Sentinel 3 will be a multi-sensor mission with a payload dedicated in particular to ocean operational monitoring. The optical payload (not detailed here) consists in a spectrometer and a radiometer. The topography-related part of this payload includes an altimeter, and also a radiometer and location systems.
SLSTR: Sea and Land Surface Temperature Radiometer
SRAL: Synthetic Aperture Radar Altimeter (derivative of SIRAL on Cryosat-1 and -2)
OLCI: Ocean and Land Colour Instrument
MWR: MicroWave Radiometer
LRR: Laser Retro-Reflector
SRAL (Synthetic Aperture Radar Altimeter) is a redundant dual-frequency (C-band and Ku-band) instrument for determining the two-way delay of the radar echo from the Earth’s surface to a very high precision: less than a nanosecond. It also measures the power and shape of the reflected radar pulses. SRAL is derived from the Cryosat SIRAL altimeter, and from Jason-2 Poseidon-3 altimeter. Operating over oceans, its measurements are used to determine the ocean topography.
MWR (Microwave Radiometer)
The main objective of the microwave radiometer (MWR) is to measure the integrated atmospheric water vapour column and cloud liquid water content, which are used as correction terms for the radar altimeter signal. Once the water content is known, we can determine the correction to be applied for radar signal path delays for the altimeter. In addition, MWR measurement data are useful for determining surface emissivity and soil moisture over land, for surface energy budget investigations to support atmospheric studies, and for ice characterisation.
The location systems onboard Sentinel 3 complement each other to measure the satellite’s position on orbit to within two centimetres on the radial component. The LRR is highly accurate, but it requires ground stations that are complex to operate, and its use can be restricted by adverse weather conditions. It is used to calibrate the GNSS receiver so that the satellite orbit can be determined as accurately as possible. A Doris receiver will also be onboard.
DORIS (Doppler location)
The DORIS system uses a ground network of orbitography beacons spread around the globe, which send signals at two frequencies to a receiver on the satellite. The relative motion of the satellite generates a shift in the signal’s frequency (called the Doppler shift) that is measured to derive the satellite’s velocity. These data are then assimilated in orbit determination models to keep permanent track of the satellite’s precise position on its orbit.
The GNSS receiver uses the Global Navigation Satellite System (including the Global Positioning System (GPS) and its European civil counterpart, Galileo) to determine the satellite’s position by triangulation, in the same way that GPS fixes are obtained on Earth. At least three GNSS satellites determine the a mobile object’s (in this case the satellite’s) exact position at a given instant. Positional data are then integrated into an orbit determination model to track the satellite’s trajectory continuously
LRR (laser tracking)
The LRR (Laser Retroreflector Array) is an array of mirrors that provides a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, we can locate where the satellite is on its orbit.