Field evaluation of a diffuse to global irradiance meter for vicarious calibration

Vicarious calibration methods have been developed to calibrate radiometric sensors in-flight. One such method, the irradiance-based method, requires the measurement of the diffuse-to-global (diffuse-to-total) irradiance ratio. A diffuse-to-global irradiance meter has recently been developed by the Remote Sensing Group at the University of Arizona. The instrument uses a baffled integrating sphere as its collector and an occulting disc to block the sun for diffuse-irradiance measurements. Data are collected at 10-nm intervals from 350 to 1100 nm. In this paper, two methods are used to evaluate this instrument. The first relies on the well-known Langley method to determine atmospheric optical thickness from measurements of the direct solar irradiance derived from the difference between the global and diffuse irradiances. Secondly, the diffuse-to-global ratio was used to predict the radiance at the top of the atmosphere. These radiance results are compared to those from the reflectance-based method as well as those derived from data collected by the AVIRIS sensor. Data were collected at Lunar Lake Nevada June 23,24,25 1997 and White Sands Missile Range on October 31 and November 1, 1997. Comparison of optical thickness obtained from data collected by a well-understood solar radiometer show differences in optical thickness ranging from 0.003 minimum to 0.018 maximum. These results are encouraging as they indicate that there are no major effects due to inhomogeneities in the spherical collector.