The behavior of the anisotropic reflectance of a pecan orchard derived from multiview and multiscale imaging spectrometer data

Bidirectional reflectances for a pecan orchard have been studied using Advanced Solid-state Array Spectroradiometer (ASAS) data acquired in the solar principal plane and perpendicular to it at altitudes of 2300 m and 5300 m above ground. In particular, the angular dependency of the bidirectional reflectance of two targets, sunlit portions of the pecan tree and composite (tree crowns and orchard floor), have been studied for viewing directions between − 45° and + 45°. The results generally indicate an increasing reflectance from the forward scattering direction to the backscattering direction for image sequences acquired in the solar principal plane and a symmetrical behavior about nadir with decreasing reflectance towards the extreme viewing angles for site passes perpendicular to the solar principal plane. These off-nadir effects vary with solar zenith angle, target type (composition and structure), and spatial scale. The largest variations were usually observed in the backscatter directions. It was also found that a combination of these factors, especially with the target characteristics, had a significant impact on the surfaceʹs bidirectional reflectance. In addition, these reflectance changes are wavelength-dependent, with the largest difference in relative reflectance occuring between the visible and infrared bands. In general, the overall behavior of the bidirectional reflectance derived from airborne data follows the trends observed in ground-based investigations.