SATELLITE LASER ALTIMETRY OF TERRESTRIAL TOPOGRAPHY - VERTICAL ACCURACY AS A FUNCTION OF SURFACE SLOPE, ROUGHNESS, AND CLOUD COVER

Satellite laser altimetry provides a method to obtain global digital t opographic data of high accuracy by measuring the round-trip time-of-f light of laser pulses reflected from the Earth's surface. Analysis of the sensitivity of laser ranging errors to surface conditions indicate s that predicted single-shot range errors are primarily dependent on s urface slope. Range errors are less sensitive to variations in surface roughness or reflectivity. Values of total surface slope and roughnes s for nine terrestrial landforms, derived from digital elevation data at a 186-m-length scale, vary from 2-degrees to 40-degrees and 0.8 to 15 m, respectively, at a 90% frequency of occurrence. This range of su rface morphologies yields a variation in single-shot laser ranging err or from 0.4 to 8 m, assuming system parameters for the proposed Topogr aphic Mapping Laser Altimeter (TMLA) and a nominal 30% surface reflect ivity. The total elevation accuracy of data obtained via satellite las er altimetry, although dominated by the range error, is also a functio n of additional error sources, including orbit ephemeris, atmospheric, and calibration errors. Averaging of multiple laser measurements impr oves the vertical accuracy of the elevation data by statistical reduct ion of random errors. During a three-year mission, two to three laser measurements will be acquired, on average, for each 200-m footprint at low to moderate latitudes, accounting for the latidudinal variation o f ground track spacing and cloud cover. For high-latitude regions, the narrow spacing of satellite ground tracks in a polar orbit will provi de frequent repeat observations yielding, on average, 4 to 25 measurem ents of each footprint over the Antarctic and Greenland ice sheets. Av eraging of these multiple repeat observations at high latitude will yi eld an improvement in vertical accuracy by a factor of two to five.