A model of satellite radar altimeter return from ice sheets

Abstract

Satellite-borne radar altimetry offers a unique opportunity for measuring the form and mass balance of the polar ice sheets. Changes in ice-sheet mass balance are intimately linked to climatic change and variations in the global mean sea level. However, previous altimeter measurements of ice-sheet topography have been made without the use of a well-validated model of the altimeter return. Here, we present a theoretical model of the return which, supported by both observational and experimental evidence, suggests that over vast areas of the higher altitude regions of the ice-sheets, significant radar penetration of the firn occurs at frequencies commonly used for space altimetry. This implies the need for a previously-neglected correction to height measurements which can be as much as 3-3 m, depending on the retracking method and location. Since the degree of radar penetration may exhibit variability over a range of time-scales, failure to account for the effect could lead to erroneous estimates of surface elevation change. The detection of variability in the degree of penetration is of considerable interest from the point of view of monitoring the processes of accumulation and ablation of snow over the ice-sheet surface, as the return is particularly sensitive to conditions within a few centimetres of the surface. The model has wider applications as it may be used in modified form to simulate altimeter return from all smooth surfaces which exhibit a combination of surface and volume scattering, including deserts and the surfaces of the terrestrial planets and their satellites.