Effects of assumed solar spectral irradiance on intercomparisons of earth-observing sensors

Recent work by the Remote Sensing Group at the University of Arizona has focused on the calibration and radiance validation of numerous sensors that are part of NASA's Earth Observation System and Earth Sciences Enterprise. The unique orbital combination of many of these sensors, both from formation flying as well as from multi-sensor platforms, provides an unprecedented opportunity with which to cross-compare results from these sensors. This work presents the results of cross-comparisons between the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), the Moderate Resolution Imaging Spectroradiometer (MODIS), the Enhanced Thematic Mapper Plus (ETM+) sensor, the MODIS-ASTER Simulator (MASTER), and the Advanced Land Imager (ALI). Differences in the cross-comparison between these sensors can be attributed to numerous causes, including spatial registration effects, spectral differences in the sensors, and temporal changes in the atmosphere and surface between sensor collections. One aspect of cross-comparison studies that is often overlooked is the solar spectral irradiance that is used by each sensor's calibration and validation teams, and with the advent of onboard diffusers the selection of the solar irradiance values becomes even more critical. This work describes the differences between typical solar irradiance spectra that are being used for the above-listed sensors and the effect of these spectra on the intercomparison of at-sensor radiances from earth-observing sensors.