Instruments on Sentinel-3

The Sentinel-3 satellites are carrying a payload of several instruments linked to altimetry: the SRAL altimeter, which measures range; the radiometer, which measures perturbations due to atmospheric water vapor; three location systems (Doris, GNSS and LRA). An ocean and land colour sensor, and a sea and land surface temperature one are also onboard (not detailed below).

SRAL altimeter

SRAL is a follow-on to Cryosat Siral 'delay-Doppler' altimeter.
The main frequency used for range measurements is the Ku-band (13.575 GHz, bandwidth 350 MHz), but, contrary to Cryosat, a second frequency is used. The C-band frequency (5.41 GHz, bandwidth 320 MHz) is used for ionospheric correction.

There are two radar modes:

•     Low-Resolution Mode (LRM): conventional altimeter pulse-limited mode
•     Synthetic Aperture Radar (SAR) mode: high along track resolution mode

On Sentinel-3, the SRAL mission is always operated at High Resolution Mode (commonly called SAR mode). Low Resolution Mode (LRM) is a back-up mode only. (see more details on the operating modes) 

Two tracking modes are provided,  closed-loop mode (autonomous positioning of the range window using the median algorithm) and open-loop mode (range window position based on a priori knowledge of terrain altitude derived from a Digital Elevation Model (DEM)). (find here a map of Sentinel-3 tracking modes per observed surfaces)

MWR radiometer

The design and specifications of the Sentinel-3 MWR have been based on those of the Envisat MWR. The instrument measures brightness temperature at 23.8 GHz and 36.5 GHz covering a bandwidth of 200 MHz in each channel. The lower frequency channel is mostly sensitive to atmospheric water vapour and the higher frequency channel to cloud liquid water.

Precise Location systems

Doris

Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) is a satellite tracking system designed in the late 1980s by Cnes as a new system to provide precise orbits on-board low Earth orbit satellites. The operation of Doris relies on a dense global network of ground stations transmitting omnidirectional signals at a known frequency.
Sentinel-3 is equipped with a new-generation (multi-channel and digital) receiver developed by Cnes, as a Customer Furnished Instrument (CFI). This type of receiver is also flying on Jason-2, Jason-3, Saral, HY-2A and Cryosat-2 and is able to track up to seven beacons simultaneously. It was shown that real-time accuracy of the Doris navigation solution (Diode) on Sentinel-3 is about 3.5 cm. Diode is used to drive the open loop tracking mechanism of the SRAL altimeter.

The offline POD performance is approximately 1 cm on the radial direction (nadir), as for the Jason satellites. The combination of both GPS and DORIS measurements in POD processing improves the final performance on the three directions.

GNSS

The Sentinel-3 GPS receiver contributes to satellite control, provides accurate on-board timing and provides the measurements for the final POD analysis.

Laser Retro Reflector (LRR)

The Sentinel-3 Laser Retro Reflector (LRR) is a passive device that acts as a target for laser tracking measurements performed by dedicated ground stations.