The primary goal of the Ice, Cloud and land Elevation Satellite (ICESat) mission is ice sheet elevation change detection. Confirmation that ICESat is achieving its stated scientific requirement of detecting spatially-averaged changes as small as 1.5 cm/year requires continual assessment of ICESat-derived elevations throughout the mission. We use a GPS-derived digital elevation model (DEM) of the salar de Uyuni, Bolivia for this purpose. Using all twelve ICESat passes over the salar survey area acquired to date, we show that the accuracy of ICESat-derived elevations is impacted by environmental effects (e.g., forward scattering and surface reflectance) and instrument effects (e.g., pointing biases, detector saturation, and variations in transmitted laser energy). We estimate that under optimal conditions at the salar de Uyuni, ICESat-derived elevations have an absolute accuracy of <2 cm and precision of <3 cm.