Sentinel-3 is a European Earth observation satellite mission designed to ensure the long-term collection of uniform quality data products generated and delivered in an operational manner to Copernicus services in the marine environment with contributions to land, atmospheric, emergency, security and cryospheric applications. The mission includes a series of satellites over a 20 year period starting with the launch of Sentinel-3a in early 2014 and during full operations two identical satellites will be maintained in the same orbit with a phase delay of 180°. Sentinel-3 measurement requirements have been derived from operational user needs to insure data continuity for the observation and monitoring of (Donlon, et al., 2011): 

Sea surface topography (SSH), significant wave height (Hs) and surface wind speed derived over the global ocean to an equivalent accuracy and precision as that presently achieved by ENVISAT Radar Altimeter-2 (RA-2). Enhanced surface topography measurements in the coastal zone, sea ice regions and over inland rivers, their tributaries and lakes. Sea surface temperature (SST) determined for oceanic and coastal waters globally to an equivalent accuracy and precision as that presently achieved by the ENVISAT Advanced Along Track Scanning Radiometer (AATSR) over the ocean (i.e. <0.3 K), at a spatial resolution of 1 km. Visible, Near Infrared, Short-Wave Infrared, and Thermal Infrared radiances for oceanic, inland and coastal waters, land surfaces including sea ice and ice sheets determined to an equivalent level of accuracy and precision as ENVISAT Medium Resolution Imaging Spectrometer (MERIS), AATSR and SPOT Vegetation data with complete ocean coverage in 2-3 days, complete land coverage in 1-2 days, a spatial resolution of ≤0.3 km and simultaneously and co-registered with SST measurements. 

The Copernicus Sentinel-3 mission addresses these requirements by implementing and operating the following components: A Synthetic Aperture Radar Altimeter (SRAL) instrument, a passive microwave radiometer (MWR), a GPS receiver and laser retro-reflector for precise orbit determination providing continuing the legacy of ENVISAT RA-2 and Cryosat. An Ocean and Land Colour Imager (OLCI) delivering multi-channel optical measurements for ocean and land providing continuity to ENVISAT MERIS. A Sea and Land Surface Temperature Radiometer (SLSTR) delivering accurate surface ocean, land and ice temperature providing continuity of ENVISAT AATSR. An operational ground segment providing access to core data product in a near real time mode and a collaborative ground segment providing additional data products for specific users and purposes in collaboration with other entities. 

Compared to ENVISAT and satellite missions currently providing data for operational services the instrumentation payload on-board Sentinel-3 contains one new instrument, i.e. the SRAL radar altimeter. The remaining instruments will continue collecting data similar to the currently operating satellites. The SRAL instrument (Le Roy et al, 2009) is a fully redundant dual-frequency nadir-looking microwave radar altimeter, which employs delay Doppler altimetry technologies (Raney, 1998) inherited from the Cryosat (Wingham, 1999) and Jason altimeter missions. SRAL may operate in a low-resolution mode (LRM) similar to the conventional radar altimeters and provide sea surface heights, significant wave heights and wind speeds that have been averaged at 1 Hz or every 7 km intervals. In addition by applying the delay Doppler technology, SRAL can operate in a SAR mode providing high resolution data along track at 20 Hz or every 300 m.