Radiance-based and Reflectance-based Retrievals of Surface Reflectance for Vicarious Calibration

A field deployable transfer radiometer has been developed as part of an effort to ensure the quality of upwelling radiance at automated test sites used for vicarious calibration in the solar reflective. Ground-based radiometer measurements of the surface combined with knowledge of the atmospheric state can be used to predict top-of-atmosphere reflectance and radiance. This work discusses two pathways with independent traceability of deriving the surface reflectance, namely a radiance-based and reflectance-based approach. The radiance-based retrieval converts the upwelling radiance reported by the radiometer to reflectance using radiative transfer calculations and atmospheric characterization data. The reflectance-based approach converts measurements by the radiometer of the surface to reflectance through ratios to data collected while viewing a diffuser reference of known reflectance. The results from both traceability paths are obtained using the same radiometer data sets, thus allowing analysis of any resulting differences. Field radiometer data collected at a desert site in the western US coincident with overpass of the Landsat 8 OLI sensor are analyzed to understand the impact of the two approaches on predictions of both the top-of-atmosphere reflectance and radiance. The comparison between the two traceability paths shows clear differences, but these differences are within the combined uncertainties of the two approaches.