Satellite Description

Copernicus Sentinel-6 is comprised of two identical satellites flown sequentially that are designed to provide near-real-time measurements of sea-surface heights, significant wave heights and wind speed, to support operational oceanography and climate monitoring.

The first satellite, Copernicus Sentinel-6 Michael Freilich, was launched on 21 November 2020 on a SpaceX Falcon 9 rocket from Vandenberg, California, USA. The second satellite is expected to be launched five years later. The satellites operate in a non-sun-synchronous orbit at a mean altitude of 1336 km, inclined at 66ยบ with a 10-day repeat cycle that overlaps with Jason-3's to ensure that the long-term altimeter data record acquired on this orbit will be continued into the 2030s.

Sentinel-6 is designed for a five-year operational lifetime and six months for commissioning activities, with consumables included for an additional two years of operation. Each satellite weighs 1191 kg including 230 kg of hydrazine monopropellant fuel.

Since the orbit is non-synchronous with a drifting orbit plan and a varying solar aspect angle, the satellite is designed with a roof shaped solar array that maximises solar power over the satellite's lifetime. A lithium ion battery stores sufficient energy to power the satellite system during eclipses.

With its satellite configuration derived from ESA's CryoSat, Copernicus Sentinel-6 features body mounted solar arrays to limit the variable drag cross section of the satellite and remove the need for large moving solar arrays.

Sentinel-6 instruments

Global Navigation Satellite System (GNSS) antennas are located on the roof of the satellite to optimise the receiving of signals from the Global Positioning System (GPS) and Galileo constellations. A large Earth-facing panel hosts the Poseidon-4 altimeter, the Laser Retroreflector Array (LRA), the Doppler Orbitography Radio-positioning Integrated by Satellite (DORIS) antenna and communications antennas. Command and control are implemented via a bi-directional S-band communication link. Scientific data collected over each orbit are stored in 576Gb solid-state mass memory, prior to downlinking via the X-band communication channel to the ground station at a data rate of 150Mbps.

The spacecraft equipment is designed to operate within a relatively narrow thermal range (typically 0-40C). To maintain this temperature range, it is necessary to emit the heat generated in operation to the outside of the spacecraft. For this, Sentinel-6 makes use of the nadir-facing panel that always points to Earth with a small radiator panel located on the top of the satellite.

The forward facing panel of the spacecraft hosts the Advanced Microwave Radiometer for Climate (AMR-C, provided by NASA) that also includes the High-Resolution Microwave Radiometer (HRMR). Star-tracker heads, the GNSS Radio Occultation (GNSS-RO) instrument antenna and the Radiation Environment Monitor (REM) instrument are all located on this panel.

Sentinel-6 uses star trackers to determine its absolute attitude in inertial space. It has three star tracker heads that point in different directions, since in flight one or the other will be blinded by sunlight or moonlight at some point in an orbit.

The GNSS-RO instrument is provided by NASA and its forward-looking antenna array is inclined towards Earth limb to track the GNSS satellite signals as they cross Earth's atmosphere and disappear below the horizon. A second array is accommodated on the aft external bulkhead panel to acquire occultation profiles from the opposite direction. The GNSS-RO also has a dedicated Precise Orbit Determination (POD) antenna mounted on the roof of the satellite, which provides additional information for primary mission POD.

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