Optimal directional view angles for remote-sensing missions

Directional reflectance measurements spanning the entire exitance hemisphere for a number of vegetation and bare soil sites were measured from the ground in NOAA-7 AVHRR bands 1 (0-58-0-68 μm) and 2 (0-73-1-1 μm) for various solar zenith angles. The normalized difference and ratio transformations of these bands were calculated. The effects of the atmosphere on the directional radiance above the atmosphere was explored by applying the average mid-latitude atmospheric model of Dave to a series of ground measurements. These data and simulations were analysed for the optimal directional view angles with respect to two strategies. The first strategy views the utility of off-nadir measurements as extending spatial and temporal coverage while the second strategy views the utility of off-nadir measurements as increasing information content about the physical characteristics of the target(s). The optimum view angles for the first strategy are view angles which minimize the change in sensor response of target(s) relative to the sensor response to the nadir direction. The optimum view angles for the second strategy are view angles which, relative to a nadir view, provide superior and/or additional information about the physical characteristics of the target(s). For the first strategy the results indicate that, from the ground level, the optimum view angles are in the azimuth direction perpendicular to the principal plane of the Sun for all targets, bands and Sun angles. The differences in reflectance between the nadir and off-nadir view angles in this azimuth plane decreased significantly with decreasing solar zenith angle for both bands and all targets. The normalized difference transformation of bands 1 and 2 is significantly less sensitive to variations of view angles than the individual bands for all Sun angles and targets. The results for the second strategy from the ground level showed that off-nadir view angles with an aximuth in the principal plane of the Sun were optimum for all targets and both bands. Finally, the results of the atmospheric simulation study indicated that a mid-latitude summer atmosphere does not significantly change the optimum directional view angles for either the first strategy or the second strategy. However, the atmosphere increased off-nadir sensor responses relative to the ground-level responses particularly in the forwardscattering direction. The normalized difference transformation of bands 1 and 2 was a good transformation for the first strategy for off-nadir viewing angles of less than 45°. The study provides valuable information for interpreting present remotely sensed off-nadir data and in designing future systems with off-nadir capabilities.