THE ACCURACY OF SATELLITE RADAR ALTIMETER DATA OVER THE GREENLAND ICE-SHEET DETERMINED FROM AIRBORNE LASER DATA

The 336 days of the geodetic phase of ERS-1 provides dense coverage, b y satellite radar altimetry, of the whole of the Greenland ice sheet. These data have been used to produce a digital elevation model of the ice sheet. The errors present in the altimeter data were investigated via a comparison with airborne laser altimeter data an absolute accura cy typically in the range 2-10 cm +/- 10 cm. Comparison of differences between the radar and laser derived elevations, showed a correlation with surface slope. The difference between the two data sets ranged fr om 84 cm +/- 79 cm for slopes below 0.1 degrees, to 10.3 m +/- 8.4 m f or a slope of 0.7 degrees ( the half power beam-width of the ERS-1 rad ar altimeter). An explanation for the behaviour of the difference as a function of surface slope is given in terms of the pattern of surface roughness on the ice sheet.