ON THE DERIVATION OF KERNELS FOR KERNEL-DRIVEN MODELS OF BIDIRECTIONAL REFLECTANCE

A new approximation to Ross' (1981) radiative transfer theory for smal l values of leaf area index (LAI) and two new approximations to Li and Strahler's (1992) geometric-optical mutual shadowing model are derive d. These, together with Roujean et al.'s (1992) approximation to Boss' theory for large LAI and their geometric-optical model of rectangular protrusions, may be used for formulating semiempirical models of the bidirectional reflectance distribution function (BRDF) of the land sur face through linear combinations. Because the functions superimposed d epend only on viewing and illumination geometry, the BRDF models deriv ed may be called kernel-driven; but Nilson and Kuusk's (1989) modified version of Walthall et al.'s (1985) model is an example of an empiric al model that belongs to this same class. The linearity of kernel-driv en models is advantageous to global BRDF and albedo processing needs i n several respects, most notably analytical invertibility, making poss ible look-up table approaches to albedo calculation, accommodation of mixed pixel situations, and spatial scaling. The models discussed here are being proposed for BRDF/albedo processing for the moderate resolu tion imaging spectroradiometer (MODIS) sensor of NASA's Earth Observin g System (EOS).