| Abstract | Techniques for geometrically and radiometrically correcting NOAA-AVHRR imagery from crop condition assessment have been developed over several years a part of the Crop Monitoring program at the Canada
Centre for Remote Sensing, Brown et al., 1982, "Radiometric and Processing of Remotely Sensed Data, West Lafayette, In.). These corrections result in a standardized image product and are important for reliable multi-temporal analysis. The
radiometric correction involves the use of standard targets at or near nadir to estimate path radiance and atmospheric extinction effects. This is followed by radiometric profiling across the scene to empirically model and correct for increasing
path radiance at off nadir scan angles. The primary reference targets used to date have been water bodies and clouds. Deep, clear water bodies serve as excellent low reflectance targets in both AVHRR channel 1 (0.58 - 0.68 µm) and channel 2 (0.725
- 1.10 µm). Clouds, on the other hand, may have an extremely wide range of reflectances causing this identification of a 'standard cloud' to be very subjective. This paper describes the investigation of a number of secondary radiometric targets that
may serve to supplement or replace those currently in use.
The study focussed on the potential use of forest, irrigated areas, and selected agricultural sites as secondary targets. Each was investigated for its radiometric stability over the
growing season using AVHRR data of Western Canada acquired in 1985. Of the three targets, coniferous forest was found to be the most stable and provided a high reflectance target in AVHRR channel 2. In AVHRR channel 1, however, the same target is
among the darkest of surface cover types and consequently not suitable as a high end reference. A variety of approaches to overcome this problem are discussed. The paper also describes how the targets investigated may be used for quality checking
the radiometric properties of the corrected output products. |