Red and near-infrared sensor response to off-nadiir viewing

Abstract NOAA-6 and 7 AVHRR visible (VIS) and near-infrared (NIR) radiance data from the advanced very high resolution radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration's (NOAA) NOAA-6, 7 and 8 satellites were simulated for a black target, for bare soil and for tow, medium and high levels of green-leaf biomass. Simulations were based on measured reflectance and Dave's atmospheric models. Radiance data were generated for viewing and illumination geometry corresponding to the summer solstice, equinox and winter solstice at a latitude of 30°. The results indicate that the radiance responses to atmosphere are less for NOAA-7 geometry than for NOAA-6 and 8. Viewing in the backscatter direction has a more constant radiance response with increasing scan angle than viewing in the forward scatter direction. These results arc discussed with respect to atmospheric composition, surface reflectance and wavelength. Normalized relative differences between NIR and VIS channels, arc also described and found to be a convenient way of summarizing the VIS and NIR AVHRR data. Normalized difference values arc greatest near nadir and slightly off-nadir in the forward scatter direction. Values decrease towards extreme off-nadir viewing angles being most pronounced in the backscatter direction. High levels of green-leaf biomass become poorly separable with low sun viewing, high aerosols and increasingly off-nadir viewing.