DUAL-FREQUENCY TOPEX ALTIMETER OBSERVATIONS OF GREENLAND

Altimetric height can be underestimated on ice sheet surfaces, because of penetration of the radar wave inside the snowpack. Estimation of t he induced bias is quite impossible, because of the large number of pa rameters involved both in surface and subsurface scattering. Indeed, a ltimetric echo of snow surface is controlled by antenna pattern, surfa ce microroughness and kilometric roughness, internal stratification, s cattering and extinction coefficient. Most of these parameters are rad ar frequency dependent, the dual-frequency Topex altimeter thus gives an opportunity to empirically detect this bias. Indeed, volume signal is clearly suggested by comparing the C-band and Ku-band altimeter hei ghts and waveform parameters. Altimeter height difference are of a few decimeters only, and have kilometric scale variations: induced volume error seems to be more linked with the waveform deformation by the su rface topography than with the intensity of the subsurface signal.