Hemispherical reflectance and albedo estimates from the accumulation of across-track sun-synchronous satellite data

The estimation of the hemispherical reflectance and the instantaneous albed o of canopies from top of canopy satellite reflectance data was investigate d. The study was designed to approximate the specifications of generic sens ors aboard satellites like NOAA, VEGETATION, MERIS, MISR, MODIS, and PRISM. These sensors acquire reflectance data in two to six wave bands distribute d along the visible, near-infrared, and middle infrared domains. Five great biomes (grassland, sparse vegetation, tropical forest, boreal forest, and bare soil) were approximated, simulating the corresponding top of canopy re flectances as observed from the satellites using well-known leaf, soil, and canopy radiative transfer models, including the effect of cloud cover that limits the actual data acquisition scheme. Albedo was accurately derived f rom the hemispherical reflectance observed in only a few wave bands. When u sing six wave bands, albedo was estimated within 1% relative accuracy. The MRPV bidirectional reflectance distribution function (BRDF) model was teste d to derive the hemispherical reflectance from the top of canopy bidirectio nal data as sampled by the generic sensors during a 32 day orbit cycle. Res ults showed that this is the main source of error, with a relative accuracy around 6%. This showed the importance of the directional sampling scheme a nd possible improvements that may be made to the model and the way it is fi tted to the observed data. The algorithm developed produced a relative accu racy less than 7% for the albedo estimation, when using the six wave bands and a +/-45 degrees across-track directional scanning capacity. The results were discussed with particular emphasis on the optimization of sensors and algorithms dedicated to albedo estimation and to the use of hemispherical reflectance as a potential normalized geophysical product for monitoring ve getation.