Altimetric height can be underestimated on ice sheet surfaces, because of penetration of the radar wave inside the snowpack. Estimation of the induced bias is quite impossible, because of the large number of parameters involved both in surface and subsurface scattering. Indeed, altimetric echo of snow surface is controlled by antenna pattern, surface microroughness and kilometric roughness, internal stratification, scattering and extinction coefficient. Most of these parameters are radar 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 heights 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 surface topography than with the intensity of the subsurface signal.