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

A new approximation to Ross' (1981) radiative transfer theory for small values of leaf area index (LAI) and two new approximations to Li and Strahler's (1992) geometric-optical mutual shadowing model are derived. 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 surface through linear combinations. Because the functions superimposed depend only on viewing and illumination geometry, the BRDF models derived 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 empirical model that belongs to this same class. The linearity of kernel-driven models is advantageous to global BRDF and albedo processing needs in several respects, most notably analytical invertibility, making possible 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 resolution imaging spectroradiometer (MODIS) sensor of NASA's Earth Observing System (EOS).