3.1.5. Delay-Doppler (or SAR) Altimetry
The Delay Doppler/SAR altimeter differs from a conventional radar altimeter in that it exploits coherent processing of groups of transmitted pulses. It is not pulse-limited like classical radar altimeters, so the full Doppler bandwidth is exploited to make the most efficient use of the power reflected from the surface.
Delay Doppler/SAR altimetrer "stares" at each resolved along-track cell as the radar passes overhead for as long as that particular cell is illuminated.
Note that each cell is viewed over a larger fraction of the antenna beam than the pulse-limited; thus more data is gathered, which leads to substantial benefits (e.g. it uses most of the power received).
The Delay Doppler/SAR altimetry processes the data such that they could be seen as having been acquired from a synthetic aperture antenna. This along track processing increases resolution and offers a multilook processing with the two independent dimensions: along-track and across-track (range).
Comparison between conventional radar altimeter (left) and Delay Doppler/SAR (right) altimety. (Credits R.K. Raney, Johns Hopkins University Applied Physics Laboratory)
Theoretical "step-by-step" building of a SAR-altimetry waveform (for a single Doppler beam). Contrary to the classical altimeter, the lighted area is not a surface-constant ring, but only part of it, which explain the peakier shape of the echo. Several such beams are used at the same time (Using material from R.K. Raney, Johns Hopkins University Applied Physics Laboratory)
The Delay Doppler/SAR mode altimeter offers many potential improvements over conventional altimetry for measurements over the oceans, coastal zones and inland waters using retrackers adapted to the specific nature of Delay Doppler/SAR altimeter echoes.
Cryosat-2 operates a Delay Doppler/SAR mode altimeter, and Sentinel-3 missions will.
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