KIMES, DS, DEERING, DW (1992). REMOTE-SENSING OF SURFACE HEMISPHERICAL REFLECTANCE (ALBEDO) USING POINTABLE MULTISPECTRAL IMAGING SPECTRORADIOMETERS. REMOTE SENSING OF ENVIRONMENT, 39(2), 85-94.
Several techniques that use multiple off-nadir view angles taken in a single azimuth plane (called a string of data) to estimate hemispherical reflectance have been developed. The range of view angles that is required in order to obtain accurate inferences of hemispherical reflectance using string techniques was studied as applicable to the Moderate Resolution Imaging Spectroradiometer (MODIS) and the High Resolution Imaging Spectroradiometer (HIRIS). Full and half strings out to 60-degrees, 45-degrees, 30-degrees, and 15-degrees were tested on ground data of a wide range of cover types and sun angles. Both a visible and near infrared band (AVHRR Bands 1 and 2) were tested using a knowledge-based system called VEG. Using string techniques that calculate a single coefficient from an internal spectral data base, the results showed small errors for the full string +/- 60-degrees (less than 4% error), HIRIS like data (less than 5% error), and MODIS like data (less than 7% error). Much larger errors occur in those cases where long extrapolations must be calculated (e.g., strings to only 15-degrees and 30-degrees). Systems that view fore and aft out to 60-degrees would be desirable to minimize errors, assuming atmospheric corrections could be made at these angles. HIRIS (views 60-degrees in forward and 30-degrees in aft directions) is a good compromise by viewing out to 60-degrees in only one direction and increases the error by approximately 2% or less. MODIS (full string +/- 45-degrees) still has a relatively small error of approximately 5% or less. Sensor systems capable of only viewing in the fore or aft directions (half strings) as opposed to both directions (full strings) greatly increase the error.