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Abstract
Abstract. We report on a previously undocumented range error in NASA's Ice, Cloud and land Elevation Satellite (ICESat) that degrades elevation precision and introduces a small but significant elevation trend over the ICESat mission period. This range error (the Gaussian-Centroid or "G-C" offset) varies on a shot-to-shot basis and exhibits increasing scatter when laser transmit energies fall below 20 mJ. Although the G-C offset is uncorrelated over periods ≤ 1 day, it evolves over the life of each of ICESat's three lasers in a series of ramps and jumps that give rise to spurious elevation trends of −0.92 to −1.90 cm yr−1, depending on the time period considered. Using ICESat data over the Ross and Filchner–Ronne ice shelves we show that (1) the G-C offset introduces significant biases in ice-shelf mass balance estimates, and (2) the mass balance bias can vary between regions because of different temporal samplings of ICESat. We can reproduce the effect of the G-C offset over these two ice shelves by fitting trends to sample-weighted mean G-C offsets for each campaign, suggesting that it may not be necessary to fully repeat earlier ICESat studies to determine the impact of the G-C offset on ice-sheet mass balance estimates.
Highlights
NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) (Schutz et al, 2005) was an Earth-orbiting laser altimeter mission that operated from 2003–2009
Using global statistics for the GC offset and case studies over the salar de Uyuni in Bolivia and two Antarctic ice shelves, we demonstrate the potential impact that the correction has on ICESat elevation accuracy and ice sheet mass balance estimates
Of greater relevance to ice-sheet mass balance studies is a secondary effect due to the fact that when ordered in time, the changes in the mean G-C offset between campaigns exhibit time-correlated behavior over the mission period (Fig. 5) that could potentially be interpreted as real surface elevation change
Summary
NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) (Schutz et al, 2005) was an Earth-orbiting laser altimeter mission that operated from 2003–2009. Many studies have used ICESat elevation data to estimate volume/mass changes of glaciers (Gardner et al, 2013), ice shelves (Pritchard et al, 2012), and ice sheets (Shepherd et al, 2012), and ICESat data have been combined with other measurements to increase the spatiotemporal coverage and resolution of surface change estimates These complementary data include airborne laser altimetry from NASA’s Operation IceBridge mission (Koenig et al, 2010; Kwok et al, 2012; Schenk and Csathó, 2012), gravity from the NASA/DLR GRACE mission (Riva et al, 2009), and elevations from ESA’s ERS-1, ERS-2 and Envisat radar altimeters (Zwally et al, 2011; Hurkmans et al, 2012). Using global statistics for the GC offset and case studies over the salar de Uyuni in Bolivia and two Antarctic ice shelves, we demonstrate the potential impact that the correction has on ICESat elevation accuracy and ice sheet mass balance estimates
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