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27 January 1997 BRDF and surface-surround effects on SPOT-HRV vicarious calibration
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Typically, the surfaces used in radiative transfer codes to predict the radiance at a satellite sensor are assumed to be lambertian and spatially-homogeneous. Of course, surface bidirectionality and surface-surround effects are second order terms, but improvements in vicarious calibration procedures, as well as more challenging accuracy requirements, require that these effects be included. This paper examines the effect of surface BRDF and spatial inhomogeneity on retrieved calibration coefficients for the SPOT HRV cameras from the well-known reflectance-based calibration approach. This calibration method has primarily relied on test sites at White Sands Missile Range in New Mexico, USA and La Crau, France. BRDF effects are studied using multispectral measurements of the bi-directional reflectances of both sites used in a Fourier series expansion in azimuth to model the surface BRDF. This Fourier series expansion follows the architecture of the successive- orders-of-scattering radiative transfer code and is easily introduced as a new boundary condition. These BRDF models are used to reprocess past calibration data and the results are compared to those obtained by assuming the surface to be lambertian. In addition to surface BRDF, spatial- inhomogeneity of the surface reflectance is examined. The surrounding area's surface reflectance is derived from the SPOT imagery and included in the radiative transfer computations suing in house modifications to the 6S code. These results are compared to those which exclude adjacency effects.
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Richard P. Santer, C. Schmectig, and Kurtis J. Thome "BRDF and surface-surround effects on SPOT-HRV vicarious calibration", Proc. SPIE 2957, Advanced and Next-Generation Satellites II, (27 January 1997);

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