RETRIEVAL OF SURFACE BRDF FROM MULTIANGLE REMOTELY-SENSED DATA

The bidirectional reflectance distribution function (BRDF) is the intr insic measure of the directional properties of the Earth's surface. Ho wever, it cannot be measured directly by remote sensing. A procedure u sing the optimum technique is applied to retrieve surface BRDF paramet ers from multiangle remotely sensed data in which a statistical BRDF m odel and an analytical model of atmospheric radiative transfer are cou pled. The atmospheric model decomposes radiant intensity received by a detector into four parts: scattered only by the (non-Lambertian) surf ace; singly scattered by the atmosphere; multiply scattered to the sur face but unscattered to the detector; and multiply scattered before an d after surface scattering. Analytical solutions are derived for the f irst two. The third is derived from a two-stream model, but incorporat es the surface BRDF explicitly. The fourth uses a two-stream model und er conditions of azimuthal independence. The six-parameter surface BRD F is modeled as the sum of a modified limacon function, which describe s the basic bowl-shape of the BRDF, and a negative exponential, which describes the hotspot. Multiangle observations from the ASAS and PARAB OLA instruments are analyzed using the coupled models. Experiments sho w that the simple statistical BRDF can summarize typical directional o bservations with errors in the range of 3-10 %.