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
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.
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.