GPS-III and past the 2020 retirement of the legacy signals. This capability significantly improves the quality and quantity of the radio occultation measurements from previous missions. The expected instrument data rate is about 53 kbit/s.
REM (Radiation Environment Monitor)
The REM instrument has been baselined lately in 2016 as a payload experiment for S-6. The REM is installed externally on the fore panel and provides all elements necessary to monitor in flight protons, electrons, and heavy ions fluxes.
Payload performances of Sentinel-6
Primary mission expected performances: As indicated, the general end-user requirement for S-6 is to "perform at least as well as Jason-2" in terms of RMS Error (RMS-E) in the retrievals of SSH (Sea Surface Height), SWH (Significant Wave Height) and wind speed. This requirement was broken down into the individual components that make up the measurement of SSH: altimeter range, orbital altitude, atmospheric corrections, and sea state bias. An analysis was done on the current state of the art, expected performances of the POS-4 altimeter, and current POD performances led to the establishment of the S-6 requirements listed in Table 3, with some more challenging goals to be met for all products later in the mission. Overall, the S-6 requirements for the RSS (Root Sum Square) sea surface height error for LRM measurements closely meet the established Jason-2 performances, whereas SAR measurements will clearly outperform Jason-2, because of the reduction in measurement noise. The only exceptions are the orbit performances, which are kept conservatively similar to the Jason-3 requirements. However, the performance goals of orbit determination are likely to be met and are at least equal to Jason-3 performances.
Although the requirements for SWH, wind speed, and backscatter have been kept somewhat less restrictive than the claimed Jason-2 performance, they are still vastly tighter than the requirements for Jason-3 and Sentinel-3, which are regarded as far too cautious.
Table 7: Overview of the requirements and actual performances of Jason-2 (NASA, 2011), the requirements for Jason-3 (Couderc, 2015) 28), the requirements for the Sentinel-3 SRAL (Ferreira, 2009; Donlon, 2011) 29) 30) and the requirements and goals for S-6/Jason-CS. In each column either a single value is presented if it applies equally to NRT, STC, and NTC. If a triplet of numbers is given, it applies to NRT/STC/NTC. Numbers are in centimeters, unless indicated otherwise.
Legend to Table 7: a After ground processing, averaged over 1 s, for 2 m wave height. b Goals from CNES system performances budget study. c Derived from Ku- and C-band range difference, averaged over 200 km. d Equal to Jason-2 actual performance. e Could also be expressed as 1% of SWH. f The RSS values for the NTC products given in (Ref.28) have been corrected in (Ref. 9) . g NRT/OGDR orbit from real-time DORIS on-board ephemeris. h Whichever is greater. i After calibration to Jason-1. j After cross-calibration with other altimeter missions. k For 0.5–8 m SWH range. l For 3-20 ms-1 wind speed range.
RO (Secondary Mission) expected performances
The GNSS-RO will observe occultations over the SLAT (Straight line Tangent Altitude) range from - 300 to 500 km, where the SLAT is the minimum elevation above the reference ellipsoid of an imaginary straight line connecting S-6 and the occulting GNSS satellite. This is negative in the lower atmosphere since the refraction bends the ray behind the horizon. As a secondary payload, the GNSS-RO will not be able to observe the upper atmosphere up to orbit altitude due to data size limitations.
The occultation tracking rates are 50 Hz for GPS and 100 Hz for GLONASS in the lower atmosphere, while higher up a 1 Hz tracking is foreseen. Open loop tracking is enabled from a configurable SLAT altitude downwards. With no ultrastable oscillator available, occultation processing will rely on single differencing with respect to a reference GNSS satellite to be tracked simultaneously.
Based on simulations with a constellation of 31 GPS and 24 GLONASS satellites and assuming an antenna coverage of ± 55º in azimuth, the S-6 satellite will be observing about 1100 occultations per day, about 600 from GPS and about 500 from GLONASS. Contrary to e.g. the EPS (EUMETSAT Polar System) and the EPS-SG (EPS-Second Generation), S-6 will fly in a non-sun-synchronous orbit, providing measurements at various local solar times, cycling through a full 24 h every 118 days.
Product Processing and Evaluation
Interleaved SAR mode: As indicated, the Poseidon-4 radar altimeter system can operate in conventional pulse-width limited (LRM) and SAR processing simultaneously. Hence, both Brown echoes and SAR radar echoes will be generated simultaneously in the ground processing. This is loosely called the interleaved operating mode, because the transmit and receive pulses are "interleaved" just like in LRM altimetry but at a much higher rate (9 kHz), Figure 18. This is in contrast to the burst mode operation of CryoSat-2 and Sentinel-3, which transmit and receive alternatively, each approximately one-third of the time. This high rate interleaved pulsing of the Jason-CS altimeter has the following advantages:
• The original (Jason-2 and -3) low-resolution processing is maintained simultaneously to higher-resolution products, thereby ensuring full continuity of services with Jason-3, based on pulse-width limited processing with an along-track resolution of approximately 7 km.
• The range noise of SAR processed altimeter echoes will be reduced by a factor of 1.7 compared to Sentinel-3 since more independent echoes are received owing to the continuous pulsing of Jason-CS compared to the burst mode of Sentinel-3 (and CryoSat-2).
• The availability of much higher along-track resolution (approximately 300 m) and, when averaged, a lower range measurement noise will enable an enhanced use especially in coastal areas.
• This enables continuous and direct comparison of LRM and SAR measurements (which is neither available from Sentinel-3 or CryoSat-2) and makes Sentinel-6 a reference for all SAR altimetry missions.
Thanks to the interleaved operating mode, S-6 will bring some unique opportunities for cross-calibrating and cross-validating LR and HR altimetry, housed on the same platform, working from the same altimeter echoes, just using different processing techniques. Also, it will be the first time that we will be able to fully process on-ground 100% of the echoes that would otherwise be averaged on-board.
Once received on ground, the raw data from the mission will be processed by the responsible institutions into NRT (Near Real Time) data and other products (Geo Physical Data Records) which are then distributed to the operational users. In addition to the European Space Agency, ESA, the French Space Agency, CNES, was involved in the previous missions as well as the world's weather and climate forecasting agencies EUMETSAT and, in view of the transatlantic cooperation, NOAA. They are also responsible for the establishment of forecasts of long term changes which affect climate and society (e.g. agriculture).
On the contrary of the NRT product, off-line products need longer post processing. Off-line means few days or weeks after data take. For Sentinel-6, NRT data will be downlinked at every X-band contact. NRT data will not be older than 3 hours under the provision that the ground station network setup (which is under customer responsibility) will be compliant.
Altimeter and Radio Occultation Product Levels: Different levels of products are distinguished in terms of readiness in the various stages of the processing:
• Level 0 products consist of raw data after restoration of the chronological sequence for each instrument and removal of data overlaps at dump boundaries and relevant quality flags related to the reception and decoding;
• Level 1 products maintain the same time structure and sampling as the Level 0. The instrument measurements are converted into recognized engineering units. For what regards the Altimeter, calibration data (radiometric and spectral calibration as well as geometric registration to geodetic Earth coordinates) are appended (Level 1a) or applied (Level 1b). Geolocation data are also appended.
In case of S-6, Level 1a products will include all the recorded individual echoes in the time domain, whereas Level 1b provides the synthesized waveforms (without geophysical corrections). A Level 1b-S product, similar to what is produced for Sentinel-3 is not envisioned; however, software will be provided to derive these from the Level 1a product after performing Delay Doppler processing.
For what regards the Radio Occultation, at Level 1a, phase and amplitude data as well as the satellite orbits of the occultation are provided. The Level 1b products will include the main variables for assimilation, such as the vertical bending angle profile.
• Level 2 products contains the geophysical measures, combined with auxiliary input data from other sources (such as geophysical corrections coming from meteorological models) to yield directly useful geophysical parameters, e.g. SSH , SWH and wind speed. The auxiliary data parameters and geophysical corrections are appended. For altimetry, Level 2 products contain measurements of SWH, wind speed, and SSH, at a high rate of 20 Hz, which are then averaged along-track to form one averaged measurement at 1 Hz. These products are thus equivalent to the GDRs (Ground Data Records) for the Jason missions. — For Radio Occultation at Level 2, temperature and water vapor profiles are provided.
• Level 2P products are enhanced Level 2 Altimeter products, aimed at harmonization between missions, e.g. applying the same geophysical corrections across the missions, or applying externally derived biases to the data in case they have not been applied yet in the operational Level 2 products.
• Level 3 products contain geophysical parameters that have been spatially and/or temporally resampled or corrected. This may include averaging over multiple orbits.
• Level 4 products are thematic data, and are generally gridded parameters that have been derived from the analysis of the satellite measurements but are not directly derived from them. These products are elaborated by service providers and users and are not delivered by the S-6 program.
Product Services and Generation Delays: Based on the synthesis of the operational applications, various product services are identified. Table 8 and Table 9 match the applications with the appropriate product levels. Three different latencies are considered: NRT (Near-Real-Time), STC (Short-Time-Critical), and NTC (Non-Time-Critical). The latencies govern the quality of the auxiliary data used in the product generation; therefore better-quality data are available after a longer elapsed time.
According to the generation latencies, the product services are:
• The Near Real Time Altimetry service [ALT-NRT, Note: equivalent to the OGDR (Operation Geophysical Data Record)service in the Jasons] delivers Level 2 products within 3 hours after data acquisition. Because of the reduced time allowed for the generation, it will often be necessary to rely on alternative data sources (e.g. predicted or climatological values) for auxiliary data like altimeter range corrections. The quality of the orbit determination will also be reduced. Nonetheless, the algorithms used for the production of Level 2 data from Level 1 are expected to be the same. In addition, to provide NRT data in the fastest possible way, data will not be provided in consolidated products with a length of half an orbit (as is the case for STC and NTC), but will rather be provided in smaller granules. The main objective of this product service is to provide information on the sea-state (SWH and wind speed, but also on SSH). It is mainly used for marine meteorology, ocean-atmosphere air-sea transfer studies and real-time operational oceanography.
• Short Time Critical Altimetry service [ALT-STC, Note: equivalent to IGDR (Interim Geophysical Data Record) service in the Jasons] delivers Level 2 products within 36 hours after data acquisition which enables consolidation of some auxiliary or ancillary data (e.g. preliminary orbit determination). These products will be produced using the same algorithms as the NTC products and they will have the same data structure. The main objective is to support operational oceanography, i.e. improve ocean state analysis, forecasts, and hindcasts produced by NOP (Numerical Ocean Prediction) systems assimilating sea surface height measurements derived from a multi-mission constellation of spaceborne altimeters. Level 3 products contain also geophysical parameters that have been spatially and/or temporally resampled or corrected. This may include averaging over multiple measurements. They are primarily intended for ocean modelling services. At this point, Level 3 data will only be provided with short time critical latency. This product service is mainly used for operational oceanography and geophysical studies.
• Non Time Critical Altimetry service [ALT-NTC, Note: equivalent ot GDR (Geophysical Data Record) service in the Jasons] delivers Level 2 products within typically 2 months after data acquisition, allowing the further consolidation of some auxiliary data (e.g. precise orbit data, radiometer calibration) leading to higher accuracy of SSH products. These products will be subject to regular reprocessing as better information about instrumentation biases, precise orbits, and geophysical corrections become available. The main objective of this product service is to provide information on ocean topography and mean sea level in support of ocean and climate monitoring services and it is mainly used for geophysical studies and operational oceanography.
• Near-Real-Time Radio Occultation product service (RO-NRT) delivers Level 1b and Level 2 products within 3 hours after sensing, for direct assimilation into NWP models. It will be provided by the US partners of the program. The main objective of the RO-NRT product service is to provide bending angles or refractivity profiles, which contain information on atmospheric temperature, pressure, and humidity. Further Level 2 products are e.g. tropopause height, planetary boundary layer height, and ionospheric information.
• Non-Time-Critical Radio Occultation product service (RO-NTC) delivers Level 1b and Level 2 products within 60 days after sensing. Longer time series of the instrument are used to obtain improved precise orbit determination and clock data, as well as using updated auxiliary data (e.g. precise orbit and clock data for the GNSS satellites). The main objective of the RO-NTC is to deliver higher precision version of the NRT data, making this service particularly valuable for climate studies, including assimilation in reanalysis models. Two parallel services will be providing these data. They both start from Level 0 and thus allow estimating uncertainties introduced by the processing set-up. On the European side, processing up to Level 1b is performed at EUMETSAT and the ROM SAF (Radio Occultation Meteorology Satellite Application Facility) is responsible for processing these data further to Level 2 (and also to Level 3 within their climate service). The other RO-NTC service will be provided by the US partners of the program.
The naming of these services (characterized by product latencies) are in common with those of the Sentinel-3 ocean surface topography mission.
Table 8: Mapping of the main application areas on the altimetry product services(Level 1 and Level 2). The mapping for Level 3 products is equivalent to the one of the Level 2 products (+ is essential; is beneficial; - is less important)
Table 9: Mapping of the main application areas on the radio occultation product services (+ is essential; is beneficial; - is less important)
In summary, the Sentinel-6 mission, consisting of two consecutively flying altimeter satellites, Sentinel-6 A and Sentinel-6 B, will ensure the continuation of the decades-long time series of sea level as recorded by TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3, from 2020 onwards. Since the RA (Radar Altimeter) will be able to serve simultaneously conventional LRM altimeter, and a SAR altimeter measurements, it does not only provide compatibility with the previous missions, that is vital for an accurate cross-calibration, but it will also improve sampling of the coastal areas with a much higher resolution, and providing the ability to measure closer to the coast line. Stability and performances of the measurements will be also improved, wrt the predecessors, to cope with day by day more demanding scientific needs.
The Sentinel-6 mission will be the first of the "reference missions" for which a wide range of Level 1, Level 2P, and Level 3 products will be provided. These are not only aiming at operational meteorological and oceanographic modelers, but are also giving the altimeter specialist the opportunity to advance further altimeter technologies that will be provided by the unique interleaved mode altimeter flown on the Sentinel-6 satellites.
Sentinel-6 will also include a secondary radio occultation payload, which makes use of GPS and GLONASS satellites occultations to measure physical properties of the atmosphere such as temperature, pressure and water vapor.
Sentinel-6 mission data, measuring how much heat is in the ocean, pinpointing where it is, and map its movement through ocean currents, will help the scientific community to better understand climate and predict future climate change.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).