Radiometric calibration of GOES-7 VISSR solar channels during the GOES pathfinder benchmark period

The GOES-7 VISSR solar channels are radiometrically calibrated for the period from June 1987 through November 1988. Space, White Sands, and the Sonora Desert are used as calibration targets. Three different calibrations were performed: 1) using the stretched data (i.e., retransmitted data operationally destriped using NOAAʹs normalization procedure) and considering individual VISSR detectors separately; 2) using the stretched data averaged over the eight VISSR detectors; and 3) using the stretched data further destriped according to Simpson et al. (1995) and then averaged over the eight VISSR detectors. The third approach provides the best results (i.e., best equalization of the detectors). Because of uncertainties in the modeling of the VISSR radiance, using separate calibration coefficients for each detector (first approach) may not reduce the striping significantly. The calibration coefficients exhibit low frequency changes that are not correlated with seasonal variations in the extraterrestrial solar irradiance. High frequency fluctuations are large, and they are due in part to NOAAʹs normalization procedure and to calibration uncertainties. Differences between the calibration coefficients of individual detectors also are large, as well as changes between consecutive VISSR acquisition times. Comparison of untampered (retransmitted data but not operationally destriped using NOAAʹs normalization procedure) “Wednesday” 1848 GMT data with stretched 1831 and 1901 GMT data indicates that NOAAʹs normalization procedure, though imperfect, reduces substantially the stripes present in the unstretched data. There is evidence, however, that NOAAʹs normalization procedure introduced artificial variations in the count squared of the targets that in one instance resulted in 30% lower calibration coefficients. The average calibration coefficients are generally higher by 15% than the values of Rossow et al. (1992); they are in better agreement with the values of Abel et al. (1992) and Whitlock et al. (1994). It is recommended that the radiometric calibration of the VISSR solar channels should be performed as frequently as possible (every day). Though the stripes in the VISSR data transmitted to Wallops Island should be reduced for operational, real-time purposes (e.g., weather analysis and forecast), the original unnormalized data should be archived for subsequent use in quantitative, scientific applications.