Albedo estimation from PolDER data

Multi-spectral and multi-directional data acquired during the ReSeDA experiment thanks to the airborne PolDER sensor were used to retrieve surface albedo over the experimental site, for 16 days over the year 1997. The data were available in four wave-bands (10 or 20 nm width) centered at 443 nm, 550 nm, 670 nm, and 865 nm. Zenith view angles ranged from 0 to 50 o. This study aimed at evaluate a procedure based on the use of multi-directional and multi-spectral information to retrieve surface albedo. Multidirectional information was extracted thanks to BRDF kerneldriven models. We compared the performances of three models (Walthall, Roujean and MRPV) in the four PolDER channels. The spectrally integrated value of the albedo was then derived from the of the hemispherical reflectance estimates in the four wave-bands, thanks to the linear regressions proposed by Weiss et al. (1999). 20 m resolution albedo maps were computed, and then compared to field measurements over several crop fields considering all days of the experiment. Results showed that PolDER retrievals overestimated ground measurements. This might be explained, at least partially, by inappropriate linear combinations used for the spectral extrapolation.