Testing a LiSK BRDF Model with in Situ Bidirectional Reflectance Factor Measurements over Semiarid Grasslands

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The non-Lambertian nature of the terrestrial surface is a major source of unexplained variability in wide-swath satellite sensor data acquired in the solar reflective wave-lengths, hindering quantitative analysis in the spectral, temporal, and locational domains. The interactions of light with the surface are governed by the bidirectional reflectance distribution function (BBDF), and modeling this is one of the most promising methods for describing and explaining this variability. Here the Roujean linear semi-empirical kernel-driven (LiSK) model was tested against two independent bidirectional reflectance factor datasets that were acquired close to ground level over seminatural semiarid grasslands in Xilingol, Inner Mongolia (People's Republic of China) and in Arizona (United States). The objectives were to determine how well the model is able to describe and explain observed bidirectional reflectance factor distributions in the red and near-infrared wavelengths, to explore its utility in correcting such data for angular variations, and the likely impact of such corrections on cover-type discrimination. The sensitivity of the model to reductions in the number and angular distribution of the bidirectional reflectance observations with which it is inverted was also evaluated. The results show that the model is able to describe the observed multiangular BRFs with good accuracy and with low sensitivity to the number of angular inputs, with observations in the forward-scattering direction shown to be important in constraining inversions. The behavior of retrieved parameters indicates that one or more of the simplifying assumptions made in the model derivation is likely to be too severe for explaining BRDF in the near-infrared region; non-negligible anisotropic multiple scattering and the assumption of an optically thick medium mean that a physical interpretation of parameters is unlikely to be valid. However, the model is shown to provide an effective means of correcting for BRDF effects, allowing greater precision and consistency than hitherto possible in the retrieval of surface spectral reflectance over semiarid grasslands and concrete improvements in cover-type discrimination.



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