Temporal Variation in Surface Bidirectional Reflectance of the Railroad Valley Vicarious Calibration Test Site in Nevada

Spectral reflectance-based vicarious calibration (VicCal) requires accurate characterization of the bidirectional reflectance distribution function (BRDF) of the ground-based target. Railroad Valley (RRV) Playa, Nevada, has been used as a VicCal test site since 1995 as it is large, appears stable over time, and has a reasonably consistent surface. This study presents the results of a diurnal measurement cycle that closely replicated illumination geometries for Earth Observing (EO) satellites over a year. By measuring the rate of change of the BRDF with respect to time, we recorded the range of BRDF effects while holding the surface constant with respect to moisture and surface condition variation. The rate of spectral reflectance change increased rapidly with view angle in the backscatter direction, reaching rates of change that are 2.3 and 10.5 times greater in the backscatter than in the forward scatter direction for view angles of 20 degrees and 40 degrees, respectively. The results show that larger off-nadir viewing angles in the backscatter direction are particularly sensitive to changes in solar/view geometries.