ERS-1 ALTIMETRY ON THE GREENLAND ICE-SHEET - PRELIMINARY INVESTIGATIONS OF ANNUAL VARIATIONS

The existence of an annual signal in the ERS-1 35-day repeat altimetry measurements over die Greenland ice sheet is investigated in this pap er. Spectral analysis from a collinear set of long tracks indicates, t hat the timevarying part of the elevation signal is rather constant at a level around 20 cm at frequencies higher than 1 cycle per degree. A t lower frequencies the presence of a stationary signal can be seen. T his signal can be estimated in a collinear analysis from the entire se t of 35-day altimetry data over the ice sheet spanning one and a half years. Separation of an annual signal revealed a signal highly correla ted with topography. The mean amplitude is around 40 cm, with amplitud es between 10 - 20 cm around the summit of the ice, rising to between 50 - 80 cm approaching the ablation zone of the ice sheet.