Brdf Correction on Avhrr Imagery for Spain

The free reception of Advanced Very High Resolution Radiometer (AVHRR) data keeps a growing interest for the remote sensing community concerned with vegetation studies on regional and global scales. However, the use of AVHRR data for long-term quantitative monitoring requires consistent surface reflectance data. This implies correcting the effects from varying sun sensor target geometries in multitemporal AVHRR data sets described by the Bidirectional Reflectance Distribution Function (BRDF). In a joint research undertaken by the Geography Department of the University of Alcalá (Spain) and the Canadian Centre for Remote Sensing (CCRS) a new semi-empirical model, the Non-linear Temporal Angular Model (NTAM), has been applied for the BRDF correction of AVHRR data for Spain. Reflectance data derived from AVHRR channels 1 and 2 were corrected for BRDF effects by normalization to a standard viewing geometry. The study period was between May and September 2002. The required correction parameters were derived by model inversion. In order to obtain a good representation of the temporal and spatial dimensions of the data to be corrected, a sampling scheme was implemented. The land cover based approach of the NTAM was accounted for by the use of CORINE Land Cover data from Spain. A cloud mask algorithm was used to exclude cloud contaminated observations in the sampling data. The evaluation of the statistical parameters obtained from model inversion showed good results for practically all land cover classes in the two channels. Derived parameters allow for visualization of angular dependence of reflectance for different land cover classes.