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The Clouds and the Earth's Radiant Energy System (CERES) spacecraft scanning thermistor bolometers measure earth- reflected solar and earth-emitted longwave radiances, at the top- of-the-atmosphere. The bolometers measure the earth radiances in the broadband shortwave solar (0.3 -5.0 µm) and total (0.3 - >100 pm) spectral bands as well as in the 8 -12 µm water vapor window spectral band over geographical footprints as small as 10 kilometers at nadir. In December 1999, the second and third sets of CERES bolometers were launched on the Earth Observing Mission Terra Spacecraft. Ground vacuum calibrations define the initial count conversion coefficients that are used to convert the bolometer output voltages into filtered earth radiances. The mirror attenuator mosaic (MAM), a solar diffuser plate, was built into the CERES instrument package calibration system in order to define in-orbit shifts or drifts in the sensor responses. The shortwave and total sensors are calibrated using the solar radiances reflected from the MAM. Each MAM consists of baffle-solar diffuser plate systems, which guide incoming solar radiances into the instrument fields of view of the shortwave and total wave sensor units. The MAM diffuser reflecting type surface consists of an array of spherical aluminum mirror segments, which are separated by a Merck Black A absorbing surface, overcoated with silicon dioxide. Thermistors are located in each MAM plate and baffle. The CERES MAM is designed to yield calibration precisions approaching 0.5 percent for the total and shortwave detectors. In this paper, the MAM solar calibration techniques are presented along with on-orbit measurements.
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