The radars on altimetry satellites, even those optimised for the ocean, continue to emit pulses while flying over land. Reception of the return echoes is more complex, since a field or a forest does not reflect radar pulses as well as water, but some conclusions can still be drawn. The amount of power received after reflection (or more precisely the backscatter coefficient) in itself yields interesting information, since the backscatter coefficient depends on the state of the observed surface, according to whether it is covered by snow, vegetation, flooded areas, etc.
|fig 1. Seasonal anomalies of the backscatter coefficient (return rate of the radar wave to the altimeter antenna) for Topex in Ku band, in winter (top) and summer (bottom) for the first ten years of measurements. Significant variations can be seen, especially in regions which are covered by snow in winter (higher than 55°N), or which have a marked rainy season (equatorial regions, India).
Many of the other unique characteristics of radar altimeters over continents are now beginning to be exploited, such as retracked waveform parameters, dual-frequency measurements or synergies with the radiometer that operates simultaneously on most altimeter platforms. These can contribute to monitoring forests, deserts and boreal regions, and their seasonal variations, e.g. information about snow beginning and end dates, and the thickness of snow or plant cover in relation to the season. This new field of altimetry should grow in importance over the next few years, as a complement to other satellite observation techniques.