A robust threshold retracking algorithm for measuring ice-sheet surface elevation change from satellite radar altimeters

A threshold retracking algorithm for processing ice-sheet altimeter data is presented. The primary purpose for developing this algorithm is detection of ice-sheet elevation change, where it is critical that a retracking algorithm produce repeatable elevations. The more consistent an algorithm is in selecting the retracking point the less likely that errors and/or biases will be introduced by the retracking scheme in the elevation-change measurement. The author performed extensive comparisons between the threshold algorithm and two widely used ice-sheet retracking algorithms on Geosat datasets comprised of over 60000 crossover points. The results show that the threshold retracking algorithm, with a 10% threshold level, produces ice-sheet surface elevations that are more repeatable than the elevations derived from the other retracking algorithms. For this reason, the threshold retracking algorithm has been adopted by NASA/GSFC as an alternative to their existing algorithm for production of ice sheet altimeter datasets under the NASA Pathfinder Program. The threshold algorithm will be used to re-process existing ice-sheet altimeter datasets and to process the datasets from future altimeter missions.