SAND DUNE RIDGE ALIGNMENT EFFECTS ON SURFACE BRF OVER LIBYA-4 CALIBRATION SITE

摘要

The Libya-4 desert area, located in the Great Sand Sea,rnis one of the most important bright desert CEOSrnpseudo-invariant calibration sites by its size andrnradiometric stability. This site is intensively used forrnradiometer drift monitoring, sensor intercalibration andrnas an absolute calibration reference based on simulatedrnradiances traceable to the SI standard. The Libya-4rnmorphology is composed of oriented sand dunes shapedrnby dominant winds. The effects of sand dune spatialrnorganization on the surface bidirectional reflectancernfactor is analyzed in this paper using Raytran, a 3Drnradiative transfer model. The topography isrncharacterized with the 30 m resolution ASTER digitalrnelevation model. Four different regions-of-interest sizes,rnranging from 10 km up to 100 km, are analyzed. Resultsrnshow that sand dunes generate more backscattering thanrnforward scattering at the surface. The mean surfacernreflectance averaged over different viewing andrnillumination angles is pretty much independent of thernsize of the selected area, though the standard deviationrndiffers. Sun azimuth position has an effect on thernsurface reflectance field, which is more pronounced forrnhigh Sun zenith angles. Such 3D azimuthal effectsrnshould be taken into account to decrease the simulatedrnradiance uncertainty over Libya-4 below 3% forrnwavelengths larger than 600 nm..