In the SENTINEL-3 SRAL mission, there are two main modes of operation:
The SRAL mission is always operated at High Resolution Mode (commonly called SAR mode). Low Resolution Mode (LRM) is a back-up mode only.
SAR mode is designed to achieve high along-track resolution over relatively flat surfaces. This property can be exploited to increase the number of independent measurements over a given area and is a prerequisite for sea-ice thickness measurements, coastal waters, ice sheet margins, land and inland waters. The scientific justification of High Resolution Mode 100% coverage over the Earth is also applicable to open ocean surfaces because studies have shown that the best performance of this mode is over open ocean surfaces where topography is homogeneous (areas at least as large as the antenna footprint).
LRM employs conventional pulse-limited altimeter operation, as used in the ERS and ENVISAT satellites.
To facilitate autonomous operations, LRM and SAR modes use one of two on-board tracking modes. Traditional autonomous, closed loop tracking of range and gain may be used where the altimeter range window is autonomously positioned based on on-board NRT analysis of previous SRAL waveforms. Alternatively, an open loop tracking mode is available where the altimeter range window is positioned using a priori knowledge of the surface height stored on-board the instrument in a one-dimensional along track Digital Elevation Model (DEM).
Figure 1: Sentinel-3 tracking modes (open loop/closed loop) per observed surface (Credit: ESA)
In the Sentinel-3 mission, SRAL transmits pulses alternatively at Ku-band (13.575 GHz, bandwidth=350 MHz) main frequency for altimeter range measurements complemented by a C-band frequency (5.41 GHz, bandwidth=320 MHz) that is used to correct range delay errors due to the varying density of electrons in the ionosphere.