AN ASSESSMENT OF THE CAPABILITY OF THE SATELLITE RADAR ALTIMETER FOR MEASURING ICE-SHEET TOPOGRAPHIC CHANGE

The monitoring of surface topography has frequently been cited as the most important application of the satellite radar altimeter over the ice sheets. The potential of the instrument in this respect is examined in detail using Seasat and Geosat altimeter data recorded over the Wilkes plateau in East Antarctica. The precision of the range measurements is optimized by the use of an improved retracking technique. However, although the r.m.s. precision of range measurements indicated at orbit crossovers, after orbit adjustment, is 20-30 cm, tilt and bias orbit adjustment techniques are shown to leave errors in the measurement of surface elevation change, with magnitudes of the order of tens of centimetres, depending on the location. Additional orbit error, which cannot be identified from crossover residuals, is almost certainly present in the difference measurements. Variable mispointing of the Geosat antenna is considered the most likely explanation for an apparent change in surface scattering properties which results in errors in the measurement of elevation differences, locally of the order of a metre. As a result of these two uncorrected sources of error remaining in the data, the best estimate for surface elevation change between 1978 and 1985, along a narrow strip 2748 km long at 72-degrees-S, is + 1.05 m +/- 2.0 m. This includes correction for a -22 cm systematic bias in the Geosat range measurements, probably resulting from antenna offpointing. The results have implications for altimeter measurements of surface elevation change over all land surfaces, and indicate that much further research needs to be carried out to make full use of data from ERS-1 and subsequent satellites.