Comparison of modeled and measured downwelling, diffuse irradiance at a ground-reference calibration test site

Vicarious calibration is a technique that provides an independent, absolute calibration of a sensor. The irradiance-based method, one of the three used by the Remote Sensing Group of the Optical Sciences Center at the University of Arizona, makes use of measurements of downwelling diffuse and global sky irradiance to predict the radiance at the sensor being calibrated. The advantage of this technique over other methods used by the Remote Sensing Group, such as the reflectance-based approach, is that it does not rely on assumptions about the aerosol size distribution and composition. In the reflectance-based method, these aerosol parameters are typically derived from solar extinction measurements and used as input into a Gauss-Seidel radiative transfer code to predict the at-sensor radiances. Differences in the reflectance- and irradiance-based approaches imply that the aerosol parameters used in the reflectance-based technique could be in error. In this work, we present the results of comparisons between measured diffuse and global irradiances to those from radiative transfer code results using aerosol parameters derived from solar extinction data at a typical test site used by the Remote Sensing Group. These comparisons are performed for the 400 nm to 1000 nm spectral range. We also show the results of a sensitivity study of these modeled irradiances to changes in aerosol index of refraction and size distribution to assess possible errors in the assumptions used in the reflectance-based method