Copernicus: Sentinel-6 

Overview

Sentinel-6 is the second component of the hybrid solution (Jason-3 + Sentinel-6) agreed to in 2009. Sentinel-6 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 Sentinel-6 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.

Background

In late 2013, following a request from the EC (European Commission), it was agreed that Sentinel-6, formerly known as Jason-CS, should become more closely associated with the other missions in the Copernicus family, and use the name Sentinel-6. A compromise was adopted so that the Sentinel-6 mission will be implemented with the Jason-CS satellite, and partner organizations are able to use either name according to circumstances.

As part of the approval process on the EUMETSAT side, the second meeting of potential program participants was held in December 2013. At this meeting, ESA announced that the new High Resolution Microwave Radiometer, which was still under technical investigation, would be suppressed for affordability reasons. The detailed technical definition continues in Phase-B2, including the selection of the subcontractor for the Mono-Propellant Propulsion System being performed according to ESA's Best Practice rules. 3)

In early December 2014, ESA selected Airbus Defence and Space as the prime contractor to develop and construct the first Sentinel-6 satellite. 4) ⁵⁾

ESA and Airbus Defence and Space then signed a contract On May 11, 2015, to develop the Sentinel-6A satellite mission for Europe's Copernicus program. 6)

TAS (Thales Alenia Space) signed the first part of a contract with Airbus Defense and Space in July 2015, to supply Poseidon-4 spaceborne radar altimeters. These instruments are installed on the Sentinel 6A and Sentinel-6B satellites developed by Airbus Defense and Space for ESA (European Space Agency), in collaboration with EUMETSAT and the European Commission, for the Copernicus program. 7)

- Drawing on a 20 year heritage of orbital operations, the Poseidon-4 altimeter features higher performance than the previous generation, because of the introduction of a new, "interleaved" SAR (Synthetic Aperture Radar) operating mode. Poseidon-4 will also feature a new architecture, improving the role of the digital functions to support higher stability of the performances, and eventually reduce development costs.

The EUMETSAT Member States have approved the development and implementation of the collaborative high precision ocean altimetry Sentinel-6 mission on September 11, 2015, involving also ESA, the European Union through its Copernicus program, and the United States, through NASA and NOAA. 8)

The Sentinel-6 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 (Sentinel-6A and Sentinel-6B) 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. 

Programmatic Setup

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 Sentinel-6 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 Sentinel-6 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 Sentinel-6B (together with the EU) and potentially part of Sentinel-6A as well.

• ESA is responsible for the development of the first Sentinel-6 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 Sentinel-6B (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. 10)

Mission Objectives

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.

Mission Characteristics

The Sentinel-6 Space Segment consists of two successive Sentinel-6 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 Sentinel-6 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 Sentinel-6 satellites will fly in the same orbit as their predecessors, TOPEX/Poseidon and the Jason missions (Table 1). 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, Sentinel-6 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

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.

- 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.

- 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. 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.

Spacecraft

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 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 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 Sentinel-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.

Sentinel-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 resulting overall satellite layout is shown in Figures 8 and 9.

The POS4 (Poseidon-4 Radar Altimeter) is the main instrument of the Sentinel-6 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 Sentinel-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 Sentinel-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 Sentinel-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 Sentinel-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 Sentinel-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.

Launches

Sentinel-6A Michael Freilich, the first satellite of the Copernicus Sentinel -6 mission, 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.. 16) 17)

The second satellite of the series, Sentinel-6B, was launched from Space Launch Complex 4 East (SLC-4E) from Vandenberg Space Force Base in California, USA, at 05:21 am UTC, aboard a Falcon 9 rocket. 71)

Orbit

The nominal orbit for Sentinel-6A and Sentinel 6B is the same of the precedent missions (TOPEX/Poseidon, Jason-1 to -3) ensuring data consistency with the previously acquired time series. The mission operates in drifting orbit from a relatively high altitude (1336 km), with an inclination of 66º. See table 1 for orbital parameters. 72)

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 11 and 12 show the intersections of reception cones of exemplary ground stations and the Sentinel-6 ground track. Considering the exemplary ground stations, the mean contact time will be 16 min with 76 min contact gap.

Mission Status

• November 17, 2025: Sentinel-6A’s twin Sentinel-6B, was launched at 05:21 am UTC from Space Launch Complex 4 East (SLC-4E) from Vandenberg Space Force Base in California, aboard SpaceX’s Falcon 9 rocket. 71)

• May 17, 2023: As global sea-surface temperatures hit record highs and an El Niño event looms, the Copernicus Sentinel-6 mission plays a crucial role in monitoring these climate shifts by providing the most accurate measurements of sea-surface height, which contributes to understanding and tracking sea-level rise. Sentinel-6 complements Sentinel-3 by delivering sea-level data every 10 days, a key factor in analyzing ocean temperature effects, including expansion due to warming. This high-precision data, along with near-real-time information, supports ongoing climate studies and allows scientists to gauge El Niño’s impact on ecosystems, weather patterns, and sea levels worldwide, helping inform strategies to mitigate these changes. ⁷⁰⁾

• June 8, 2022: Satellite altimetry missions, such as Copernicus Sentinel-6, are instrumental in tracking global and regional sea level changes, a critical metric in understanding the impacts of rising sea levels. The island of Crete, Greece, supports these efforts through the ESA’s Permanent Facility for Altimetry Calibration, which utilises transponders on the White Mountains to validate satellite altimeters following their launch. A documentary detailing this process features insights from Craig Donlon of ESA and Stelios Mertikas from the Technical University of Crete, explaining how these calibration efforts help ensure accurate sea height data for scientific and environmental analysis. ¹⁹⁾

• March 22, 2022: Sentinel-6 Michael Freilich has become the official reference for global sea level measurements, meaning other satellites will now compare their data to ensure accuracy. Continuing a nearly 30-year mission lineage started by TOPEX/Poseidon, Sentinel-6 Michael Freilich delivers precise sea surface height data critical for climate monitoring. Its predecessor, Jason-3, will soon move into an interleaved orbit, doubling measurement frequency alongside Sentinel-6, enhancing spatial resolution. The mission, supported by scientists like NASA's Josh Willis and EUMETSAT's Julia Figa Saldana, aims to improve understanding of rising sea levels and their effects on coastal communities. ²⁰⁾

• December 13, 2021: La Niña has re-emerged in the Eastern Pacific Ocean for the second consecutive year, with forecasts predicting its presence until at least spring 2022. This phenomenon intensifies easterly trade winds, which bring cooler, upwelled water to the eastern and central Pacific Ocean, affecting global atmospheric circulation and creating distinct weather patterns. La Niña typically results in increased rainfall in Indonesia and Australia, drier conditions in Brazil and Argentina, and cooler, stormier weather in the Pacific Northwest, while the southern U.S. tends to experience warmer and drier conditions. This current La Niña coincides with a cool phase of the Pacific Decadal Oscillation, a trend that has contributed to prolonged drought in the American Southwest. Observations from the Sentinel-6 Michael Freilich satellite are providing detailed insights into La Niña's effects, as scientists work to align this new data with historical sea level records. 21)

• November 29, 2021: The Copernicus Sentinel-6 Michael Freilich satellite delivers precise, high-resolution data on sea-surface height, leveraging EUMETSAT's operational expertise to manage and distribute these observations. Equipped with the advanced Poseidon-4 altimeter, Sentinel-6 provides simultaneous low- and high-resolution radar measurements. This dual capability enhances continuity with earlier missions like Jason-3 while enabling detailed tracking of ocean currents, wind speeds, and wave heights. The mission includes a 12-month tandem flight with Jason-3, which allows for precise cross-calibration to within 1 mm, ensuring consistent measurements in the long-term sea level record. Now fully operational, the data from Sentinel-6 is vital for forecasting models, hurricane tracking, and understanding climate change impacts on the oceans with unprecedented accuracy. 22)

• June 21, 2021: The Copernicus Sentinel-6 Michael Freilich satellite is now operational after more than six months of calibration alongside Jason-3, achieving a remarkable consistency with differences under 2mm, verified through ESA’s transponder on Crete. Sentinel-6’s data streams are now available: the first within hours of collection, aimed at time-sensitive uses like weather forecasting, and a more refined stream a few days later. A third, ultra-precise stream is expected soon, enhancing accuracy for long-term sea-level rise monitoring. With new high-resolution radar data from the Poseidon-4 altimeter, Sentinel-6 supports the transition from Jason-3’s measurements, ensuring seamless continuity in sea-level records. 24) 25)

• February 8, 2021: After its launch in November 2020, the Copernicus Sentinel-6 Michael Freilich satellite successfully completed the 'in-orbit verification phase' in January 2021, confirming its altimeter and subsystems are functioning accurately. ESA’s team managed early operations, including Launch and Early Orbit Phase (LEOP), before transferring control to EUMETSAT for ongoing operations. In testing, the Poseidon-4 altimeter demonstrated high accuracy, exceeding previous Sentinel-3 and Jason-3 missions, with cross-calibration at ESA's Crete facility confirming its precision. The satellite, in a trailing orbit 30 seconds behind Jason-3, will assume the primary role in sea-level monitoring after a 12-month tandem period, providing enhanced data to track climate-related sea-level changes. 26)

• November 2020: EUMETSAT took control of Sentinel-6 Michael Freilich’s flight operations in late November 2020, positioning the satellite to fly 30 seconds behind Jason-3 on the same orbit for cross-calibration of sea-level data. This alignment enables seamless continuity of sea-level records as EUMETSAT activates all instruments and coordinates initial data processing with ESA and NASA. The organization, in collaboration with global partners, aims to complete system calibration and release near-real-time products by June, with the highest-accuracy climate data to follow in an additional six months. ³⁰⁾

• November 25, 2020: Following a successful launch on November 21, 2020, aboard a SpaceX Falcon 9 rocket, the Copernicus Sentinel-6 Michael Freilich satellite entered orbit and transmitted its first signal to ESA’s mission control in Germany, confirming its readiness for operations. During the crucial Launch and Early Orbit Phase (LEOP), ESA's flight control team carefully managed the satellite as it deployed solar arrays, completed functional tests, and performed two maneuvers to begin its journey toward its final orbit. With LEOP successfully concluded, ESA handed over control to EUMETSAT, which will finalize orbit acquisition, manage routine operations, and distribute the satellite’s ocean monitoring data. ³¹⁾

• November 21, 2020: Sentinel-6A Michael Freilich was launched aboard SpaceX’s Falcon 9 rocket at 17:17 UTC, from Space Launch Complex 4 (SLC-4A) from Vandenberg Space Force Base in California,USA. 16)

• 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. ⁴⁵⁾

Figure 24: 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 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). ⁴⁶⁾

- 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.

Figure 27: 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)

• October 29, 2020: In preparation for liftoff on 10 November, the Copernicus Sentinel-6 Michael Freilich satellite has been fuelled. ⁴⁷⁾

Figure 28: 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.

• 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. ⁴⁸⁾

- 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.

- 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.

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.

- 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.

• 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. ⁴⁹⁾

Figure 32: 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)

• 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.⁵⁰⁾

- Preparations are ramping up for upcoming 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.

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.

- 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.

Figure 34: 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. ^{51) 52)}​

- 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. 

• 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. ⁵³⁾

- 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.

- 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.

Figure 36: 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)

• 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. ⁵⁴⁾

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. ⁵⁵⁾

• 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. ⁵⁶⁾

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.

• 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. ⁵⁷⁾

- 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.

- 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.

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. ⁵⁹⁾

- 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.

- 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.

- 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.

• 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. ^{60) 61)}

Figure 42: 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

During Freilich's NASA tenure, the agency increased the pace of Earth science mission launches and in 2014 alone sent five missions to space to study our home planet. The missions balanced many objectives from research to applications and technology development activities. Freilich also led NASA's response to the National Academy of Sciences' first-ever Earth Science and Applications from Space decadal survey in 2007, which expanded NASA's innovative Earth-observing programs and continues to guide the agency's global Earth observation efforts.

Freilich also established the sustained Venture Class program of low-cost space and airborne science missions that is now a central feature of the NASA Earth Science Division's portfolio. He pioneered the broad use of the International Space Station as a platform for Earth-observing instruments, a unique observing platform for the Earth system. Unlike many of the traditional Earth observation platforms, the space station orbits the Earth in an inclined equatorial orbit that is not Sun-synchronous. This means that the space station passes over locations between 52 degrees north and 52 degrees south latitude at different times of day and night, and under varying illumination conditions. This is particularly important for collecting imagery of unexpected natural hazard and disaster events such as volcanic eruptions, earthquakes, flooding and tsunamis, as well as for cross-calibrating other satellites in Sun-synchronous polar orbits.

Freilich also inaugurated a NASA activity to use data products from private sector, small-satellite constellations and commercial partners to supplement traditional government data sources. Under Freilich's leadership, NASA looked at new ways to carry out its critical mission and established cutting-edge programs to use small satellites and payloads hosted on commercial satellites to advance Earth science research and to demonstrate new technologies.

All told, during Freilich's time at NASA Headquarters, he oversaw 16 successful major mission and instrument launches and eight CubeSat/small-satellite launches. The agency's Earth Science Division has 14 Earth-observing missions in development for launch by 2023, which includes eight major hosted instruments on other nations' satellites.

Freilich's career as an oceanographer spanned nearly four decades and integrated research on Earth's oceans, leading satellite mission development, and helping to train and inspire the next generation of scientific leaders. His training was in ocean physics, but his vision encompasses the full spectrum of Earth's dynamics.

• 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. ⁶²⁾

Figure 43: 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 mission, will be the longest-running mission dedicated to answering the question: How much will Earth's oceans rise by 2030? ⁶³⁾

- 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.

• 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. ⁶⁴⁾

- 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.

- 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.

• 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. ^{65) 66)}

• 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. ⁶⁷⁾

- 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.

• 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. ⁶⁸⁾

- 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. 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.

- 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 mission had its first meeting in June. ⁶⁹⁾