G-function and HOt SpoT (GHOST) reflectance model: application to multi-scale airborne POLDER measurements

The complex architecture of conifer species affects the radiation scattering processes that influence the bidirectional reflectance distribution function (BRDF) characteristics. An interesting BRDF signature is the hot spot effect that yields a maximum of reflectance in the sun direction. It has been shown that the hot spot phenomenon is sensitive to the foliage structure. The present work introduces the G-function and HOt SpoT (GHOST) model that simulates the BRDF of boreal forest in accounting for the arrangement and the structure of the stems. The model represents the medium by a distribution of conical crowns whose size and shape are accounted for in the G-function expression. GHOST compares quite well with airborne POLarization and Directionality of Earth Reflectances (POLDER) reflectances acquired during the BOREAS experiment at various scale resolutions ranging from 2 (helicopter) up to 35 m (aircraft). The inversion procedure exploits the multidirectionality of measurements with emphasis on the hot spot observations and the angular range of dominant influence of each parameter. The retrieved parameters translate optical and structural properties of the canopy. They are used to calculate a new architectural parameter X that allows to get the clumping index of the patch vegetation.