RETRIEVAL OF SURFACE BRDF FROM MULTIANGLE REMOTELY-SENSED DATA

The bidirectional reflectance distribution function (BRDF) is the intrinsic measure of the directional properties of the Earth's surface. However, it cannot be measured directly by remote sensing. A procedure using the optimum technique is applied to retrieve surface BRDF parameters from multiangle remotely sensed data in which a statistical BRDF model and an analytical model of atmospheric radiative transfer are coupled. The atmospheric model decomposes radiant intensity received by a detector into four parts: scattered only by the (non-Lambertian) surface; singly scattered by the atmosphere; multiply scattered to the surface but unscattered to the detector; and multiply scattered before and after surface scattering. Analytical solutions are derived for the first two. The third is derived from a two-stream model, but incorporates the surface BRDF explicitly. The fourth uses a two-stream model under conditions of azimuthal independence. The six-parameter surface BRDF is modeled as the sum of a modified limacon function, which describes the basic bowl-shape of the BRDF, and a negative exponential, which describes the hotspot. Multiangle observations from the ASAS and PARABOLA instruments are analyzed using the coupled models. Experiments show that the simple statistical BRDF can summarize typical directional observations with errors in the range of 3-10 %.