Empirical Correction of Residual Error in the ICESat-1 Altimetry Time Series at Lake Vostok

Antarctica’s subglacial Lake Vostok is a large (~15,690 km2), relatively flat feature that is located in a broader, stable ice sheet surface. These characteristics make it well suited for calibration of satellite altimetry time series data and, critically, for assessment of elevation accuracy. Lake Vostok’s stability is documented by analysis of multi-year GPS data from a network of stations around Vostok Station (Richter et al., 2008). The lake surface has been crossed by numerous Ice, Cloud, and land Elevation Satellite (ICESat) laser altimeter profiles during 2003-2009. ICESat data is collected in ~month long observation periods 2-3 times a year. Our study uses all ICESat crossovers to assess elevation changes on and off the subglacial lake area within an 87,500 km2 study area to assess elevation biases between observation periods to characterize instrumental artifacts. Our preliminary elevation change results from altimetry are compatible with the results from the GPS network; both data sets suggest that no lake level changes have occurred (the ICESat data on the lake is differenced from the elevations off the lake). However, the ICESat data also reveals the difficulty in determining elevations with cm-level accuracy. The primary problem is a known detector saturation that distorts the received waveforms (Sun et al., 2003). This error is a function of received energy and leads to 10s of cm of range error (elevations are too low) in central Antarctica for portions of the ICESat record. Our analysis shows that the available saturation correction algorithm partially compensates for this range error but leaves residual elevation errors of up to ~20 cm for the highest received energy operations periods. In addition, these errors are a function of each ICESat laser's operational behavior. Mathematical functions derived from the Lake Vostok study area crossover data indicate that the majority of the residual error can be accounted for but additional analyses are needed to evaluate this problem across larger and cloudier portions of the ice sheet and we are continuing to investigate the problem. Richter, A., S. Popov, R. Dietrich, V. Lukin, M. Fritsche, V. Y. Lipenkov, A. Matveev, J. Wendt, A. Yuskevich, and V. Masolov (2008), Observational evidence on the stability of the hydro-glaciological regime of subglacial Lake Vostok, Geophysical Research Letters, doi:10.1029/2008GL033397 Sun, X., J. B. Abshire, and D. Yi (2003), GLAS - Characteristics and performance of the altimeter receiver, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract C32A-0432.