Evaluation of a CCD camera system for BRDF retrieval for remote sensing applications for vicarious calibration

Abstract

A CCD-camera based system for the retreival of bidirectional reflectance distribution function (BRDF) data has been evaluated for vicarious calibration applications. This evaluation is done by assessing the calibration requirements necessary to retrieve BRDF data for the improvement of the vicarious calibration approach, and then by examining the calibration problem itself. A sensitivity analysis shows that for a top of the atmosphere (TOA) radiance accurate to 0.1%, instrumental biases must be under 5% while pixel-to-pixel gain variations may be as great as 10%. A method for achieving the calibration requirements using a CCD-based BRDF camera system constructed by the Remote Sensing Group (RSG) at the University of Arizona Optical Sciences Center is presented. A relative calibration level of approximately 1% across the camera array is found to be achievable given the laboratory facilities of the RSG. Software designed to extract BRDF data from the BRDF camera system output and convert the data into a form usable in the RSG's radiative transfer code are described and demonstrated on example data sets. A diffuse-light correction algorithm and software to perform the correction on BRDF camera data are described, and the software is tested against several example data sets to evaluate the retrieval accuracy of the code. Retrieval accuracies of better than 0.5% in phase and better than 0.01% in radiance have been achieved with this code using modeled data and at a 45-degree solar zenith angle. Based on these results, CCD-camera based systems can be used to improve the level of accuracy of TOA radiance calculations for vicarious calibration.