Exploitation of MODTRAN 4 capabilities to predict at-sensor radiance

Top-of-atmosphere radiance is computed between 350 and 2500 nm for atmospheres containing one of three aerosol models (rural, maritime and dust) inherent to MODTRAN, over different surface reflectance values, and compared with those computed using a model of the same aerosol species derived from measurements by a global network of ground-based radiometers (AERONET). It is observed that even over high reflectance targets (R=0.5), care must be taken in the prescription of aerosol optical properties so as to limit uncertainty resulting from aerosols in the top-of-atmosphere radiance to less than 2%. It is found that, for grass and desert sites, use of a simple power law size distribution to compute aerosol optical properties reduces uncertainty in the computed satellite radiance to less than 1.5%. Uncertainty in the computed top-of-atmosphere radiance during vicarious sensor calibration over desert sites that may result from this simple prescription of the aerosol size distribution is thus less than uncertainty in the TOA radiance resulting from measurements of the site reflectance. The new aerosol and multiple scattering capabilities of the most recent version of MODTRAN have made such studies possible and are promising for attempts to use MODTRAN in the vicarious calibration of airborne and spaceborne sensors.