RETRIEVING LAND-SURFACE REFLECTANCES USING THE ATSR-2 - A THEORETICAL-STUDY

The measurement of absolute land reflectance from Earth-orbiting satel lites requires the separation of the signal due to the reflection of s olar radiation by the surface from that due to scattering by the atmos phere. Thus for accurate but generally applicable quasi-operational re trieval techniques it is necessary to derive atmospheric parameters, s uch as the aerosol type and loading, simultaneously with the surface r eflectances. The assumption of large-scale horizontal homogeneity acro ss an image, or the presence of very low reflectance pixels within an image, is avoided, since these are not necessarily applicable in the g eneral case. In this paper, results of simulated retrievals using the visible and near-infrared channels of the ATSR-2 satellite radiometer are presented on the basis of techniques which exploit the dual look a nd other features of the instrument. The pixel-based retrieval scheme comprises a fast representation of atmospheric scattering and a simple surface reflectance approximation. The effect of the atmospheric appr oximation on the accuracy of retrieval of the surface reflectance and atmospheric optical depth is tested by using a full multiple-scatterin g radiative transfer model with simpler surface model. The effect on r etrieval accuracy of the surface model is tested using a full surface vegetation canopy reflectance model with inclusion of the hot-spot eff ect, combined with a successive orders of scattering atmospheric radia tive transfer model, taken to second order. The results indicate that the scheme could provide accurate retrieval of surface reflectances ev en where no extraneous information on the surface bidirectional reflec tance or on the atmospheric optical thickness is available. The scheme is also applicable to the AATSR instrument to be launched on the ENVI SAT (Environmental Satellite) and, more widely, to other satellite rad iometers with multiple views of the Earth's surface.