MODTRAN® scattering: Extracting spherical-refractive path contributions from plane-parallel disort

The well-established and extensively validated atmospheric radiative transfer model MODTRAN® 1 is widely used by the remote sensing community to define the mapping from surface spectral reflectance to solar scatter radiance as a function of atmosphere definition. This mapping is subsequently used in atmospheric correction/compensation models to define the reverse mapping from multi- and hyperspectral images to ground surface spectral reflectances. Since MODTRAN employs a stratified spherical shell atmosphere and accounts for spherical refraction, its applicability extends to low sun and highly off-nadir scenarios. The discrete ordinate module DISORT solves the RTE to compute multiple scattering solar path radiances for MODTRAN. However, there is a problem: DISORT is a plane-parallel atmosphere model. In this paper, a method for modifying DISORT to compute the curved-path segment radiance contributions required by MODTRAN is described.