ALTIMETRIC OBSERVATIONS OF SURFACE CHARACTERISTICS OF THE ANTARCTIC ICE-SHEET

The aim of this paper is to investigate the geophysical characteristic s of the Antarctic ice sheet using radar altimetric observations. To d o this, we use an altimetric waveform simulator, in situ observations, ERS-1 (European remote-sensing satellite) data and SPOT (Satellite po ur l'observation de la terre) images. The small-scale study takes plac e at Dome C, Terre Adelie, which is a relatively flat region with gent le undulations and low wind speed. Despite this, the altimetric wavefo rm parameters (height, energy leading edge and trailing edge) are high ly noisy. The effect of undulations on the waveform parameters is foun d to be dominant. The combination of a subsurface signal and a rough s urface produces a linear effect on the altimetric backscattering or on the trailing edge of the waveform, but a strongly non-linear effect o n the leading edge of the waveform or height estimation. As a conseque nce, the height measurement is very sensitive to the altimeter technic al or orbital characteristics and is not reproducible from one mission to another. Observations show sastrugi fields that enhance the leadin g edge and affect the whole waveform. Observed local backscattering ch anges, probably due to local variations in surface microroughness, enh ance the backscattered energy and may artificially create a topographi c signal. The continental-scale study shows coherent patterns. Even if both surface and subsurface components affect the altimetric observat ion, the large-scale signal is mostly controlled by surface backscatte ring variations. The surface or near-subsurface characteristics of the snowpack may then be reached by altimetric observations.