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SPACECRAFT AND
INSTRUMENTS

The Sentinel-6 Michael Freilich spacecraft undergoes tests

The Sentinel-6 Michael Freilich spacecraft undergoes tests at an Airbus facility in Friedrichshafen, Germany, in 2019.
Image credit: Airbus

Satellite Bus

The Sentinel-6 Michael Freilich spacecraft was built by Airbus Defence and Space in Friedrichshafen, Germany. It is 16 feet, 11 inches (5.15 meters) long, 7 feet, 7 inches (2.35 meters) high, 8 feet, 5 inches (2.58 meters) wide and weighs 2,628 pounds (1,192 kilograms) including onboard propellant.



POWER SYSTEM

The electrical system of the Sentinel-6 Michael Freilich satellite comprises all the necessary hardware to operate the spacecraft while allowing the onboard systems to execute the software. The electrical power subsystem generates energy via sunlight collected by the 185.1-square-foot (17.5-square-meter) body-mounted solar arrays. Each array consists of gallium arsenide (GaAs) solar cells that cover the top and sides of the satellite like a tent. Excess energy is stored in a lithium-ion battery (based on 1,152 cells, split into two modules) with a total capacity of approximately 200 amp hours. The system provides an average of 1 kilowatt of electrical power in orbit.


THERMAL CONTROL

A spacecraft's thermal control subsystems keep it, and the science instruments it carries, within allowable temperature limits. The Sentinel-6 Michael Freilich spacecraft utilizes a combination of passive- and active-control elements to achieve this. The passive elements include multilayer insulation blankets and dedicated radiators covered with secondary surface mirrors that radiate heat away from the spacecraft. The main structure is partly painted black internally to minimize temperature gradients inside the spacecraft. For active temperature control, heaters are installed in dedicated areas.


TELECOMMUNICATIONS

Communication between the satellite and the ground is accomplished using microwave S-band and X-band transmitters and antennas located on the nadir (Earth-facing) panel of the spacecraft.

The tracking, telemetry, and command (TT&C) system is composed of two permanently active receivers and two transmitters (one that is permanently active and one that acts as backup, and is used only in contingencies) that allow conventional S-band communications with Earth, providing an uplink data rate of up to 32 kilobits per second and a downlink data rate of 1 megabit per second.

In addition, the payload data handling and transmission (PDHT) system has its own X-band antenna that is only used to transmit scientific and telemetry data to the ground at a downlink data rate of 150 Mbit/s.

ONBOARD DATA HANDLING

The Sentinel-6 Michael Freilich onboard data handling systems provide the central processor and mass memory software resources for the spacecraft and management of the science data.

The data handling subsystem is in charge of the overall spacecraft command and control. It provides necessary input and output capabilities for the attitude and orbit control system as well as for power and thermal systems operations. In addition, it performs spacecraft health functions, including fault detection, isolation, and recovery operations.

The PDHT system includes the mass memory and formatting unit (MMFU), a standalone solid mass memory unit that is based on SDRAM (synchronous dynamic random-access memory) technology, providing 352 GB of data storage. The MMFU also processes the science data and links it to the X-band subsystem (XBS), which then transmits it to ground stations via the X-band antenna.


ATTITUDE AND ORBIT CONTROL SUBSYSTEM

The satellite's "attitude," or orientation and orbit control, is managed by a system consisting of sensors, actuators, and software. Subsystems include reaction wheels, magnetic torquers, magnetometers, a coarse Earth and Sun sensor, a rate measurement unit, a star tracker, and precise orbit determination (POD) instruments. They work together to provide three-axis stabilized Earth-pointing attitude control during all mission modes, and they measure spacecraft rates and orbital position.

The POD instruments include a global navigation satellite system (GNSS) and precise orbit determination receiver (GNSS-POD), a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) instrument, and a laser retroreflector array (LRA). These instruments work in concert to determine the exact orbital position of the satellite so that sea level measurements can be made by the altimeter to a high degree of accuracy and precision. Although not required to meet mission requirements, the GNSS-Radio Occultation (GNSS-RO) instrument also produces data that can be optionally used by scientists to further improve the estimate of the satellite orbit.

Poseidon-4 SAR Altimeter Instrument

Sentinel-6 Michael Freilich carries the Poseidon-4 synthetic-aperture radar (SAR) altimeter that measures mean sea levels by bouncing radio wave pulses off the ocean surface, and precisely timing how long those pulses take to travel back to the spacecraft. The instrument is also used to determine significant wave height and wind speed. Because the radar pulses travel through the ionosphere, which contains electrons that interfere with the propagation of radio waves, the altimeter uses two separate frequencies (the C-band and Ku-band) to correct for ionospheric interference. This was developed by ESA.


HERITAGE AND EVOLUTION

Similar altimeters have been used aboard Sentinel-6 Michael Freilich's predecessors (Poseidon-2, 3A, and 3B were used by the Jason-1, 2, and 3 missions, respectively), but Poseidon-4 marks a significant evolution. By inheriting the SAR High Resolution Altimeter mode of the CryoSat-2 mission's SIRAL (SAR Interferometry Radar Altimeter) and the Sentinel-3 mission's SRAL (SAR Radar Altimeter), much higher along-track resolution can be achieved. With support of the precise orbit determination (POD) package (which determines the position of the satellite above the Earth) and the Advanced Microwave Radiometer for Climate (AMR-C) instrument (which estimates the quantity of atmospheric water vapor, and has new features to improve stability), improved measurements of sea level height are possible (to a global mean sea level stability of 1 millimeter over a one-year period).

The Poseidon-4 radar altimeter

The Poseidon-4 radar altimeter, which will measure the ocean surface topography, is located on the bottom of the spacecraft (the cone-shaped instrument shown here during testing at the IAGB space test center in Ottobrunn near Munich, Germany).
Image credit: ESA

Advanced Microwave Radiometer for Climate (AMR-C)

The Advanced Microwave Radiometer for Climate (AMR-C) instrument incorporates AMR technology evolved from the Jason-2 and Jason-3 missions. The purpose of the system is to measure the amount of water vapor between the satellite and the ocean. Water vapor affects the propagation of the radar pulses from the Poseidon-4 radar altimeter, which can make the ocean look higher or lower than it actually is. AMR measurements are therefore required to correct for this effect, thus preventing over- or underestimation of sea level measurements. The AMR-C includes a new onboard calibration system to improve the multiyear stability of the radar delay measurement compared to the predecessor missions, ensuring the altimeter system is capable of tracking changes in the global mean sea level extremely accurately. The AMR-C instrument was developed by NASA-JPL.


This photograph shows the AMR-C instrument undergoing tests at NASA's Jet Propulsion Laboratory in Southern California.
Image credit: NASA/JPL-Caltech


HIGHER RESOLUTION

The AMR-C instrument also includes an experimental High-Resolution Microwave Radiometer (HRMR) that will provide the radar delay measurements to a higher resolution, therefore allowing radar delay measurements along coastal areas, which is not currently possible with other missions.

This artist's impression shows the aft of the satellite, including sensors of the GNSS-RO instrument (the grey rectangle to the left)
Image credit: NASA/JPL-Caltech

Global Navigation Satellite System – Radio Occultation (GNSS-RO)

The Global Navigation Satellite System – Radio Occultation (GNSS-RO) experiment is used to measure the physical properties of the atmosphere, such as temperature, pressure, and water vapor. To achieve this, GNSS-RO detects the occultation of global navigation satellites' radio signals as they disappear beyond the limb of the Earth from the perspective of Sentinel-6 Michael Freilich orbit.

The instrument is composed of three antennas: one that is used for precise orbit determination and two others that are directed toward the Earth's limb. As the signal from a global navigation satellite is acquired by the fore or aft antennas, it will drop toward or rise from the horizon. As it does so, the satellite's radio signal will travel through the atmosphere. These signals can be detected through the vertical extent of the atmosphere – even through thick clouds – from the very top and almost all the way to the ground. Measurements of the amount of refraction (or change in frequency) of these signals reveal the atmosphere's physical properties and may be used to improve weather forecasting, provide information about the ionosphere, and support climate studies. The GNSS-RO instrument was developed by NASA-JPL.


UNLIMITED AND INDEPENDENT

Like the GNSS precise orbit determination (GNSS-POD) receiver, GNSS-RO is not limited to only tracking GPS satellites; navigation satellites from other networks (such as the GLONASS and Galileo constellations) can also be tracked. As a secondary mission instrument, its operation is independent from sea level measurements made by Sentinel-6 Michael Freilich's radar altimeter.

Precise Orbit Determination (POD) Package

Precisely determining the position of the Sentinel-6 Michael Freilich spacecraft in orbit is of paramount importance when recording extremely small variations in sea level data (on the millimeter scale). To achieve this, Sentinel-6 Michael Freilich carries a state-of-the-art precise orbit determination package that works in conjunction with the mission's scientific instruments to accurately define its position in space and time. This is accomplished by the following key instruments:



GLOBAL NAVIGATION SATELLITE SYSTEM – PRECISE ORBIT DETERMINATION (GNSS-POD)

The Global Navigation System – Precise Orbit Determination (GNSS-POD) antennas are attached to Sentinel-6 Michael Freilich's zenith panel (facing away from Earth) and support the precise determination of the spacecraft's position in orbit. While GPS and GNSS systems work in a similar way, GNSS instruments can use navigational satellites from other networks and are not limited to GPS. In the case of GNSS-POD, navigation data from the GPS and Galileo satellite constellations can be accessed arbitrarily, boosting positioning measurements to an accuracy of a few centimeters. This instrument was provided by ESA.


DOPPLER ORBITOGRAPHY AND RADIOPOSITIONING INTEGRATED BY SATELLITE (DORIS)

The Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) instrument measures the radio signals from 55 global ground stations that compose the International DORIS Service (IDS). Each ground station acts as a beacon to broadcast two stable radio frequencies at 2036.25 MHz (S-band) and 401.25 MHz (VHF). Every 10 seconds, the DORIS instrument aboard Sentinel-6 Michael Freilich measures the Doppler shift of the radio beacons' frequencies to precisely determine its line-of-sight velocity. Over time, an accurate 3D position of the satellite can be determined. This instrument was provided by ESA.

LASER RETROREFLECTOR ARRAY (LRA)

Located on the Earth-facing (nadir) plate of the spacecraft, the laser retroreflector array (LRA) is a completely passive component of the navigation instrumentation on Sentinel-6 Michael Freilich. The LRA consists of nine precisely shaped mirrors that reflect laser beams back to their point of origin on the ground. Ground-based laser-ranging stations can then determine how long the laser beam took to travel to the satellite, reflect off the LRA, and return to the station. Doing this enables a measurement of the distance between the station and satellite to be made. Over time, many ground-based laser-ranging stations will combine their distance estimates, and the spacecraft's orbit can be reconstructed and tracked, thereby supporting measurements made by the spacecraft’s other navigation systems. This instrument was provided by NASA-JPL.


GLOBAL NAVIGATION SATELLITE SYSTEM – RADIO OCCULTATION (GNSS-RO)

Although the NASA-provided GNSS-RO instrument's purpose is for atmospheric sounding measurements completely independent of the primary altimetry mission, the GNSS-RO instrument also records measurements that can be used by scientists to determine the satellite's precise orbit. This is not required to meet the baseline mission requirements, but it is anticipated that some in the scientific community will take advantage of the GNSS-RO data to further improve the POD estimates.

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