Atmospheric multiple scattering effects on GLAS altimetry. II. Analysis of expected errors in Antarctic altitude measurements

For pt.I see ibid., vol.39, no.1, p.92-101 (2001). The altimetry bias in the Geoscience Laser Altimeter System (GLAS) or other laser altimeters resulting from atmospheric multiple scattering is studied in relationship to current knowledge of cloud properties over the Antarctic Plateau. Estimates of seasonal and interannual changes in the bias are presented. Results show the bias in altitude from multiple scattering in clouds would be a significant error source without correction. The selective use of low-optical-depth clouds or cloud-free observations, as well as improved analysis of the return pulse such as by the Gaussian method used here, is necessary to minimize the surface altitude errors. The magnitude of the bias is affected by variations in cloud height, cloud effective particle size, and optical depth. Interannual variations in these properties as well as in cloud cover fraction could lead to significant year-to-year variations in the altitude bias. Although cloud-free observations reduce biases in surface elevation measurements from space, over Antarctica these may often include near-surface blowing snow, also a source of scattering-induced delay. With careful selection and analysis of data, laser altimetry specifications can be met.