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

The Clouds and the Earth´s Radiant Energy System (CERES) spacecraft scanning thermistor bolometer sensors measure Earth radiances in the broadband shortwave solar (0.3-5.0 μm) and total (0.3->100 μm) spectral bands as well as in the 8-12-μm 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 orbit, and 30 days later, the sensors initiated operational measurements of the Earth radiance fields. In 1998, the Earth Observing System morning (EOS-AM1) spacecraft will place the second and third sensor sets into orbit. The prelaunch CERES sensors´ count conversion coefficients (gains and zero-radiance offsets) were determined in vacuum ground facilities. The gains were tied radiometrically to the International Temperature 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 systems 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. Analyses 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 calibrations