The spatial sampling of the altimeter measurements can be divided into two dimensions. The along-track sampling is the along-track distance between two measurements. The range sampling is the distance between two samples within a waveform.
Figure 1: LRM and SAR Along Track Sampling at 1 Hz and 20 Hz Measurement Rate (left) and Range Sampling (right) (Credit: Thales Alenia Spazio)
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). This mode facilitates acquisition over rough terrain and ensures continuous acquisitions across sea/land and sea/ice transition zones. A key advantage of open-loop tracking is that the data loss, typical of conventional closed-loop tracking due to mode switching, and loss of track during transitions or over variable terrain, are minimised.