EUMETSAT Copernicus marine missions, sensors and products

Copernicus Sentinel-3:

Copernicus Sentinel-3 carries three primary instruments, the Ocean and Land Colour Instrument (OLCI), Sea and Land Surface Temperature Radiometer (SLSTR) and Synthetic Aperture Radar Altimeter (SRAL). The latter is support by the Microwave Radiometer (MWR) and Precise Orbital Determination package (POD), consisting of the DORIS antenna, GOS and laser retroreflector array. Together, SRAL,  MWR and the POD package form the surface topography mission (STM). Each of the primary sensors is discussed in more detail below.

The Ocean and Land Colour Instrument (OLCI):

The key mission driver for the Sentinel-3 OLCI instrument is continuity of the Envisat MERIS instrument capability. As such, it is a push-broom imaging spectrometer with 5 camera modules sharing the Field Of View (FOV):

• The five cameras’ FOV are arranged in a fan-shaped configuration in the vertical plane perpendicular to the platform velocity,
• Each camera has an individual FOV of 14.2 degrees with a 0.6 degree overlap with its
• neighbours,
• The whole FOV is shifted westwards across-track by 12.58 degrees away from the Sun to minimise the Sun glint impact.

OLCI has the following characteristics:

• 21 spectral bands, 
• Improved SNR compared with previous sensors and a 14-bit analogue to digital converter,
• A coverage of the global ocean with 2 satellites flying (3A & 3B): < 3 days, ignoring the effect of clouds.
• A long time-series of data created by launching 4 missions (A to D) over 20+ years.
• A fast data delivery timeliness for L1B and L2 products,
• 100% overlap with SLSTR swath, and simultaneous acquisitions facilitating the use of OLCI and SLSTR in synergy.

The 21 wavelengths used to measure TOA radiance are as shown in the table below, alongside the SNR for a reference radiance (L_ref) that approximates the expected signal from an open-ocean water pixel. The measured spectral response functions, which described the true shape of the wavebands, are available from the ESA’s Sentinel’s Copernicus website. Links to an external site.

Despite sharing the same industrial design, OLCI-A and OLCI-B do not share the same exact spectral characterisation nor radiometric and geometric calibrations. Tiny differences can bring significant impacts, for that reason some a djustments need to be made to homogenise and harmonise the two series of measurements for mission continuity.

In the following video we will expand on some of the above points;

For much more information on OLCI and ocean colour, see our dedicated knowledge base. Links to an external site.

The Sea and Land Surface Temperature Radiometer (SLSTR):

The instrument responsible for taking sea (and land) surface temperature measurements on Sentinel-3A and B is the Sea and Land Surface Temperature Radiometer (SLSTR). SLSTR is designed to continue the legacy of the ATSR series of instruments, most recently deployed on ENVISAT (AATSR), but makes a number of significant improvements.

SLSTR is a multi-channel imaging radiometer. It has three visible channels (0.55 µm; 0.659 µm; 0.865 µm), three short-wavelength infrared (SWIR) channels (1.375 µm; 1.610 µm; 2.25 µm - channels in bold were not on the AATSR), three medium-wavelength / thermal infrared (MWIR/TIR) channels (3.74 µm; 10.85 µm; 12 µm) and two new channels dedicated to fire detection (also at 3.74 µm; 10.85 µm, but with differing radiometric resolution to the MWIR/TIR channels and a specific design to avoid saturation). The addition of new SWIR channels aids in cloud clearing for improved SST retrieval.

SLSTR independently scans swaths at two positions, each with its own scanning mirror. The nadir position scans an asymmetric swath 1400 km directly below the satellite, but offset to the right of the direction of travel. The along track position scans a narrower, symmetric, 740 km swath at an oblique angle (55o) behind the direction of travel. Each of these scans intersects the blackbody calibrators. After each scan cycle (nadir & along track), visible calibration is also performed. The increased width of the the nadir swath gives enhanced coastal resolution. As SLSTR measures the same surface point twice (once at nadir, once at along track), it employs a dual-view approach to to provide a more robust atmospheric correction for the common swath. The mean global revisit time for a dual-view swath is 1.9 days at the equator for one satellite.

Physically, SLSTR is separated into two units, and optical scanning unit, and a separate control and processor unit, both mounted on a single plate. Given its use in gathering data for climate studies, where sensor drift must be minimised, SLSTR has a temporal stability of 0.1 K/decade. It retrieves global sea-surface skin temperatures with zero bias and an uncertainty of ± 0.3 K for a 5° by 5° latitude–longitude area.

For much more information on SLSTR and sea surface temperature colour, see our dedicated knowledge base Links to an external site..

The Surface Topography Mission:

For much more information on the Sentinel-3 surface topography mission, see our dedicated knowledge base Links to an external site..

Copernicus Sentinel-6A Michael-Freilich:

Sentinel-6 is a collaborative Copernicus Links to an external site. mission implemented and co-funded by the European Commission, the European Space Agency, EUMETSAT and the US, through NASA and the National Oceanic and Atmospheric Administration (NOAA).

Two Sentinel-6 satellites will fly the same specific non-synchronous low-Earth orbit as the series of European/US Topex-Poseidon and Jason satellites to continue the high-precision ocean altimetry mission delivered for more than 27 years. EUMETSAT is responsible for satellite operations, as well as the altimetry data processing and distribution. Sentinel-6 also has a second mission providing atmospheric temperature and moisture profiles through radio occultation measurements.

Sentinel-6 Michael Freilich was launched in November 2020. Sentinel-6B is planned for launch in 2025.

There have been an altimetry-dedicated mission on the same orbit, providing data continuously since October 1992 (first Topex/Poseidon data, see History Links to an external site. section). All follow-on missions have been cross-calibrated on each other (Jason-1 on Topex/Poseidon, Jason-2 on Jason-1, Jason-3 on Jason-2 and lately Sentinel-6 Michael Freilich on Jason-3).

This enables to provide a virtually uninterrupted time series over now more than 30 years, and is important for Mean Sea Level computation over the long term, as well as all long-term studies requiring homogeneity.

For much more information on the Sentinel-6 surface topography mission, see our dedicated knowledge base Links to an external site..