Radar altimeters are one of the main tools for measuring elevation changes across the Antarctic and Greenland ice sheets and larger ice caps. A ground-based radar was deployed in autumn 2004 and spring 2006 in the percolation zone of the Greenland Ice Sheet. This radar is a high bandwidth system operating in the Ku band, the same frequency as several satellite altimeters. Measurements were made over an elevation range of 1795 to 2350 m, along with snow pit and shallow core studies. These measurements demonstrate the spatial and temporal variations in the backscatter. Relative strengths of surface and volume reflections change dramatically between spring and autumn and there is also high spatial variability across the percolation zone. The extent of percolation will affect elevation estimates made by radar altimeters.