3.1.2.4. Altimetric measurements over lands

Envisat's 'ICE2' retracker: Continental ice retracking can also be used successfully on continental surfaces

The parameters extracted from the waveforms over continental surfaces are similar to the ones for ice:

epoch at mid-height: this gives the time delay of the expected return of the radar pulse (estimated by the tracker algorithm), and thus the time the radar pulse took to travel the satellite-surface distance (or 'range') and back again.
backscatter coefficient (or sigma0): gives information on the nature of the surface.
leading edge amplitude.
leading edge width: this is related to the penetration into the medium and the surface roughness of the target.
trailing edge slope: this gives information on antenna mispointing, and also on the signal penetration into the medium.

For Ku and S band, the backscatter is low in mountainous regions (e.g. e.g. <7 dB in Ku band, <14 dB in S band) as a direct result of the presence of topographic slopes. For both bands, the backscatter values are high on very flat surfaces, such as deserts, large river basins or wetlands (e.g. >15 dB in Ku band and >20 dB in S band), due to the specularity of the return radar echo.

The leading edge width values are high in desert areas due to the strong penetration of the wave and the dunes generated by the winds. Low values, related to weak penetration, correspond to dense vegetated areas, such as tropical or boreal forests, or to large river basins or flooded regions. In contrast to the backscatter coefficient, the use of only one frequency gives a characteristic signature on continental surfaces, providing good discrimination of forests, deserts, etc.

Examples of real waveforms (from the Topex altimeter), for a river (Amazon) and a desert (Sahara)