Using Lunar Observations to Validate Clouds and the Earth's Radiant Energy System Pointing Accuracy

To make measurements of the Earth's radiation budget, a pair of Clouds and the Earth's Radiant Energy System (CERES) instruments, i.e., Flight Models (FM) 1 and 2, have flown on the Terra spacecraft since December 1999, and a pair, i.e., FM-3 and FM-4, have flown on the Aqua spacecraft since June 2002. To produce accurate radiation fluxes at the top of the atmosphere and at various levels within the atmosphere and at the surface, CERES data are combined with higher resolution imager data. Validation is necessary to ensure that the accuracy with which the CERES footprints are located on the Earth will be adequate to use the imager data. The Moon provides a useful target for determining the pointing accuracy of the three channels of CERES. Near full moon, the CERES instruments can be turned to look at the Moon as the host spacecraft passes near the pole. The instrument scans the Moon in a raster-like pattern for a few minutes during the orbit when the Moon is in position. A technique has been developed by which these data can be used to compute accurately the direction in which the instrument is pointed in terms of azimuth and elevation angles when it views the Moon. The difference between this direction and the computed direction of the Moon is taken to be the pointing error of the instrument. The technique has been applied to each of the three channels of all four CERES instruments using lunar observation data from 2006 to present. The maximum error was found to be 0.05° in azimuth and 0.03° in elevation angle. This corresponds to an error in geolocation of 0.37 km near nadir. These results agree with those from the coastline detection method within one standard deviation for all but one case, where the difference was one-and-a-half standard deviations. The lunar and coastline techniques supplement each other for computing pixel location errors away from nadir. The alignment of the three channels in each instrument is evaluated as the differences of- azimuth and elevation angles of the shortwave and window channels from those of the total channel. The alignment was within 0.1° for all cases and within 0.02° for most cases.