PRELAUNCH CALIBRATIONS OF THE CLOUDS AND THE EARTHS RADIANT ENERGY SYSTEM (CERES) TROPICAL RAINFALL MEASURING MISSION AND EARTH OBSERVING SYSTEM MORNING (EOS-AM1) SPACECRAFT THERMISTOR BOLOMETER SENSORS

The Clouds and the Earth's Radiant Energy System (CERES) spacecraft sc anning thermistor bolometer sensors measure earth radiances in the bro adband shortwave solar (0.3-5.0 mu m) and total (0.3-->100 mu m) spect ral bands as well as in the 8-12-mu m water vapor window spectral band . On November 27, 1997, the launch of the Tropical Rainfall Measuring Mission (TRMM) spacecraft placed the first set of CERES sensors into o rbit, and 30 days later, the sensors initiated operational measurement s of the earth radiance fields, In 1998, the Earth Observing System mo rning (EOS-AM1) spacecraft will place the second and third sensor sets into orbit. The prelaunch CERES sensors' count conversion coefficient s (gains and zero-radiance offsets) were determined in vacuum ground f acilities. The gains were tied radiometrically to the International Te mperature Scale of 1990 (ITS-90), The gain determinations included the spectral properties (reflectance, transmittance, emittance, etc.) of both the sources and sensors as well as the in-field-of-view (FOV) and out-of-FOV sensor responses. The resulting prelaunch coefficients for the TRMM and EOS-AM1 sensors are presented. Inflight calibration syst ems and on-orbit calibration approaches are described, which are being used to determine the temporal stabilities of the sensors' gains and offsets from prelaunch calibrations through on-orbit measurements. Ana lyses of the TRMM prelaunch and on-orbit calibration results indicate that the sensors have retained their ties to ITS-90 at accuracy levels better than +/-0.3% between the 1995 prelaunch and 1997 on-orbit cali brations.