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Greenland for measuring ice sheet surface elevation and thickness. This ability, if combined with present efforts at detecting the distribution of shallow layers to understand the spatial variations in accumulation, could produce an unmatched tool for completing the ice thickness map of Antarctica. Tying measurements from an accumulation radar to shallow core sites where background rates and variability have been determined in both Greenland and Antarctica would greatly enhance efforts to determine appropriate forcing for ice sheet models.

Altimetry offers the most promising approach to sea ice thickness mapping for change detection; this approach is the subject of an upcoming European Space Agency mission. The utility of ICESAT data for this application needs to be investigated, along with integrated approaches that use altimetric and ice motion time series, such as AVHRR and RGPS methods in the still exploratory stages. Validation and error analysis for these products should be pursued in an integrated manner as well.

NASA has no plans for a polar radar altimeter to measure ocean surface elevation beyond the latitudinal range of Topex-Poseidon. NASA should consider participation in future high-latitude radar altimetry programs, perhaps in conjunction with the satellite agencies of other nations.

Finally, the Earth Science Enterprise research activities of the polar community will be limited by the lack of appropriate SAR coverage, which may require a U.S. SAR. Many of the recent NASA-funded advances in cryospheric research have come from interferometric analysis of time series of SAR data. The processes that are studied are time-variable and so require relatively frequent repeat observations. Both sea ice and ice sheet research will suffer without access to SAR data acquired at appropriate intervals. The absence of a U.S. SAR satellite and/or MOUs with entities flying SARs is of great concern to the polar ocean and ice sheet communities.

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