EFFECT OF VIEWING GEOMETRY ON REMOTELY SENSED NET RADIATION MEASUREMENTS FOR VEGETATED AND NON-VEGETATED SURFACES

Increased use of remotely sensed data for estimation of land-surface climatological variables such as the radiation and energy balance has underscored the need to understand errors inherent in remote sensing systems. The effects of viewing geometry on the signal received at the sensor have been examined for a variety of surface types, but systematic study of how sensor viewing geometry may introduce error into remote measurements of surface climate parameters is lacking. In this paper, close-range remote sensing is used to assess error caused by viewing geometry in remote measurements of net radiation, an important surface climate variable. Estimates of net radiation were made by inserting remotely sensed values of albedo and surface temperature into the surface radiation budget equation. Two contrasting surfaces, one vegetated and one bare, were evaluated at seven view angles and five azimuth angles. The remote net radiation measurements for each combination of view and azimuth angle then were compared to reference measurements to assess error. Results indicate that, under cloud-free conditions, view angle can affect measurement accuracy substantially, and that off-nadir view angles ranging from 15° to 30° in the forward-scatter direction are optimal. The potential impact of these findings on studies of the spatial variation of the surface radiation budget from spaceborne remote sensing platforms is discussed. Key words: land-surface climatology, remote sensing, radiation budget, net radiation.]