In the Sentinel-3 SRAL mission, there are two main modes of operation:
- High Resolution Mode, also known as Synthetic Aperture Radar (SAR) or delay Doppler altimetry (DDA)
- Low Resolution Mode (LRM)
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. Scientific studies have shown that HRM (or SAR) mode provides the best performance over open ocean surfaces where topography is homogeneous, as well as over highly varying surfaces.The instrument needs to position its data recording window at the likely time of the return echo. To facilitate autonomous operations, LRM and SAR modes use one of two on board tracking modes.
LRM employs conventional pulse-limited altimeter operation, as used in the ERS and ENVISAT satellites.
Figure 1: S3A and S3B timeseries of LRM and SAR operational modes
The instrument needs to position its data recording window at the likely time of the return echo. 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 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). Until March 2019, Sentinel 3A and 3B follow different acquisition mode masks. With the update of the OLTC version on Sentinel 3 A, performed on 13th March 2019 its acquisition mode mask is now identical to that for Sentinel 3B.
Figure 2: Sentinel-3 tracking modes (open loop/closed loop) per observed surface (Credit: ESA)
In the Sentinel-3 mission, SRAL transmits pulses interleaved 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.