Pre-launch Characterization of Spectral Response Functions for the Clouds and Earth's Radiant Energy System (CERES) Instrument Sensors

The Clouds and Earth's Radiant Energy System (CERES) uses four instruments onboard two spacecraft to make measurements of the earth's reflected shortwave and emitted longwave radiation, which constitute two components of earth radiation budget. Flight Models 1 and 2 (FM1, FM2) are onboard the TERRA spacecraft and Flight Models 3 and 4 (FM3, FM4) are onboard the AQUA spacecraft. The measurements are made using three radiometric sensor channels on each instrument-shortwave channel (0.3-5 microns), window channel (8-12 microns) and total channel (0.3-200 microns). This paper describes the process of obtaining the estimates of the spectral response functions of the three sensor channels using pre-launch measurements. The shortwave spectral response function (0.3-2 microns) for the shortwave channel as well as the shortwave region of the total channel is obtained through measurements of the components in the optical path. The longwave responses (>2 microns) are obtained using a Fourier Transform Spectrometer (FTS) system. The CERES sensors as well as a reference detector, which is used to account for any measurement or background noise in the system, acquire measurements of the FTS. By the use of various sources and beam-splitters in the spectrometer, the entire spectral range of the broadband total channel is covered. The spectral estimates are obtained in smaller, overlapping bands, which are then tied together appropriately to obtain the end-to-end spectral response function for each of the sensors for all CERES instruments.