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Post-launch calibration and characterization of Geostationary Earth Observing (GEO) satellite sensors, which lack on-board visible calibration, is a keenly felt need in many applications for studying long-term global climate changes. This paper proposes a vicarious technique of calibrating GEO visible sensors for the Clouds and the Earth's Radiant Energy System (CERES) project, using a kernel-based bidirectional reflectance distribution function (BRDF) model derived over an invariant desert site. The technique is illustrated with two Meteosat-9 visible channels, 0.65um and 0.86um, whose radiometric gains have been computed using a BRDF model of Libya-4 desert site. The Libya-4 BRDF model is TOA based and is derived from ten years of clear-sky Aqua observations over this site. The calibration results are validated by a direct comparison with the corresponding gains derived from the GEO-to-MODIS ray-matching technique.