On-orbit solar calibrations using the Clouds and Earth's Radiant Energy System (CERES) in-flight calibration system

The Clouds and Earth's Radiant Energy System (CERES) scanning thermistor bolometers measure earth-reflected solar and earth-emitted longwaveradiances, at the topof-the-atmosphere. The bolometers measure the earthradiances in the broadband shortwave solar (0.3-5.0 microns) and total (0.3->100 microns) spectral bands as well as in the 8->12 microns water vapor window spectral band over geographical footprints as small as 10 kilometers at nadir. December 1999, the second and third set of CERES bolometers was launchedon the Earth Observing Mission Terra Spacecraft. May 2003, the fourth and fifth set of bolometers was launched on the Earth Observing Mission Aqua 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 shortwave part of total sensors are calibrated using the solar radiances reflected from the MAM's. 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 SIOx. Thermistors are located in each MAM plate and the total channel baffle. The CERES MAM is designed to yield calibration precisions approaching .5 percent for the total and shortwave detectors. However, in their first year of operation the Terra and Aqua MAMs showed shifts in their calibrations larger than expected. Shifts of this nature have been seen in other Solar viewing instruments in the past. A possible explanation has attributed the changes to pre-orbit or on-orbit contamination combined with solar ultraviolet/atomic oxygen induced chemical changes to the contaminant during solar exposure. In the subsequent year of operation all instruments begin to stabilize within the .5 percent precision range. In this presentation, the MAM solar calibration procedures will be presented along with on-orbit measurements for the nine years the CERES instruments have been on-orbit. A switch to an azimuth rotation raster scan of the Sun rather than an elevation scan will be discussed. The implementation of a thermal correction to the shortwave channel will also be discussed. Comparisons are also made between the Terra CERES instruments and the Aqua instruments during their MAM solar calibrations and total solar irradiance experimental results to determine how precise the CERES solar calibration facilities are at tracking the sun's irradiance.