Simulation of ATSR-2 optical data and estimates of land surface reflectance using simple atmospheric corrections

The author presents simulations of ATSR-2 optical data using a combined orbit and scan model and model atmospheres. The atmospheric profiles are taken from LOWTRAN, but the author has converted these into a form suitable for a discrete ordinates radiative transfer code. All scattering processes are taken into account. The major approximations used are that the atmosphere is plane parallel, and that polarization is not taken into account. Cloud-free conditions are assumed. The author assumes ATSR-2 observations over the Earth with a constant atmosphere, and different types of reflecting ground in order to see what the implications are for retrieving ground reflectances. It was found that in the winter months, surface reflectance estimation will be difficult except at low latitudes. In the northern winter, the ATSR-2 forward scan sees large reflectances from aerosols in the atmosphere. These are not seen in the southern winter, but at 55° S, the Sun is more than 80° from the zenith point at ATSR-2 crossing times, and the atmosphere is still bright. The results from two simple reflectance retrieval algorithms and their region of validity are also examined . First, the author looks at a simulated retrieval for a Lambertian reflectance covering the globe at all times of year. Then the author looks at a simulated retrieval of a BRDF at a particular location using pairs of overpasses over a period of one year. It is predicted that the simple algorithms work well for ATSR-2 Sun/Sensor configurations except in the far northern and southern winters. The failure in the southern winter might be avoided if the Rayleigh/aerosol interaction is taken into account. It is als predicted that the forward scan data for the northern winter is unlikely to be useful for land surface studies, but might be useful for aerosol sounding, provided this is restricted to areas where there is no snow or frost. In this study of the BRDF at one location, it was found that if certain types of model BRDF are used, Sun synchronous orbits will severely limit the kinds of information that can be estimated