From a high-inclination Sun-synchronous polar orbit, at 814.5 km over geoid, the pair of operational Sentinel-3 satellites will provide global coverage in two days.
Each satellite has an approximate seven-year operational life span.
Benefiting from a proven heritage the three-axis stabilised platform, Sentinel-3 will carry several instruments:
A topography system, which includes a dual-band Ku- and C-band SAR radar altimeter based on technologies used on ESA's Earth Explorer CryoSat mission; a microwave radiometer for atmospheric correction, and a Precise Orbit Positioning package, including a GPS receiver, a DORIS receiver and a Laser Retro-Reflector.
An Ocean Land Colour Instrument (OLCI), which is based on heritage from Envisat's Medium Resolution Imaging Spectrometer MERIS instrument. The OLCI operates across 21 wavelength bands from ultraviolet to near-infrared and uses optimised camera’s pointing to reduce the effects of sun glint.
A surface temperature instrument called Sea Land Surface Temperature Radiometer (SLSTR), which is based on heritage from Envisat's Advanced Along Track Scanning Radiometer (AATSR). The SLSTR uses a dual viewing technique and operates across eight wavelength bands, providing better coverage than AATSR because of a wider swath width.
The European Space Agency (ESA) Sentinel webpages have more information on Sentinel-3.