Estimation of ice sheet surface elevation change from the geoscience laser altimeter satellite crossover simulation.

摘要

This study presented a determination of ice sheet surface change for a (100 km){dollar}sp2{dollar} area with a slope of about 0.6{dollar}spcirc.{dollar} The Geoscience Laser Altimeter System (GLAS) ice-sheet surface height measurements at laser-footprint-track crossover points were simulated, where a 50% data outage from clouds was assumed. A technique known as geolocation was developed to compute the location and height of laser footprints. The (100 km){dollar}sp2{dollar} area was part of a (900 km){dollar}sp2{dollar} area located in the Ross ice streams, West Antarctica. The (900 km){dollar}sp2{dollar} area was modeled as the sum of a constant topography and a time-varying part of the ice surface. The time-varying part contained both the long-term trend and the short-term variability. One of the ice sheet surfaces adopted for the study was the "undulating (100 km){dollar}sp2{dollar} area" with a slope of about 0.6{dollar}spcirc.{dollar} The simulation introduced the systematic nature of orbit and pointing error sources: the orbit error was obtained from a complete orbit simulation, and the pointing error was generated from a Gauss-Markov process. The effects of these orbit and pointing errors were analyzed in detail for the "undulating (100 km){dollar}sp2{dollar} area." Results from a 0.5-year crossover data set showed that the major error source affecting the accuracy in the estimation of surface change was the pointing enror. To obtain a better assessment of the pointing error characteristics and ice-sheet surface change, a Monte-Carlo Simulation of the pointing error was performed. Six of these 0.5-year crossovers were combined to obtain a 3-year data set, which then was used to obtain a 1.1 cm/year accuracy in the determination of surface change. This precision for the 3-year data set satisfied one of the most stringent GLAS science requirements over ice sheets: a 1.5 cm/year accuracy for (100 km){dollar}sp2{dollar} areas in the Ross ice stream with surface slopes less than 0.6{dollar}spcirc.{dollar}