Sensitivity of space-borne SAR data to forest parameters over sloping terrain. Theory and experiment

Recently, SAR data proved to be useful for the retrieval of forest biomass. However, the effects of terrain slope must be addressed towards the genera lization of biomass retrieval for varied forest and environmental condition s. To this aim, we developed experimental and theoretical approaches allowi ng the study of multi-frequency/multi-polarization forest backscatter of a given forest type, as a function of forest parameters and SAR local inciden ce angle over the relief. The experimental results showed that the sensitiv ity of SAR data to biomass was similar to that obtained over a flat terrain , only if the backscatter data were calibrated for slope effects. Moreover, the backscatter must also be corrected for its angular decrease, which can be removed using a simple angular model developed under assumptions of the oretical equations. The highest correlation of corrected backscatter with f orest parameters related to aboveground biomass (such as stand age and bole volume) was achieved at L-HV 55 degrees (R-2=0.8) with results similar to those for flat terrain. Modelling results validate the semiempirical model developed to remove the angular decrease of forest backscatter, which is du e to variable optical depth with terrain slope. Results also indicated that the simple angular model may encounter problems for forested areas with lo w optical depth (tau <0.5) located over extreme topographic conditions (loc al incidence angle < 10 degrees or > 80 degrees).