CERES instrument consists of three sensor units. The first one is a Shortwave (SW) sensor that measures reflected solar energy in the 0.3 to 5.0 micron region. The second sensor called the Total measures both reflected and emitted thermal radiances covering 0.3 to >100 microns and the third one is a narrow band thermal window sensor making measurements in the 8 to 12 micron wavelength region. The pre-launch accuracy goal for the CERES instrument measurements is to have the emitted longwave radiances within 0.5% and the shortwave radiances within 1.0%.
The radiometric gains and the spectral responses of the sensors were determined from pre-launch sensor calibrations performed using the primary sources in the Radiometric Calibration Facility (RCF) at the Northrop Grumman Aerospace Systems in Redondo Beach, CA . In addition to the calibration with the RCF sources, the CERES instrument also performed calibrations using on-board sources during the pre-launch vacuum testing. The on-board calibration system known as the Internal Calibration Module (ICM) consists of two blackbody sources to calibrate the Total and window sensors and a tungsten lamp for the SW sensor calibration. The ICM calibrations serve as a traceability standard to carry the ground determined sensor radiometric gains to orbit.
In addition to the ICM sources, CERES also carries a second on-board calibrator caller Mirror Attenuator Mosaic (MAM). MAM is a solar diffusor plate which guide incoming solar radiances into the fields of view of shortwave and total sensor units. MAM is instrumental in determining any response changes to the shorter wavelength regions of shortwave sensor and the shortwave portion of the total sensor.
Several validation studies utilizing the intercomparison of three CERES sensors' measurements viewing various targets were used to evaluate the performance of the sensors over time. Steady targets such as tropical ocean and deep convective clouds are used in these evaluations. The Tropical Mean (TM) value is the monthly average of all nadir longwave radiance measurement over the tropical ocean in the 400 latitude zone centered at the equator. The longwave radiance at nighttime is primarily derived from the total sensor measurement. Thus the TM value at night provide an excellent tool to montor any changes in the total sensor. The day time LW radiance is derived from the measured values of the total and the shortwave sensors. The day night difference in the TM longwave value in comparison with the day-night difference of longwave radiance from the trained window sensor for the same region highlight any changes that occur in the shortwave sensor or shortwave portion of the total sensor of the instrument. Similar comparison study using nadir measurements viewing Deep Convective Couds (DCC) also provide insights to variations that may occur either in the shortwave sensor and shortwave portion of total sensor.
This paper covers the on-orbit calibration and validation studies performed for the CERES FM5 instrument. The sensors' stability based on in-flight calibrations are presented. The results from the validation studies to determine any changes occurring on CERES sensors will also be discussed.
 Wielicki, B. A., B. R. Barkstrom, E. F. Harrison, R. B. Lee III, G. L. Smith and J. E. Cooper: Clouds and the Earth's Radiant Energy System (CERES): An Earth Observing System Experiment, Bull. Amer. Met. Soc., 77, 853-868, 1996.
 Lee III, R. B., Barkstrom, B. R., Bitting, H. C., Crommelynck, D. H., Paden, J., Pandey, D. K., Priestley, K. J., Smith, G. L., Thomas, S., Thornhill, K. L., and Wilson, R. S., Prelaunch calibrations of the Clouds and the Earth's Radiant Energy System (CERES) Tropical Rainfall Measuring Mission and Earth Observing System Morning (EOS-AM1) spacecraft thermistor bolometer sensors, IEEE Transactions on Geoscience and Remote Sensing, Vol 36, No. 4, 1173 - 1185, 1998