Remote Sensing of Angular Characteristics of Canopy Reflectances

The influence of the atmosphere on the remotely sensed angular distribution of canopy reflectance is studied by radiative transfer calculations with both a coupled atmosphere-canopy model and a pure atmosphere model with the canopy replaced by an equivalent bidirectional reflectance distribution function (BRDF). The canopy model, although one-dimensional, is able to reproduce typical canopy features like the "bowl shape" and the canopy "hot spot." In the decoupled mode, it can be used to compute canopy-equivalent BRDF's. The atmospheric perturbation of the angular reflectance pattern of a Lambertian, a mixed Lambertian/specular BRDF, and of the measured BRDF's of savannah and coniferous forest canopies is studied using one aerosol-free and two polluted atmospheres with surface visual ranges of V0 = 23 km and V0 = 5 km. It is shown that for surface albedoes > 10%, local extremes in the angular distribution of the surface reflectance and dependencies on the view azimuth angle are still detectable above the atmosphere and are nearly invariant to atmospheric perturbations. This result leads to the recommendation that off-nadir satellite observations in the near-infrared may contribute additional valuable information to crop identification. However, canopy reflectance variations with varying view zenith angles are dominated by atmospheric perturbations even for relatively clear atmospheres.