AN OFF-NADIR-POINTING IMAGING SPECTRORADIOMETER FOR TERRESTRIAL ECOSYSTEM STUDIES

The Advanced Solid-State Array Spectroradiometer (ASAS) is an airborne, off-nadir-pointing imaging spectroradiometer used to acquire bidirectional radiance data for terrestrial targets. As its platform aircraft flies over a target, the sensor can image the target through a sequence of at least seven fore-to-aft view directions ranging up to 45-degrees on either side of nadir. ASAS acquires data for 29 spectral bands in the visible and near-infrared portions of the spectrum (465 to 871 nm) with a resolution of 15 nm. The basic ASAS data product is a sequence of digital images acquired from multiple view directions and consisting of calibrated spectral radiance values. Examples of ASAS data acquired for recent field experiments are presented. The data demonstrate the combined effects of reflectance anisotropy and increased atmospheric path length on off-nadir observations. One result of these effects is a variation in vegetation indices as a function of view direction. Normalized-difference-vegetation-indices for prairie grass, coniferous forest, and deciduous forest canopies varied up to 14, 23, and 6%, respectively, relative to the nadir as a function of view zenith angle along the solar principal plane. These ASAS data are comparable to data anticipated from several sensors proposed for the Earth Observing System.