MODELING BIDIRECTIONAL RADIANCE MEASUREMENTS COLLECTED BY THE ADVANCED SOLID-STATE ARRAY SPECTRORADIOMETER (ASAS) OVER OREGON TRANSECT CONIFER FORESTS

A geometric-optical model of the bidirectional reflectance of a forest canopy, developed by Li and Strahler, fits observed directional radia nce measurements with good accuracy. This model treats the forest cove r as a scene of discrete, three-dimensional objects (trees) that are i lluminated and viewed from different positions in the hemisphere. The shapes of the objects, their count densities and patterns of placement , are the driving variables, and they condition the mixture Of sunlit and shaded objects and background that are observed from a particular viewing direction, given a direction of illumination. This mixture, in turn, controls the brightness apparent to an observer or a radiometri c instrument. The Advanced Solid-State Array Spectroradiometer (ASAS) was used to validate this model. This aircraft sensor presently acquir es images in 29 spectral bands in the range (465-871 nm) and is pointa ble fore-and-aft, allowing directional measurements of radiance as a t arget is approached and imaged at view angles ranging +/- 45-degrees f rom nadir. Through atmospheric correction, ASAS radiances were reduced to bidirectional reflectance factors (BRFs). These were compared to c orresponding BRF values computed from the Li-Strahler model using, whe rever possible, ground measured component BRFs for calibration. The co mparisons showed a good match between the modeled and measured reflect ance factors for four of the five Oregon Transect Sites. Thus, the geo metric-optical approach provides a realistic model for the bidirection al reflectance distribution function of such natural vegetation canopi es. Further modifications are suggested to improve the predicted BRFs and yield still better results.