Fine tuning of the SVC method for airborne hyperspectral sensors: the BRDF correction of the calibration nets targets

摘要

AbstractThis paper presents measurements of the azimuth and zenith angle-dependence of the targets that are used for supervised vicarious calibration (SVC) methodology of hyperspectral sensors. Surface radiance measure at-sensor from airborne instruments, such as the AisaDUAL and HyMap sensors, show an apparent noise and shift that limit the information content and even impair the collected data (after removing the atmospheric attenuation). The drift from actual radiance is contributed by various sources; e.g., atmospheric gases and aerosols, and sensor stability. Yet one of the major contributors to it during nadir scanning is the directional effect of target reflectance at variable illumination geometry, also known as Bidirectional Reflectance Distribution Function (BRDF). The SVC method, which is based on ground dark nets of varying densities, was able to correct for the above factors and recalibrate the sensor's radiance during data acquisition. The main assumption of the SVC method is that the net targets are a Lambertian surface. In this paper, the BRDF measurements of the SVC net targets were carried out using a goniometer and controlled laboratory conditions. These measurements revealed anisotropic properties of the nets targets, which were further introduced to the radiometric calibration process are significant, especially with the high density nets (>50% black net cover). The paper presents a look-up-table of the BRDF correction coefficient factors for each net target (identified and separated by its density and mixing level once placed over a bright background) at various zenith (from ?10° to ?30°) and azimuth (from 0° to 360° at intervals of 10°) angles. The