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Definition:
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A technique in which glacier mass variations are calculated from direct measurements of Earth's gravity field. Satellite gravimetry is at present the most feasible method for determining glacier mass balance from changes in gravity. The Gravity Recovery and Climate Experiment (GRACE) consists of two polar-orbiting satellites separated by about 200 km along-track, and is the primary mission for this work to date. Precise measurements of range and range rate are used to construct local gravity fields after correcting for non-gravitational accelerations. Suitable models are used to remove gravity variations resulting from atmospheric, hydrospheric and lithospheric mass variations, leaving a time series that represents the glacier mass balance (usually summed and shown as the cumulative mass balance). GRACE spatial and temporal resolutions as good as 2 arc degrees and 10 days have been achieved. Satellite gravimetry is limited by the quality of observations used to constrain the models of non-glacial mass variations, and at present it can resolve only large and rapidly changing glacier complexes or glacierized regions. A distinctive advantage of the method is that it yields a direct measure of mass and does not require density corrections such as those required for geodetic methods. Glacier mass balance has also been estimated using ground-based gravimeters. Measurements at two or more times yield the change in absolute gravity that results from the change in vertical position of a sensor on the glacier surface or at a fixed position above it, and from changes in glacier mass. This technique may become more widely developed as gravimeter resolution, precision and portability improve.
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