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NASA's ICESat (Ice, Cloud, and Land Elevation Satellite) was launched into near-polar orbit in February 2003 carrying a laser altimeter (22) that operated during February-March and September-November 2003 and during February-March 2004. Some of our aircraft surveys were along ICESat orbits, and all crossed orbit tracks at many locations. Overlapping (by 50%) planar surfaces, or "platelets," were fit to the -1200 aircraft measurements on each side of the aircraft within a 70-m along-track distance, generally with root-mean-square fit of 10 cm or less. These were compared with ICESat footprint elevations by extrapolating elevations from any platelet within 200 m distance (using the platelet slope), yielding almost 7000 comparisons. The major source of aircraft survey errors was laser pointing, with errors magnified by the 15° off-nadir scan angle. For the ≃1000-m altitude flown and a typical maximum roll error of 0.05° (effects of pitch errors are averaged out in the platelet calculation), the resulting elevation error is <0.3 m, with opposite signs on each side of the surveyed swath. Because we averaged about four adjacent ICESat/aircraft comparisons that included data from each side of the aircraft, effects of these roll errors should be largely canceled. Other error sources contribute about ±0.2 m. Accuracy of ICESat measurements is determined mainly by errors in laser pointing and by forward scattering in thin clouds, with the latter edited from the data on the basis of distortion of laser return waveforms. We estimated pointing errors for the September-November 2003 and the 2004 measurements to be <5 arc sec by comparing ICESat data with aircraft surveys over undulating terrain in Antarctica, Greenland, and arid parts of the western United States. Most of our Amundsen glacier surveys were over slopes less than 2°, resulting in slope-induced errors less than ±0.5 m. Consequently, estimated (largely random) errors in elevation changes are less than ±0.6 m. ICESat pointing errors for February-March 2003 data were larger (up to 40 arc sec) and are not included in this analysis.
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We dedicate this work to the memory of Niels Gundestrup, who did much to make the project possible despite serious illness. We thank pilots, crew, technicians, and staff from the Armada de Chile, University of Kansas, NASA/WFF, and CECS who helped make the surveys over Antarctica; the British Antarctic Survey, NSF, U.S. Geological Survey. NOAA. Dirección Meteorológica de Chile, and the field team at Base Carvajal for providing weather reports/forecasts and GPS data that made possible the flights and subsequent accurate trajectory calculations: A. Shepherd for estimates of elevation change rates (7); NASA's ICESat Project for satellite laser altimeter elevation measurements; and T. Hughes and an anonymous reviewer for suggesting improvements to the paper. Supported by CECS, through Fundación Andes and the Millennium Science Initiative, and by NASA's Cryospheric Processes Program. E.R. performed his work at JPL under a contract with this program.
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