Ground measurements of surface BRF and HDRF using PARABOLA III

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Abstract
The ground‐based Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA), version 3, provides multiangle measurements of sky and ground radiances on a spherical grid of 5° in the zenith‐to‐nadir and azimuthal planes in eight spectral channels. The hemispherical directional reflectance factor (HDRF) can be measured directly by comparing the radiance reflected by the surface in given direction to that reflected from a reference surface simultaneously observed by the PARABOLA 3. The surface bidirectional reflectance factor (BRF) cannot be measured directly, however, because of the presence of the sky diffuse illumination. The contribution of the diffuse sky radiance to the radiance reflected by the natural target surface is computed, and removed, using an iterative technique. Two approaches are employed: the first requires knowledge of the atmospheric optical depth, and the second requires the simultaneous measurements of the radiance reflected by a standard surface panel under the same atmospheric and illumination conditions. Ground measurements of the BRF and HDRF for dry lake surfaces were obtained from the PARABOLA 3 observations with better than ±10% accuracy. The results described in this work are used primarily for the vicarious calibration of the Multiangle Imaging Spectroradiometer (MISR) onboard the Earth Observing System (EOS) Terra platform and for validation of MISR BRF retrievals of selected Earth surface targets.