Sentinel-6 Michael Freilich
Copernicus: Sentinel-6 Michael Freilich Mission — formerly 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)
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.
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 4). 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 23). 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).
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.
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).
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)
• November 19, 2020: Learn about sea-level rise and Copernicus Sentinel-6. 16)
Figure 11: Learn how climate change is causing our seas to rise and how satellites have been measuring the height of the sea surface systematically since 1992. With global sea level now rising fast, Copernicus Sentinel-6 Michael Freilich picks up the baton as the latest satellite mission to extend the legacy of sea-surface height measurements (video credit: ESA)
• November 12, 2020: As global temperatures continue to rise, coastal areas will increasingly bear the brunt of storm surges and more frequent, intense weather events. Sea level is rising at 3.6 cm per decade and this trend is accelerating, compounding the threats faced by coastal communities: with every centimeter another three million people are put at risk of annual coastal flooding. Scheduled to be launched on 21 November, the Copernicus Sentinel-6 Michael Freilich satellite is set to continue the long-term record of sea-level measurements that are needed for protect our coasts. 17)
• November 6, 2020: As preparations for the launch of Copernicus Sentinel-6 Michael Freilich continue, the team at the Vandenberg Air Force Base in California has bid farewell to the satellite as it is sealed from view within the two half-shells of its Falcon 9 rocket fairing. Liftoff is now set for 21 November at 17:17 GMT (18:17 CET; 09:17 PST). 18)
- Since its arrival at the launch site at the end of September, Sentinel-6 Michael Freilich has been thoroughly tested, fuelled and joined to the launch adapter. Now safely tucked up inside the rocket fairing that will protect it during liftoff, the next steps include roll out to the launch tower and fitting to the rest of the rocket.
- Once launched, this new mission will take the role of radar altimetry reference mission, continuing the long-term record of measurements of sea-surface height started in 1992 by the French–US Topex Poseidon and then the Jason series of satellite missions.
- Sea-level rise is one of the biggest threats we face as a consequence of climate change.
- Satellite data show that global mean sea level has risen, on average, by just over 3 mm every year since 1993. Even more worryingly, this rate of rise has increased in recent years. The role of Copernicus Sentinel-6 is not only to continue this critical ‘gold standard' record for climate studies, but also to measure sea-surface height with greater precision than before.
- 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 vulnerable low-lying areas.
Figure 12: The Copernicus Sentinel-6 Michael Freilich launch campaign team in front of the satellite (image credit: NASA, Randy Beaudoin)
Figure 13: Sentinel-6 orbit: The Copernicus Sentinel-6 satellites reach 66ºN and 66ºS – a specific orbit occupied by the earlier missions that supplied the reference sea-surface height data over the last three decades. This orbit allows 95% of Earth's ice-free ocean to be mapped every 10 days. As the next radar altimetry reference mission, Copernicus Sentinel-6 is continuing the long-term record of sea-surface height measurements that were started in 1992 by the French–US Topex Poseidon satellite and then by the Jason series of satellite missions. Copernicus Sentinel-6 comprises two identical satellites launched five years apart. Firstly, Copernicus Sentinel-6 Michael Freilich in 2020 and then Copernicus Sentinel-6B in 2025 to supply measurements until at least 2030 (video credit: ESA/ATG medialab)
Figure 14: The video shows the satellite being spun around on its frame and then moved out of the cleanroom. The satellite was subsequently fuelled. Everything went very smoothly, with the team completing this somewhat hazardous task in just one day. The fuelling team followed up to check that there were no leaks and then sealed the fill and drain valves (video credit: NASA)
- The next task is to join the satellite to the launch adapter before it is finally encapsulated in the Falcon 9 rocket fairing. Liftoff from the Vandenberg Air Force base in California has been confirmed for 19:29:39 GMT (20:29:39 CET) on 10 November.
- Once safely in orbit, Copernicus Sentinel-6 will continue the long-term record of reference sea-surface height measurements that were started in 1992 by the French–US Topex Poseidon satellite and then by the Jason series of satellite missions. The mission comprises two identical satellites launched five years apart. Firstly, Copernicus Sentinel-6 Michael Freilich launching in few weeks, and then Copernicus Sentinel-6B in 2025 to supply measurements until at least 2030.
- Since sea-level rise is a key indicator of climate change, accurately monitoring the changing height of the sea surface over decades is essential for climate science, for policy-making and, ultimately, for protecting those in low-lying regions at risk.
- 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.
• October 26, 2020: Teams at ESA's mission control centre are getting ready to ensure a new Sentinel-6 Earth Observation mission safely arrives in its correct orbit, from where it will map, measure and monitor rising sea levels after its launch on 10 November. 20)
- The 1.5-ton Copernicus Sentinel-6 ‘Michael Freilich' spacecraft will launch on a Space X Falcon 9 rocket from Vandenberg, California, in the United States. Once safely in orbit, ESA's ESOC Operations Centre in Darmstadt, Germany, will take over the reins.
- Over the subsequent three days, the Sentinel-6 mission control team will guide the fledgling mission through the ‘Launch and Early Orbit Phase' – the riskiest phase of its life.
- Like a bird hatching from the egg, this is the period in which the new spacecraft unfurls its solar arrays, wakes up to test its core functioning and maneuvers into the correct path, all the while at its most vulnerable to the hazards of space.
Figure 15: On 30 September 2020, the Sentinel-6 control team at ESA/ESOC in Darmstadt, Germany, practiced for liftoff. In one of many 'contingency simulations' they worked through scenarios in which the Launch and Early Orbit Phase doesn't go to plan. This way, they are as prepared as can be for every eventuality (image credit: ESA)
- Sentinel-6 Michael Freilich is the first of two spacecraft being launched to ensure the ‘continuity of service' of the Jason missions currently providing data on Earth's changing oceans, but reaching the end of their lives. This adds a layer of complexity to already tricky operations, as the new Sentinel needs to fly in tandem with the Jason-3 spacecraft it will replace, until the latter is moved to a different orbit.
Figure 16: Six key facts about Copernicus Sentinel-6. The satellite is taking on the role of radar altimetry reference mission, continuing the long-term record of measurements of sea-surface height started in 1992 by the French–US Topex Poseidon and then the Jason series of satellite missions (image credit: ESA)
- The target orbit for the new mission is a polar orbit bringing the mission high over Earth's icy poles at about 1300 km altitude. Timing here is extremely important, as Sentinel-6 needs to fly in tandem with the Jason 3 spacecraft, falling into position behind it with a separation of just 30 seconds, or about 230 km.
- Teams at ESOC will perform two orbit maneuvers during the first few days, edging the spacecraft closer to where it needs to be. But as Sentinel-6 takes over from Jason, so too will EUMETSAT, the European Organization for the Exploitation of Meteorological Satellites, take over the satellite command and control from ESA, after the third day.
- Once the Sentinel is through the critical early phase and drifting towards its target orbit, EUMETSAT will complete the final ‘orbit acquisition' and take on responsibility for commissioning, routine operations and distribution of the mission's vital data.
Simulating success during a pandemic
- Control teams are used to preparing for unexpected eventualities. In fact a large part of the job involves going through real-time simulations in which they are subjected to all manner of potential problems - from all kinds of spacecraft anomalies to computers crashing and even avoiding space debris.
- Now, they are rehearsing in the midst of a very real pandemic on Earth.
- "Of course, preparation for the Sentinel-6 launch has been affected by COVID-19, and we have put all measures in place to ensure success in this difficult situation. We must always keep a safe distance from each other, we have plexiglass walls separating everyone in the control rooms, masks worn at all times and the numbers of people on site are limited to those strictly needed to support operations" explains Massimo Romanazzo, Spacecraft Operations Manager for the mission.
- "We're doing all we can to ensure the health and safety of our teams and fortunately, despite the odds, we have not experienced any delays and are on schedule for launch on 10 November."
- The team has two more ‘contingency simulations' to go in which problems are injected into the launch sequence, and two final ‘nominal simulations' in which everything runs according to the ‘nominal' operations timeline.
- A couple of days before launch, they will then go through the dress rehearsal when they run through the launch sequence, but this time connected to the spacecraft in Vandenberg sitting on top of its Falcon 9, getting live data from the satellite.
Supported from the ground
- Sentinel-6 will join a fleet of Earth-monitoring spacecraft in one of the busiest space highways, low-Earth orbit. ESA's Space Debris Office based at ESOC will be on hand throughout the critical early days, monitoring and calculating the risk of collisions with swirling space debris and advising on how best to keep the mission safe.
- ESA's Kiruna ground station will track the spacecraft's first days, while the North Pole Satellite Station in Alaska is expected to catch its first signals from space after separation from the launcher.
- 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.
- "The Sentinel-6 mission perfectly brings together the best aspects of operating in space; international cooperation, cutting edge technology and a desire to bring benefits down to Earth from the unique vantage point of near-Earth orbit," says Simon Plum, ESA's new Head of Mission Operations.
- "And guiding a spacecraft through its most risky early days showcases what teams at ESOC do best as they put their years of training and experience to practise, all the while under additional constraints due to the COVID-19 pandemic. I am very proud to join a team with such professionalism and commitment, and look forward to my first launch here at ESA mission control."
Figure 17: The Kiruna S- and X-band station supports ESA's Earth observation missions. The station is located at Salmijärvi, 38 km east of Kiruna, in northern Sweden. The station is equipped for tracking, telemetry and command operations as well as for reception, recording, processing and dissemination of data (image credit: ESA, S. Corvaja)
• October 19, 2020: With less than a month to go before a SpaceX Falcon 9 takes Copernicus Sentinel-6 Michael Freilich into orbit to chart sea-level rise, preparations are forging ahead at the launch site. 21)
- Since sea-level rise is a key indicator of climate change, accurately monitoring the changing height of the sea surface over decades is essential for climate science, for policy-making and, ultimately, for protecting those in low-lying regions at risk.
- Satellites tracking the changing height of the ocean surface show that global mean sea level has risen, on average, by just over 3 mm every year since 1993. Even more worryingly, this rate of rise has increased in recent years. The role of Copernicus Sentinel-6 is not only to continue this critical ‘gold standard' record for climate studies, but also to measure sea-surface height with greater precision than before.
- 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.
- A radar altimeter derives the height of the satellite above Earth by accurately and precisely measuring the time it takes for a transmitted radar pulse to reflect Earth's surface. The returned echo pulse from the sea surface provides a waveform that is processed to determine 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 (determined from the strength of the power returns).
Figure 18: Copernicus Sentinel-6 in action. Sentinel-6 uses radar pulses that are transmitted and received using a timing arrangement that allows both conventional ‘pulse-limited' (low-resolution mode) data to be acquired simultaneously with high-resolution ‘delay-Doppler' measurements. This arrangement allows unfocussed synthetic aperture radar (SAR) data processing to be performed where the altimeter synthesizes a large antenna as it flies forward by exploiting the Doppler characteristics of the return echoes (video credit: ESA/ATG medialab)
- The processor then steers synthetic azimuth beams to specific locations on Earth's surface to build a ‘stack' of waveforms that can be processed to achieve better performance. Unfocussed SAR processing improves the precision of sea-surface measurements by averaging the stack of waveforms to reduce noise and improves the along track resolution from several kilometers to about 300 meters. SAR processing requires a much larger amount of data than conventional altimetry and the satellite implements a dedicated onboard processor to reduce the data rate that will be sent back to ground by a factor of two.
- 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.
• October 16, 2020: The Sentinel-6 Michael Freilich spacecraft will soon be heading into orbit to monitor the height of the ocean for nearly the entire globe.22)
- Preparations are ramping up for the Nov. 10 launch of the world's latest sea level satellite. Since arriving in a giant cargo plane at Vandenberg Air Force Base in California last month, Sentinel-6 Michael Freilich has been undergoing final checks, including visual inspections, to make sure it's fit to head into orbit.
- Surviving the bone-rattling vibrations and sounds of launch atop a Falcon 9 rocket is just the start of the mission. Once in orbit some 830 miles (1,336 km) above Earth, Sentinel-6 Michael Freilich has the task of collecting sea level measurements with an accuracy of a few centimeters (for a single measurement) for more than 90% of the world's oceans. And it will be making those measurements while repeatedly flying through an area of intense radiation known as the South Atlantic Anomaly, which can scramble electronics.
- That's why engineers and researchers have put Sentinel-6 Michael Freilich through a battery of tests to ensure that the spacecraft will survive launch and the harsh environment of space. But how will the mission pull the rest of it off? With sophisticated instruments, global navigation satellites, and lasers - lots of lasers. They'll all work in concert to enable the spacecraft to carry out its task of observing the ocean.
- Given the challenges and goals of the mission, the satellite's moniker is appropriate: It's named after noted researcher Dr. Michael Freilich, the former director of NASA's Earth Science Division.
- A second spacecraft identical to Sentinel-6 Michael Freilich, Sentinel-6B, will launch in 2025 to continue the work after its sibling's five-and-a-half-year prime mission ends. Together, the satellites make up the Sentinel-6/Jason-CS (Continuity of Service) mission, which is a partnership between NASA, ESA (the European Space Agency), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and the National Oceanic and Atmospheric Administration (NOAA).
- Collectively, the satellites will add a decade's worth of the most accurate satellite data yet on ocean height to a nearly 30-year record documenting how our oceans are rising in response to climate change. Both spacecraft will also collect data on atmospheric temperature and humidity that will help to improve weather forecasts as well as atmospheric and climate models.
- This is where those sophisticated instruments, global navigation satellites, and lasers come in.
How It Works
- To accurately measure extremely small variations in sea level, Sentinel-6 Michael Freilich will rely on a suite of three instruments that provide scientists information to determine the spacecraft's exact position in orbit.
- One component of this positioning package is the laser retroreflector array, a set of nine small, precisely shaped mirrors. Lasers are directed at them from ground stations on Earth, and they reflect the (harmless) beams right back to their point of origin. These laser-emitting ranging stations, as they're known, calculate how long the laser takes to bounce off the reflectors and return, which gives the distance between the satellite and the station.
- Another instrument, the Global Navigation Satellite System - Precise Orbit Determination (GNSS-POD), tracks GPS and Galileo navigation signals. Researchers analyze these signals to help determine the satellite's position.
- The third instrument in the positioning package is the Doppler Orbitography and Radioposition Integrated by Satellite (DORIS). It analyzes radio signals from 55 global ground stations, measuring the Doppler shift of the radio signals' frequencies to determine the 3D position of the satellite over time. When used together, these instruments provide the data needed to ascertain the precise position of the satellite, which in turn helps to determine the height of the sea surface.
- On the science side are two instruments that work in concert to determine sea level and a third that collects atmospheric data. The Poseidon-4 radar altimeter measures ocean height by bouncing radar pulses off the water's surface and calculating the time it takes for the signal to return to the satellite. However, water vapor in the atmosphere affects the propagation of the radar pulses from the altimeter, which can make the ocean appear higher or lower than it actually is. To correct for this affect, an instrument called the Advanced Microwave Radiometer for Climate (AMR-C) measures the amount of water vapor between the spacecraft and the ocean.
- "AMR-C is the next generation of AMR instruments, and it includes new components that will enable more accurate measurements along coastlines and throughout the mission," said Shannon Statham, AMR-C integration and test lead at NASA's Jet Propulsion Laboratory in Southern California.
Figure 19: The Sentinel-6 Michael Freilich satellite undergoes final preparations in a clean room at Vandenberg Air Force Base in California for an early November launch (image credit: ESA/Bill Simpson)
- For information on the atmosphere, the Global Navigation Satellite System - Radio Occultation (GNSS-RO) instrument gathers data on temperature and humidity that can help to improve weather forecasts. GNSS-RO analyzes radio signals from global navigational satellites as they appear and disappear beyond the limb of the Earth - the hazy blue edge of the atmosphere that's visible when you look at pictures of our planet in space. As these radio signals travel through different layers of the atmosphere, they bend and slow by varying degrees. Sentinel-6 Michael Freilich and satellites like it use GNSS-RO technology to measure these changes, enabling researchers to then extract atmospheric characteristics like temperature and humidity at different altitudes.
- All the instruments, power systems, telecommunications - everything that makes Sentinel-6 Michael Freilich tick - must work together to accomplish the mission's science goals, much like the international partners have worked together to get this satellite ready for launch.
- "Copernicus Sentinel-6 Michael Freilich is a great contribution to climate change, environment monitoring, and to the Digital Twin Earth. Sentinel-6 is a reference model of the cooperation between the U.S. and Europe on Earth Observation and represents a good foundation for future projects," said Josef Aschbacher, ESA director of Earth Observation Programs.
Figure 20: Behind the Spacecraft – Sentinel-6 Michael Freilich – Sea Level Scout. Our planet is changing. Our ocean is rising. And it affects us all. That's why a new international satellite will continue the decades-long watch over our global ocean and help us better understand how climate change is reshaping our planet. Meet some of the talented people behind Sentinel-6 Michael Freilich and get to know the satellite (video credit: NASA/JPL-Caltech)
• September 25, 2020: The world's latest ocean-monitoring satellite has arrived at Vandenberg Air Force Base in Central California to be prepared for its Nov. 10 launch. The product of a historic U.S.-European partnership, the Sentinel-6 Michael Freilich spacecraft touched down at Vandenberg in an Antonov 124 aircraft at around 10:40 a.m. PDT (1:40 p.m. EDT) on Sept. 24 after a two-day journey from an IABG engineering facility near Munich, Germany. 23) 24)
Figure 21: New Sea Level Satellite Arrives at California Launch Site. A shipping container containing the Sentinel-6 Michael Freilich satellite is removed from an Antonov 124 aircraft at Vandenberg Air Force Base in California on Sept. 24, 2020, after its two-day journey from an IABG engineering facility near Munich, Germany (image credit: 30th Space Wing)
- "The spacecraft had a smooth trip from Europe and is in good shape," said Parag Vaze, the mission's project manager at NASA's Jet Propulsion Laboratory in Southern California. "Final preparations are under way to see the satellite safely into Earth orbit in a little under seven weeks."
- The satellite is named after Dr. Michael Freilich, the former director of NASA's Earth Science Division and an instrumental figure in advancing ocean observations from space. Sentinel-6 Michael Freilich is one of two identical spacecraft that compose the Sentinel-6/Jason-CS (Continuity of Service) mission developed in partnership with ESA (the European Space Agency). ESA is developing the new Sentinel family of missions to support the operational needs of the European Union's Copernicus program, the EU's Earth observation program managed by the European Commission. The spacecraft's twin, Sentinel-6B, will launch in 2025.
- "It has been a long journey of planning, development, and testing for the mission team," said Pierrik Vuilleumier, the mission's project manager at ESA. "We are proud to work with our international partners on such a critical mission for sea level studies and are looking forward to many years of Sentinel-6 Michael Freilich taking critical sea level and atmospheric data from orbit."
- Once in orbit, each satellite will collect sea surface height measurements down to the centimeter for more than 90% of the world's oceans. They'll be contributing to a nearly 30-year-long dataset built by an uninterrupted series of spacecraft that started with the TOPEX/Poseidon mission in the early 1990s and that continues today with Jason-3. Instruments aboard the spacecraft will also provide atmospheric data that will improve weather forecasts, help to track hurricanes, and bolster climate models.
- Although Sentinel-6 Michael Freilich has already undergone rigorous testing, it will go through a final checkout at the SpaceX payload processing facility at Vandenberg to verify that the satellite is healthy and ready for launch.
- Once tests are complete, Sentinel-6 Michael Freilich will be mounted atop a SpaceX Falcon 9 rocket at Vandenberg Air Force Base's Space Launch Complex 4E. The launch is scheduled for 11:31 a.m. PST (2:31 p.m. EST) on Nov. 10.
• September 4, 2020: When the Sentinel-6 Michael Freilich launches this November, its primary focus will be to monitor sea level rise with extreme precision. But an instrument aboard the spacecraft will also provide atmospheric data that will improve weather forecasts, track hurricanes, and bolster climate models. 25)
- "Our fundamental goal with Sentinel-6 is to measure the oceans, but the more value we can add, the better," said Josh Willis, the mission's project scientist at NASA's Jet Propulsion Laboratory in Southern California. "It's not every day that we get to launch a satellite, so collecting more useful data about our oceans and atmosphere is a bonus."
- A U.S.-European collaboration, Sentinel-6 Michael Freilich is actually one of two satellites that compose the Copernicus Sentinel-6/Jason-CS (Continuity of Service) mission. The satellite's twin, Sentinel-6B, will launch in 2025 to take over for its predecessor. Together, the spacecraft will join TOPEX/Poseidon and the Jason series of satellites, which have been gathering precise sea level measurements for nearly three decades. Once in orbit, each Sentinel-6 satellite will collect sea level measurements down to the centimeter for 90% of the world's oceans.
- Meanwhile, they'll also peer deep into Earth's atmosphere with what's called Global Navigation Satellite System - Radio Occultation (GNSS-RO) to collect highly accurate global temperature and humidity information. Developed by JPL, the spacecraft's GNSS-RO instrument tracks radio signals from navigation satellites to measure the physical properties of Earth's atmosphere. As a radio signal passes through the atmosphere, it slows, its frequency changes, and its path bends. Called refraction, this effect can be used by scientists to measure minute changes in atmospheric physical properties, such as density, temperature, and moisture content.
- The precise global atmospheric measurements made by Sentinel-6 Michael Freilich will complement atmospheric observations by other GNSS-RO instruments already in space. Specifically, the National Oceanic and Atmospheric Administration's National Weather Service meteorologists will use insights from Sentinel 6's GNSS-RO to improve weather forecasts. Also, the GNSS-RO information will provide long-term data that can be used both to monitor how our atmosphere is changing and to refine models used for making projections of future climate. Data from this mission will help track the formation of hurricanes and support models to predict the direction storms may travel. The more data we gather about hurricane formation (and where a storm might make landfall), the better in terms of helping local efforts to mitigate damage and support evacuation plans.
How It Works
- Radio occultation was first used by NASA's Mariner 4 mission in 1965 when the spacecraft flew past Mars. As it passed behind the Red Planet from our perspective, scientists on Earth detected slight delays in its radio transmissions as they traveled through atmospheric gases. By measuring these radio signal delays, they were able to gain the first measurements of the Martian atmosphere and discover just how thin it was compared to Earth's.
- By the 1980s, scientists had started to measure the slight delays in radio signals from Earth-orbiting navigation satellites to better understand our planet's atmosphere. Since then, many radio occultation instruments have been launched; Sentinel-6 Michael Freilich will join the six COSMIC-2 satellites as the most advanced GNSS-RO instruments among them.
- "The Sentinel-6 instrument is essentially the same as COSMIC-2's. Compared to other radio occultation instruments, they have higher measurement precision and greater atmospheric penetration depth," said Chi Ao, the instrument scientist for GNSS-RO at JPL.
- The GNSS-RO instrument's receivers track navigation satellite radio signals as they dip below, or rise from, the horizon. They can detect these signals through the vertical extent of the atmosphere - through thick clouds - from the very top and almost all the way to the ground. This is important, because weather phenomena emerge from all layers of the atmosphere, not just from near Earth's surface where we experience their effects.
- "Tiny changes in the radio signal can be measured by the instrument, which relate to the density of the atmosphere," added Ao. "We can then precisely determine the temperature, pressure, and humidity through the layers of the atmosphere, which give us incredible insights to our planet's dynamic climate and weather."
Figure 22: With the help of JPL's GNSS-RO principal investigator Chi Ao and NOAA's National Weather Service meteorologist Mark Jackson, this video explains how the GNSS-RO instrument aboard Sentinel-6 Michael Freilich will be used by meteorologists to improve weather forecasting predictions (video credit: NASA/JPL-Caltech)
- But there's another reason why probing the entire vertical profile of the atmosphere from orbit is so important: accuracy. Meteorologists typically gather information from a variety of sources - from weather balloons to instruments aboard aircraft. But sometimes scientists need to compensate for biases in the data. For example, air temperature readings from a thermometer on an airplane can be skewed by heat radiating from parts of the aircraft.
- GNSS-RO data is different. The instrument collects navigation satellite signals at the top of the atmosphere, in what is close to a vacuum. Although there are sources of error in every scientific measurement, at that altitude, there's no refraction of the signal, which means there's an almost bias-free baseline to which atmospheric measurements can be compared in order to minimize noise in data collection.
- And as one of the most advanced GNSS radio occultation instruments in orbit, said Ao, it will also be one of the most accurate atmospheric thermometers in space.
• On August 5, 2020, NASA Administrator Jim Bridenstine announced in a statement the passing of Mike Freilich (1954-2020), passionate explorer and former director of NASA's Earth Science Division 26)
Table 2: NASA Administrator Jim Bridenstine statement about the passing of Michael Freilich
• July 21, 2020: Like students all over the world currently awaiting exam grades, the Copernicus Sentinel-6 Michael Freilich satellite has also been put through a series of strenuous tests leaving the eyes of the teams involved in this international mission set firmly on its final results. Happily, Sentinel-6 has passed with flying colors and engineers can now prepare it for shipment to the US for liftoff on a SpaceX Falcon-9, which is scheduled for 10 November. 27)
- Renamed in honor of Michael H. Freilich, who led NASA's work in Earth science, Copernicus Sentinel-6 Michael Freilich will assume the critical role of monitoring sea-level change by extending the long-term measurement record of global mean sea level from space.
- With millions of people living in coastal communities around the world, 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 for authorities to take action to protect vulnerable communities.
Figure 23: On average, between 1993 and 2018 sea level has risen by 3.2 mm but there are regional differences within this trend. This map is based on measurements from satellite altimeters and shows regional sea-level trends [image credit: CNES/LEGOS/CLS/EU Copernicus Marine Service/contains modified Copernicus Sentinel data (2018)]
- Over the last three decades, the French–US Topex-Poseidon and Jason missions served as reference missions, and in combination with ESA's earlier ERS and Envisat satellites, as well as today's CryoSat and Copernicus Sentinel-3, they have shown how sea level has risen about 3.2 mm on average a year. More alarmingly, the rate of rise has been accelerating over the last few years. It is now rising at 4.8 mm a year.
- Now it is time for the Copernicus Sentinel-6 mission to pick up the baton and extend this dataset that is the ‘gold standard' for climate studies – and following the positive outcome of the technical ‘qualification acceptance review' stating that the satellite has passed all of its tests, the satellite can be packed up for shipment to the launch site.
Figure 24: Ready to measure sea-surface height. Copernicus Sentinel-6 carries a radar altimeter to observe changes in sea-surface topography with centimeter precision, providing insights into global sea levels. These measurements are not only critical for monitoring our rising seas, but also for climate forecasting, sustainable ocean-resource management, coastal management and environmental protection, the fishing industry, and more. The Copernicus Sentinel-6 mission will assume the critical role of monitoring sea-level change by extending the long-term measurement record of global mean sea level from space (image credit: ESA, S. Corvaja)
- Pierrik Vuilleumier, ESA's Copernicus Sentinel-6 project manager, said, "This review is an important milestone and the plan now is to have the satellite packed up by the end of the month for shipment from IABG's center near Munich in Germany to the Vandenberg launch site in California in the US. Given the COVID-19 situation, all those involved have worked brilliantly to keep to schedule.
- "We plan to ship to Vandenberg on 23 September, following a few other reviews related to the readiness of the launch site and spacecraft operations."
- The mission, which comprises two satellites launched sequentially, is a true example of international cooperation: it has been jointly developed by ESA, NASA, EUMETSAT and NOAA, with support from CNES.
- Each satellite carries 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.
- The satellites' instrument package also includes an advanced microwave radiometer that accounts for the amount of water vapor in atmosphere, which affects the speed of the altimeter's radar pulses.
• July 7, 2020: Over the course of nearly three decades, an uninterrupted series of satellites has circled our planet, diligently measuring sea levels. The continuous record of ocean height that they've built has helped researchers reveal the inner workings of weather phenomena like El Niño and to forecast how much the ocean could encroach on coastlines around the world. Now, engineers and scientists are preparing two identical satellites to add to this legacy, extending the dataset another decade. 28)
- Both spacecraft are a part of the Sentinel-6/Jason-CS (Continuity of Service) mission, a U.S.-European collaboration that aims to make some of the most accurate measurements of sea levels around the world. The first satellite to launch, Sentinel-6 Michael Freilich, will lift off in November. Its twin, Sentinel-6B, will launch in 2025. Both will assess sea levels by sending electromagnetic signals down to the ocean and measuring how long it takes for them to return to the spacecraft.
Figure 25: This chart shows the rise in global average sea level from January 1993 to January 2020. The measurement is made using data collected by the Sentinel-6/Jason-CS mission's predecessors, the TOPEX/Poseidon, Jason-1, OSTM/Jason-2, and Jason-3 satellite missions (image credit: NASA Goddard Space Flight Center)
- "This mission will continue the invaluable work of accurately measuring sea surface height," said Karen St. Germain, director of NASA's Earth Science Division. "These measurements enable us to understand and predict sea level changes that will affect people living in coastal regions around the world."
- The satellite will build on efforts that began in 1992 with the launch of the TOPEX/Poseidon mission and that continued with three more missions over the years: Jason-1, OSTM/Jason-2, and Jason-3. Sentinel-6/Jason-CS aims to extend the nearly 30-year sea level dataset that these previous missions built by another 10 years.
- Measuring the height of the ocean gives scientists a real-time indication of how Earth's climate is changing, said Josh Willis, the mission's project scientist at NASA's Jet Propulsion Laboratory in Southern California. The oceans absorb about 90% of the excess heat from the planet's warming climate. Seawater expands as it heats up, resulting in about a third of the modern-day global average sea level rise. Melting ice from land-based sources like glaciers and ice sheets accounts for the rest.
- To understand how rising seas will affect humanity, researchers need to know how fast this is happening, said Willis. "Satellites are the most important tool to tell us this rate," he explained. "They're kind of a bellwether for this creeping global warming impact that's going to inundate coastlines around the world and affect hundreds of millions of people."
- Currently, sea levels rise an average of 0.13 inches (3.3 millimeters) per year, more than twice the rate at the start of the 20th century. "By 2050, we'll have a different coastline than we do today," said Willis.
- "As more and more people move to coastal regions, and coastal megacities continue to develop, the impact of sea level change will be more profound on those societies," said Craig Donlon, mission project scientist at the European Space Agency.
• June 11, 2020: A team of engineers in the U.S. and Europe subjected the Sentinel-6 Michael Freilich spacecraft to a battery of trials to ready it for liftoff later this year. 29)
Figure 26: The test chamber, which covers an area of 100 m2 and is fitted with huge loudspeakers, is hermetically sealed during sound tests. This is to ensure that the high decibels associated with liftoff won't damage the spacecraft (image credit: Airbus) 30)
- Once the state-of-the-art Sentinel-6 Michael Freilich satellite launches in November, it will collect the most accurate data yet on sea level — a key indicator of how Earth's warming climate is affecting the oceans, weather and coastlines. But first, engineers need to ensure that the spacecraft can survive the rigors of launch and of operating in the harsh environment of space. That's where meticulous testing comes in.
- At the end of May, engineers finished putting the spacecraft — which is being built in Germany — through a battery of tests that began in November 2019. "If it can survive all the abuse we deliberately put it through on the ground, then it's ready for space," said John Oswald, the mission's deputy project manager at NASA's Jet Propulsion Laboratory in Southern California.
- The Sentinel-6 Michael Freilich spacecraft is a part of the Copernicus Sentinel-6/Jason-CS (Continuity of Service) mission, a joint U.S.-European effort in which two identical satellites will be launched five years apart. The spacecraft will join the Copernicus constellation of satellites that constitutes the European Union's Earth Observation Program. Once in orbit, each satellite will collect sea level measurements down to the centimeter for 90% of the world's oceans. The data will add to almost 30 years of information gathered by an uninterrupted series of joint U.S.-European satellites, creating an unprecedented — and unbroken — 40-year sea level dataset. The spacecraft will also measure the temperature and humidity of Earth's atmosphere, which can be used to help improve weather forecasts and hurricane predictions.
- These measurements are important because the oceans and atmosphere are tightly connected. "We're changing our climate, and the clearest signal of that is the rising oceans," said Josh Willis, the mission's project scientist at JPL. "More than 90% of the heat trapped by greenhouse gases is going into the ocean." That heat causes seawater to expand, accounting for about one-third of the global average of modern-day sea level rise. Meltwater from glaciers and ice sheets account for the rest.
- "For climate science, what we need to know is not just sea level today, but sea level compared to 20 years ago. We need long records to do climate science," said Willis.
- Six scientific instruments are key to that task. Two of them will work in concert to measure the distance from the satellite to the ocean's surface. That information — combined with data from three other instruments that precisely establish the satellite's position in orbit and a sixth that will measure vertical slices of the atmosphere for temperature and humidity — will help determine sea levels around the world.
Put Through Their Paces
- To ensure that the scientific instruments will work once they get into space, engineers sent the Sentinel-6 Michael Freilich to a testing facility IABG) near Munich and ran the satellite through a gauntlet starting in November 2019.
- First up: the vibration test, where the engineers subjected the Sentinel-6 Michael Freilich satellite to the kinds of shaking it will experience while attached to a SpaceX Falcon 9 rocket blasting into orbit. Then in December, engineers tested the spacecraft in a big vacuum chamber and exposed it to the extreme temperatures that it will encounter in space, ranging from 149 to minus 292 degrees Fahrenheit (65 to minus 180 degrees Celsius).
- The next two trials took place in late April and May. The acoustics test, performed in April, made sure the satellite could withstand the loud noises that occur during launch. Engineers placed the spacecraft in a 100 m2 chamber outfitted with enormous speakers. Then they blasted the satellite with four 60-second bursts of sound, with the loudest peaking around 140 decibels. That's like standing next to a jet's engine as the plane takes off.
- Finally, in the last week of May, engineers performed an electromagnetic compatibility test to ensure that the sensors and electronics on the satellite wouldn't interfere with one another, or with the data collection. The mission uses state-of-the-art instruments to make precise measurements, so the smallest interference could compromise that data.
- Normally, JPL engineers would help to conduct these tests in person, but two of the trials took place after social-distancing safety measures had been established due to the coronavirus pandemic. So team members worked out a system to support their counterparts in Germany remotely.
- To account for the nine-hour time-zone difference, engineers in California pulled shifts from midnight to 10 A.M. for several weeks, consulting with colleagues in Germany through phone calls, video conferences, chat rooms and text messages. "It was confusing sometimes, keeping all the channels and groups going at the same time in the middle of the night, but I was impressed with our team," said Oswald.
- The upshot of all that effort? "The tests are complete and the preliminary results look good," Oswald said. Team members will spend the next several weeks completing the analysis of the test results and then preparing the satellite for shipment to Vandenberg Air Force Base in California for launch this fall.
• May 4, 2020: During these unprecedented times of the COVID-19 (Corona Virus Disease-19) lockdown, trying to work poses huge challenges for us all. For those that can, remote working is now pretty much the norm, but this is obviously not possible for everybody. One might assume that like many industries, the construction and testing of satellites has been put on hold, but engineers and scientists are finding ways of continuing to prepare Europe's upcoming satellite missions such as the next Copernicus Sentinels. 31)
Figure 27: With liftoff still scheduled for the end of 2020, the Copernicus Sentinel-6 Michael Freilich satellite is currently being tested to ensure that it will withstand the rigors of launch and the harsh environment of space during its life in orbit around Earth. The constraints imposed by the COVID-19 crisis mean that there are far fewer engineers in the cleanroom testing the satellite at IABG's center in Ottobrunn near Munich in Germany – but work continues (image credit: Airbus DS)
- For example, with liftoff still scheduled for the end of this year, the Copernicus Sentinel-6 Michael Freilich satellite is currently being tested to ensure that it will withstand the rigors of launch and the harsh environment of space during its life in orbit around Earth.
- This new satellite will assume the role as a reference mission to provide critical data for the long-term record of sea-surface height measurements.
- As one of the most severe consequences of climate change, global sea level is rising – putting millions of people at risk. It is essential to continue measuring the changing height of the sea surface to monitor this worrying trend so that decision-makers are equipped to take appropriate mitigating action.
- The constraints imposed by the COVID-19 crisis mean that there are far fewer engineers in the cleanroom testing the satellite at IABG's center near Munich in Germany.
- Pierrik Vuilleumier, ESA's Copernicus Sentinel-6 mission project manager, said, "The current situation has meant that many of us are having to follow the test campaign remotely. Since this is an international mission, people are scattered across Europe and the US.
- "Remarkably, we have reached an important milestone completing the acoustic vibration tests, which simulate the noisy environment of liftoff and ascent through the atmosphere. This just shows how the team is determined to meet the launch date in November, despite the difficult circumstances."
- Copernicus Sentinel-6 is now set for the next set of tests, which includes the ‘electromagnetic compatibility' tests. With these complete, at the end of September, it will be transported to the Vandenberg Air Force Base in California for liftoff on a NASA-provided Space-X Falcon 9 rocket.
- Sentinel-6 Michael Freilich is being jointly developed by ESA, NASA, EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) and NOAA (National Oceanic and Atmospheric Administration), with support from CNES (Centre National d'Etudes Spatiales).
• On January 28, 2020, NASA and its partners announced they have renamed a key ocean observation satellite launching this fall in honor of Earth scientist Michael Freilich, who retired last year as head of NASA's Earth Science division, a position he held since 2006. 32) 33)
Figure 28: A key ocean observation satellite launching this fall has been named after Earth scientist Michael Freilich, as announced Jan. 28 by NASA, ESA (European Space Agency), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and the National Oceanic and Atmospheric Administration (NOAA), video credit: NASA
Table 3: Some background: NASA, Partners Name Ocean Studying Satellite for Noted Earth Scientist, namely: Sentinel-6 Michael Freilich
• December 5, 2019: In a cleanroom in Ottobrunn, Germany, the latest Copernicus Sentinel satellite is ready for final testing before it is packed up and shipped to the US for liftoff next year. Designed and built to chart changing sea level, it is the first of two identical Sentinel-6 satellites that will be launched consecutively to continue the time series of sea-level measurements. This new mission builds on heritage from previous ocean topography satellites, including the French–US Topex-Poseidon and Jason missions, previous ESA missions such as the ERS satellites, Envisat and CryoSat, as well as Copernicus Sentinel-3. With millions of people around the world at risk from rising seas, it is essential to continue measuring the changing height of the sea surface so that decision-makers are equipped to take appropriate mitigating action – as is being currently highlighted at the COP-25 Climate Change Conference in Spain. 34)
Figure 29: In a cleanroom in Ottobrunn, Germany, the latest Copernicus Sentinel satellite is ready for final testing before it is packed up and shipped to the US for liftoff next year (video credit: ESA)
• November 20, 2019: For the first time, U.S and European agencies are preparing to launch a 10-year satellite mission to continue to study the clearest sign of global warming - rising sea levels. The Sentinel-6/Jason-CS mission (short for Jason-Continuity of Service), will be the longest-running mission dedicated to answering the question: How much will Earth's oceans rise by 2030? 35)
- By 2030, Sentinel-6/Jason-CS will add to nearly 40 years of sea level records, providing us with the clearest, most sensitive measure of how humans are changing the planet and its climate.
- The mission consists of two identical satellites, Sentinel-6A and Sentinel-6B, launching five years apart. The Sentinel-6A spacecraft was on display for the media on 15 November for a last look in its clean room in Germany's IABG space test center. The satellite is being prepared for a scheduled launch in November 2020 from Vandenberg Air Force Base in California on a SpaceX Falcon 9 rocket.
- Sentinel-6/Jason-CS follows in the footsteps of four other joint U.S.-European satellite missions - TOPEX/Poseidon and Jason-1, Ocean Surface Topography/Jason-2, and Jason-3 - that have measured sea level rise over the past three decades. The data gathered by those missions have shown that Earth's oceans are rising by an average of 3 mm/year.
- Sentinel-6/Jason-CS will continue that work, studying not just sea level change but also changes in ocean circulation, climate variability such as El Niño and La Niña, and weather patterns, including hurricanes and storms.
- "Global sea level rise is, in a way, the most complete measure of how humans are changing the climate," said Josh Willis, the mission's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "If you think about it, global sea level rise means that 70% of Earth's surface is getting taller - 70% of the planet is changing its shape and growing. So it's the whole planet changing. That's what we're really measuring."
- As the oceans warm, they expand, increasing the volume of water; the trapped heat also melts ice sheets and glaciers, contributing further to sea level rise. The rate at which it is rising has accelerated over the past 25 years and is expected to continue accelerating in years to come.
- Along with measuring sea level rise, the mission will provide datasets that can help with weather predictions, assessing temperature changes in the atmosphere and collecting high-resolution vertical profiles of temperature and humidity.
- As with its Jason-series predecessors, Sentinel-6/Jason-CS will gather global ocean data every 10 days, providing insights into large ocean features like El Niño events. However, unlike previous Jason-series missions, its higher-resolution instruments will also be able to provide data on smaller ocean features - including complex currents - that will benefit navigation and fishing communities.
Figure 30: The Jason-CS/Sentinel-6 mission that will track sea level rise, one of the clearest signs of global warming, for the next 10 years. Sentinel-6A, the first of the mission's two satellites, is shown in its clean room in Germany and is scheduled to launch in November 2020 (image credit: IABG)
• November 15, 2019: Media representatives and mission partners gathered today in Germany to see a new satellite, which will take the lead in charting sea-level change, before it undergoes final testing and is packed up for shipment to the US for lift-off next year. 36)
- Copernicus Sentinel-6 was on full display at the IAGB space test center in Ottobrunn near Munich, giving media and partners in the mission a unique opportunity to see this remarkable new satellite up close.
Figure 31: The Copernicus Sentinel-6/Jason CS stands on display at the IAGB space test center. It will map up to 95% of Earth's ice-free ocean every 10 days in order to monitor sea level variability. The radar altimeter will also measure the ocean surface topography – the hills and valleys of the ocean – that help us to map ocean currents. In addition, it will provide estimates of wind speed and wave height for maritime safety (image credit: ESA, S. Corvaja)
- ESA's Director of Earth Observation Programs, Josef Aschbacher, said, "We are all extremely proud to see the complete satellite on show here in the cleanroom. With global sea level rising at shocking rates, Copernicus Sentinel-6 will take the lead in providing systematic measurements of sea level so that the worrying trend in sea-level rise can be closely monitored and key information provided for important policy decisions."
- Sentinel-6 is realized thanks to cooperation between ESA, NASA, the European Commission, EUMETSAT and NOAA.
- "The mission has been developed thanks to the outstanding international cooperation with our US partners. Sentinel-6 is indeed a model case of pan-European and US–European cooperation, taking advantage of a 26-year history in altimetry measurements from space on both sides of the Atlantic."
- Sentinel-6 builds on heritage from previous ocean topography satellites, including the French–US Topex-Poseidon and Jason missions, previous ESA missions such as the ERS satellites, Envisat and CryoSat, as well as Copernicus Sentinel-3.
- These missions have shown how sea level rose by about 3.2 mm on average a year between 1993 and 2018, but more alarmingly, that the rate of rise has been accelerating over the last few years. It is now rising at 4.8 mm a year.
- Caused mainly by warming ocean waters, melting glaciers and diminishing ice sheets, sea-level rise is one of the most severe consequences of climate change. With millions of people around the world at risk from rising seas, it is essential to continue measuring the changing height of the sea surface so that decision-makers are equipped to take appropriate mitigating action.
- The Copernicus Sentinel-6 satellite will be launched in November 2020 from the Vandenberg Air Force Base in California, US on a Falcon-9. It will be the first time ESA cooperates, through NASA, with the private US aerospace manufacturer SpaceX, which was founded in 2002 by Elon Musk.
• 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. 37) 38)
Figure 32: 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. 39)
- 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 33: 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 36).
- 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 34: 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. 40)
- 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 35). 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 35: 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.
- 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. 41)
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. 42) 43)
Figure 37: 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.44)
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 4.
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 38 and 39 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.