Retrieval of land surface bidirectional reflectance and aerosol opacity from ATSR-2 multiangle imagery

New satellite instruments that sample top-of-atmosphere radiance at a number of view angles offer the potential for improved retrieval of atmospheric aerosol opacity, land surface bidirectional reflectance, and biophysical parameters. This paper presents a method for simultaneous retrieval of aerosol opacity and land surface bidirectional reflectance, which utilizes the dual view capability of the second Along-Track Scanning Radiometer (ATSR-2). Analysis of a physically based model of light scattering results in two simple equations defining possible spectral variation of land surface bidirectional reflectance distribution function (BRDF). These are used as constraints to anew inversion of a model of atmospheric scattering to simultaneously retrieve atmospheric aerosol opacity and bidirectional reflectance from top-of-atmosphere radiance. The inversion assumes no a priori knowledge of the land surface cover. Sensitivity is evaluated using both simulated and field-measured data to reproduce expected ATSR-2 observations. Where an atmosphere of known aerosol scattering properties, but of unknown optical depth, is available, results show mean absolute error in retrieval of aerosol opacity of the greater of 0.02 or 15% relative error and bidirectional reflectance retrieval at 55 nm to an accuracy of <0.01. Where a number of candidate aerosol models are available, results show discrimination of dominant aerosol type is possible in 95% of cases considered. The methods perform best over dark surfaces, such as vegetation, but show accurate retrieval over soil and pixels containing a number of cover types.