Calibration of NOAA-7 AVHRR,GOES-5, and GOES-6 VISSR/VAS solar channels

The NOAA-7, GOES-5, and GOES-6 VISSR/VAS solar channels have been calibrated for the periods from October 1983 through January 1985 (NOAA-7, GOES-6) and from October 1983 through July 1984 (GOES-5). Space and the White Sands National Monument area in Mexico, whose reflectance properties are well known, are used as calibration targets. The shortwave reflected terrestrial radiance that is measured at satellite altitude is computed using a fairly accurate radiative transfer model which accounts for multiple scattering and bidirectional effects (Tanré et al., 1979). The relevant atmospheric characteristics are estimated from climatological data (ozone amount, aerosol size-frequency distribution, and refractive index) and observations at the nearest meteorological sites (water vapor amount, visibility). The approach produces accuracies of 8–13% depending on the channel considered. For both types of instruments, no drift in the solar channels in detected during the 15-month period. The gain changes, about 15% of the mean values, are largely attributed to inhomogeneities of the ground target (shading effects due to the presence of dunes). No systematic effect of the normalization procedure applied by NOAA to the raw VISSR/VAS data is detected. There is some evidence that the GOES-5 solar channels gradually deteriorated from March 1984 until the satellite failure in July 1984. Comparisons between gains determined in orbit and those before launch show that the NOAA-7 solar channels read higher by about 15%. The disparities, however, cannot be explained by model errors and must have occurred before the time period analyzed here.