Development of a full-disk ratioing radiometer (FDRR) for calibration of the advanced baseline imager's visible and near IR reflective spectral bands

The Geostationary Operational Environmental Satellite-R (GOES-R), scheduled for launch in 2014, will be the first in a series of next generation weather satellites. It will be 3-axis stabilized in geostationary orbit, and will have an Advanced Baseline Imager (ABI) that can make full-Earth disk images, covering most of the Western Hemisphere, in spectral bands ranging from 0.47-13.3 mu m. We are now designing a Full-Disk Ratioing Radiometer (FDRR) to determine the ratio of the full-disk irradiance to the solar irradiance in spectral bands that match the four shortest wavelengths of the ABI's Visible and Near IR (VNIR) spectral bands. When hard-mounted to the nadir face of a GOES-R satellite, this FDRR can determine the full-disk albedo in each band, with the added benefit that the ABI's corresponding channels can be calibrated by comparison of these measurements to the Earth's irradiance measurements derived from simultaneous full disk images made by the ABI. The FDRR uses an integrating sphere with two baffled pinholes. One pinhole has a baffle that restricts its field-of-view (FOV) to a circle 20.1 degrees in diameter, centered at nadir, viewing the Earth's full disk continuously throughout its daily cycle. This baffle has a shutter that allows it to be closed for dark current measurements during the day and to prevent solar intrusion at night. The second pinhole, with a much smaller diameter, has a baffle that restricts its FOV to about 1 degrees in the East-West direction and +/-25 degrees in the North/South direction, allowing the direct solar irradiance to enter the sphere for a brief interval once each night. A radiation-hardened fiber optic light pipe couples the output of the sphere to filters and detectors in an electronics box. These filters and detectors have spectral bands matched to those in the ABI. This technique measures the ratio of the full-disk irradiance to the direct solar irradiance, determining and the fibers, and the sphere's reflectivity.