Abstract
The goal of the Global Space-Based Inter-Calibration System (GSICS) and the future Climate Absolute Radiance and Refractivity Observatory (CLARREO) missions are to provide consistent calibration coefficients across satellite platforms in order to provide reliable, climate-quality retrievals. A calibration transfer that ties hyper-spectral radiances to an absolute, or traceable, calibration reference is preferred. Hyper-spectral data can be used to easily account for the varying spectral bands of operational sensors. Currently, GSICS uses the well-calibrated Infrared Atmospheric Sounding Interferometer (IASA) hyper-spectral radiances to calibrate geostationary satellite (GEO) IR radiances. To date, no such hyper-spectral instrument has been used as a direct GEO visible calibration reference. The ENVISAT Scanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) hyper-spectral data have the potential to function as a transfer medium for the absolute calibration reference of the MODerate-resolution Imaging Spectroradiometer (MODIS) and GEO imagers. Additionally, SCIAMACHY can be used to adjust for imager spectral response function (SRF) differences. The SCIAMACHY absolute calibration and stability are evaluated by cross-calibrating SCIAMACHY with Aqua-MODIS at the ground track intersects near the poles, thereby serving as a calibration transfer standard. Preliminary results indicate excellent calibration transfer given that direct comparisons of Aqua-MODIS and SCIAMACHY-convolved-with-Aqua-MODIS-SRF radiances have a monthly temporal uncertainty of 0.44%, and a relative trend of -0.2% per decade. The number of ray-matched, or bore-sighted, SCIAMACHY and GEO radiance pairs is limited by the size of the SCIAMACHY footprint, the number of footprints along the cross-track, and the nadir/limb view duty cycle. Relaxing the ray-matching criteria allows sufficient sampling for seasonal inter-calibration without detriment to the calibration transfer. This calibration approach reveals that the SCIAMACHY-to-Meteosat-9 0.65-µm calibration transfer is within 1% of other Meteosat-9 calibration techniques. Furthermore, this method yields one of the lowest temporal uncertainties of all approaches, most likely because no SRF adjustment is required.
Using Hyper-Spectral SCIAMACHY Radiances to Uniformly Calibrate Contemporary Geostationary Visible Sensors
The goal of the Global Space-Based Inter-Calibration System (GSICS) and the future Climate Absolute Radiance and Refractivity Observatory (CLARREO) missions are to provide consistent calibration coefficients across satellite platforms in order to provide reliable, climate-quality retrievals. A calibration transfer that ties hyper-spectral radiances to an absolute, or traceable, calibration reference is preferred. Hyper-spectral data can be used to easily account for the varying spectral bands of operational sensors. Currently, GSICS uses the well-calibrated Infrared Atmospheric Sounding Interferometer (IASA) hyper-spectral radiances to calibrate geostationary satellite (GEO) IR radiances. To date, no such hyper-spectral instrument has been used as a direct GEO visible calibration reference. The ENVISAT Scanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) hyper-spectral data have the potential to function as a transfer medium for the absolute calibration reference of the MODerate-resolution Imaging Spectroradiometer (MODIS) and GEO imagers. Additionally, SCIAMACHY can be used to adjust for imager spectral response function (SRF) differences. The SCIAMACHY absolute calibration and stability are evaluated by cross-calibrating SCIAMACHY with Aqua-MODIS at the ground track intersects near the poles, thereby serving as a calibration transfer standard. Preliminary results indicate excellent calibration transfer given that direct comparisons of Aqua-MODIS and SCIAMACHY-convolved-with-Aqua-MODIS-SRF radiances have a monthly temporal uncertainty of 0.44%, and a relative trend of -0.2% per decade. The number of ray-matched, or bore-sighted, SCIAMACHY and GEO radiance pairs is limited by the size of the SCIAMACHY footprint, the number of footprints along the cross-track, and the nadir/limb view duty cycle. Relaxing the ray-matching criteria allows sufficient sampling for seasonal inter-calibration without detriment to the calibration transfer. This calibration approach reveals that the SCIAMACHY-to-Meteosat-9 0.65-µm calibration transfer is within 1% of other Meteosat-9 calibration techniques. Furthermore, this method yields one of the lowest temporal uncertainties of all approaches, most likely because no SRF adjustment is required.