| Quantifying the state of the polar ice sheet is of great importance since uncertainty in ice sheet mass balance directly affects uncertainty in the rate of change of global sea level. In addition, the behavior of the polar ice has significant consequences on the change of global climate. Despite significant progress, there is still uncertainty about the mass balance of the two large grounded ice sheets in Greenland and Antarctica. While ice mass change is the fundamental quantity of interest, ice sheet elevation change is the quantity which we estimate using data from a radar altimeter originally developed to study sea surface elevation. Due to the roughness of ice surface compared to the sea surface, radar altimeter measurements cannot be used without significant on-ground reprocessing. We study several techniques and determine that for the purpose of ice sheet "height change" estimates, a simple model-independent retracking algorithm, along with our waveform cross-correlation editing procedure, is the most efficient and effective technique in obtaining more precise radar altimeter measurements. Pairs of these processed measurements and waveforms at the satellite ground-track cross-over points are used to compute an estimate of ice sheet elevation changes over time. From approximately 2.5 years of ERS-1 data over the Antarctic Ice Sheet spanning the period of September 1992 and March 1995, there is a continent-average seasonal elevation fluctuation of about 8 cm, corresponding to the maximum and minimum height around Austral winter and summer respectively. The West Antarctic region shows the largest amplitude fluctuation, up to 50 cm, which may be a combination of ice height changes and other effects. With careful analysis of radar altimeter waveform pairs at the cross-over points, it is possible to estimate surface height changes with useful accuracy at a seasonal time scale. Monitoring long term behavior of the ice sheet should be possible by combining radar altimeter data with laser altimeter data from the future ICESAT mission to create longer time series. |
