Deriving albedo maps for HAPEX-Sahel from ASAS data using kernel-driven BRDF models

This paper describes the application and testing of a method for deriving s patial estimates of albedo from multi-angle remote sensing data. Linear ker nel-driven models of surface bi-directional reflectance have been inverted against high spatial resolution multi-angular, multi-spectral airborne data of the principal cover types within the HAPEX-Sahel study site in Niger, W est Africa. The airborne data are obtained from the NASA Airborne Solid-sta te Imaging Spectrometer (ASAS) instrument, flown in Niger in September and October 1992. The maps of model parameters produced are used to estimate in tegrated reflectance properties related to spectral albedo. Broadband albed o has been estimated from this by weighting the spectral albedo for each pi xel within the map as a function of the appropriate spectral solar irradian ce and proportion of direct and diffuse illumination. Partial validation of the results was performed by comparing ASAS reflectan ce and derived directional-hemispherical reflectance with simulations of a miller canopy made with a complex geometric canopy reflectance model, the B otanical Plant Modelling System (BPMS). Both were found to agree well in ma gnitude. Broadband albedo values derived from the ASAS data were compared w ith ground-based (point sample) albedo measurements and found to agree extr emely well. These results indicate chat the linear kernel-driven modelling approach, which is to be used operationally to produce global 16 day, 1 km albedo maps from forthcoming NASA Earth Observing System spaceborne data, i s both sound and practical for the estimation of angle-integrated spectral reflectance quantities related to albedo. Results for broadband albedo are dependent on spectral sampling and on obtaining the correct spectral weight ings.