Efficacy of the CERES Internal Calibration System

The objective of the Clouds and Earth Radiant Energy System (CERES) scanning radiometer is to measure solar radiation reflected from the Earth, outgoing longwave radiation (OLR) from the Earth and radiation from the 10 to 12 micron longwave window. It has a shortwave channel, a total channel and a window channel. A Shortwave Incandescent Calibration Source (SWICS) is used to calibrate the Shortwave channel and Internal Black Bodies are used for the total and window channels. These three devises make up the Internal Calibration System (ICS) for the instrument. There is no on-board method for checking the shortwave response of the total channel. This response must be deduced by use of vicarious targets such as deep convective clouds. The CERES Flight Models 1 and 2 have been operating in orbit on the Terra spacecraft since 2000 and FM-3 and -4 since 2002 on Aqua. FM-5 flew in 2011 on Suomi-NPP. The last CERES instrument, FM-6, went into orbit in 2017 on the NOAA-20 satellite. There are now adequate data from the in-orbit calibration and validation work to evaluate the efficacy of the Internal Calibration System. After the calibrations are changed and following a period of operation, validation studies are used to produce revised calibrations. Validation methods include comparing measurements among the instruments as they observe the same scenes from the same directions within a short time. Another method is to compute the Tropical Mean, which is the average OLR between 30 degrees N and 30 degrees S. The Tropical Mean has been noted to be constant over the data period. The difference between the revised calibrations and the calibrations indicated by the ICS is a measure of the efficacy of the ICS. The ICS results and the calibrations from the broader study are compared herein. These differences are of the order of 1/4%.