Sentinel-3 is carrying a suite of instruments. The altimetry payload consists of a radar altimeter (SRAL), a microwave radiometer (MWR) and a triple precise orbit determination system (DORIS, GNSS and LRR). The satellite also has a dedicated ocean colour and surface temperature instrument payload (SLSTR and OLCI).
Main Sentinel-3 instruments
SRAL (SAR Radar Altimeter) is based on the same design as the SIRAL Doppler altimeter on CryoSat.
The main range-measuring frequency is in Ku band (13.575 GHz, with a bandwidth of 350 MHz) but, unlike for CryoSat, a second frequency is also employed in C band (5.41 GHz, bandwidth 320 MHz) for ionospheric corrections.
The radar operates in two modes:
- Low-Resolution Mode (LRM), like a conventional altimeter
- SAR high-resolution mode, along track
LRM mode was only used during in-orbit commissioning. SAR mode, with an along-track resolution on the order of 300 metres, is the operational mode used continuously since the start of May 2016.
Two tracking loop modes are planned, one closed (in which the measurement window is positioned autonomously using a median algorithm) and one open (in which the position is based on the surface height derived from a digital elevation model (DEM/OLTC)). This mode is used continuously over oceans and certain land surfaces—notably in mountainous terrain—to improve data coverage.
Microwave radiometer (MWR)
The design and specifications of Sentinel-3’s Microwave Radiometer (MWR) are the same as for the instrument used on Envisat. MWR measures brightness temperature at 23.8 and 36.5 GHz, with a bandwidth of 200 MHz for each frequency. The lower frequency is more sensitive to atmospheric water vapour, and the higher frequency to cloud water content.
Precise positioning systems
Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) is a satellite positioning system designed in the late 1980s by CNES to provide precise orbit determination for satellites in low-Earth orbit. DORIS is supported by a dense network of ground stations emitting an omnidirectional signal of known frequency (2 GHz and 400 MHz).
Sentinel-3 is equipped with a new-generation multi-channel digital receiver developed by CNES. The same kind of receiver is also operating on Jason-2, Jason-3, SARAL and CryoSat. It is capable of tracking 7 orbitography beacons simultaneously. The accuracy of the orbit provided by DORIS for Sentinel-3 is better than 5 centimetres on the radial component—the most accurate in the world. This orbit is also used to control the open-loop tracking mode of the SRAL altimeter.
The expected orbit accuracy from DORIS off line is on the order of 2 centimetres, and combining GPS and DORIS during orbit determination processing further improves final performance.
Sentinel-3’s GPS receiver contributes to satellite control, provides accurate onboard timing and controls the open-loop tracking mode of the SRAL altimeter by supplying a real-time radial position and measurements for the final precise orbit. It has 8 channels to track signals from the GPS constellation.
Laser Retro Reflector (LRR)
Sentinel-3’s Laser Retro Reflector (LRR) is a passive instrument that serves as a target for laser beams from ground stations.
surface colour and temperature instruments
Sea and Land Surface Temperature Radiometer (SLSTR)
The Sea and Land Surface Temperature Radiometer (SLSTR) measures temperatures continuously with an accuracy better than 0.3 K. Following on from Envisat’s AATSR (Advanced Along Track Scanning Radiometer) instrument, it is a conical scanning radiometer employing an along-track dual-scan technique to acquire imagery with a spatial resolution of 500 metres in the visible, near-infrared and shortwave infrared (SWIR) spectrum, and 1 kilometre in the thermal infrared.
SLSTR also features two dedicated thermal infrared channels optimized for detecting fires and measuring their radiative power, a key parameter for Copernicus Emergency Response and Climate services.
Ocean and Land Colour Instrument (OLCI)
The Ocean and Land Colour Instrument (OLCI) draws on the heritage of Envisat’s MERIS (Medium Resolution Imaging Spectrometer) instrument and has 21 separate spectral bands spanning 0.4 to 1.02 μm (400 to 1,020 nm) designed for observing ocean colour and vegetation, and tailored to atmospheric correction requirements. It has a spatial resolution of 300 metres and a swath width of 1,270 kilometres, overlapping that of the SLSTR instrument. The new perspective of Earth afforded by OLCI will enable monitoring of ocean ecosystems, support for crop management and estimations of atmospheric aerosols and clouds, which will inform decision-making to bring significant societal benefits.