Measuring sea-ice thickness using satellite radar altimetry provides basin-wide coverage and has been used to examine long-term trends in Arctic sea-ice thickness. This technique requires knowledge of the radar penetration characteristics through the snow cover on the sea ice. The radar is assumed to penetrate to the snow/ice interface in the Arctic, where the winter snow cover is cold and dry, in light of supporting laboratory experiments. However, this assumption has not been tested in the field. In the Antarctic, where the snow cover on sea ice is often flooded and can include complex layering even during the winter months, the radar penetration characteristics are likely from those over Arctic sea ice. This presentation shows measurements of radar penetration using sledge-borne, airborne and satellite radar altimeters. A ground-based study showed radar penetration into snow cover on Antarctic sea ice was variable, dependent on the snow characteristics. Preliminary results on the penetration into Arctic snow cover using airborne CryoVEx measurements are presented. We also present the results of an investigation into Ku-S band range differences over Antarctic sea ice.