Accuracy of ground-reference calibration of imaging spectroradiometers at large sensor view angles

The Remote Sensing Group at the University of Arizona has been successfully using vicarious calibration techniques since the mid-1980s to calibrate both airborne and satellite-based imaging spectroradiometers using vicarious techniques. These approaches use ground-based measurements of atmospheric and surface properties of a selected test site as input to a radiative transfer code to predict at-sensor radiances at 1-nm intervals from 350-2500 nm for a given sensor overpass. Past work has focused on sensors with view angles less than 30 degrees from nadir but recently-developed sensors use much larger view angles and these sensors will still benefit from vicarious calibrations. However, calibrations at such angles require more accurate atmospheric and surface characterizations. This paper examines the sensitivity of vicarious calibrations at large view angles to uncertainties in the atmospheric characterization and surface bi-directional reflectance. The results show that the inclusion of surface BRDF effects are critical to ensuring accurate results. Furthermore, the uncertainty in the vicarious calibration of a large view angle sensor will be of the same level as or less than that of the near-nadir case when aerosol optical thickness is less than 0.10, the aerosols have low imaginary index, and the solar zenith angle is less than 50 degrees. From the results of this study it is found that currently-used test sites are adequate for use in the vicarious calibration of large view-angle sensors and should give reflectance-based results with uncertainties less than 5%.