Pre-launch sensor characterization of the CERES Flight Model 5 (FM5) instrument on NPP mission

Clouds and the Earth's Radiant Energy System (CERES) instrument was designed to measure broadband radiances in reflected shortwave and emitted outgoing longwave energy. The 3-sensor CERES instrument measure radiances in 0.3 to 5.0 micron region with Shortwave sensor, 0.3 to >100 microns with Total sensor and 8 to 12 micron region with Window sensor. Flight Model 5 (FM5), the sixth of the CERES instruments is scheduled to launch aboard the NPP spacecraft on October 2011. An accurate determination of the radiometric gains and spectral responsivity of CERES FM5 sensors was accomplished through rigorous calibrations at Northrop Grumman Aerospace Systems' (NGAS) Radiometric Calibration Facility (RCF). The longwave calibration of the total and window sensors are achieved using the Narrow Field-of-View Blackbody (NFBB) source which is tied to International Scale of 1990 (ITS '90). A Shortwave Reference Source (SWRS) along with the Transfer Active Cavity radiometer (TACR) which acts as the transfer standard of NFBB source, is used to determine the radiometric responsivity and spectral response estimates of the SW sensor and shortwave portion of the Total sensor. The spectral responsivity in longwave region is determined using a Fourier Transform Spectrometer (FTS) system. CERES instrument also perform calibrations using on-board sources during pre-launch testing which serve as a traceability standard to carry the ground determined sensor radiometric gains to orbit. This paper covers the calibration philosophy and the results from ground calibration testing of FM5 sensors conducted in 2008. The sensor radiometric gain responses calculated using primary sources and performance of the sensors using on-board sources will be discussed.