A test system and set of procedures have been developed to fully test and calibrate the Atmospheric Infrared Sounder (AIRS), a facility instrument on NASA's EOS PM platform. The system has been used to test and calibrate, under simulated space conditions, the spatial, spectral, radiometric and polarization characteristics of AIRS for each of its 2378 spectral bands covering the IR range from 3.7 and 15.4 micrometer. Unique challenges included spectral line shape and out of band response characterizations over 3 decades of the response function, accurate radiometric response calibration for IR source brightness temperatures from 195 to 357 K, spectral channel center and width determinations to 3 ppm of wavelength, spatial co-registration determination of the 2378 bands to 0.002 degrees and polarization response determination over all bands. Measured sensitivity, spectral response and polarization response for the AIRS instrument met or bettered requirements. The test systems developed to meet these objectives are described and the procedures and summary results of testing are presented.
The Atmospheric Infrared Sounder (AIRS) has been developed for the NASA Earth Observing System (EOS) program with a scheduled launch on the first post meridian (PM-1) platform in December 2000. AIRS is designed to provide both new and more accurate data about the atmosphere, land and oceans for application to climate studies and weather predictions. Among the important parameters to be derived from AIRS observations are atmospheric temperature profiles with an average accuracy of 1 K in 1 kilometer (km) layers in the troposphere and surface temperatures with an average accuracy of 0.5 K. The AIRS measurement technique is based on passive infrared remote sensing using a precisely calibrated, high spectral resolution grating spectrometer providing high sensitivity operation over the 3.7 micrometer - 15.4 micrometer region. To meet the challenge of high performance over this broad wavelength range, the spectrometer is cooled to 155 K using a passive two-stage radiative cooler and the HgCdTe focal plane is cooled to 58 K using a state-of-the-art long life, low vibration Stirling/pulse tube cryocooler. Electronics waste heat is removed through a spacecraft provided heat rejection system based on heat pipe technology. All of these functions combine to make AIRS thermal management a key aspect of the overall instrument design. Additionally, the thermal operating constraints place challenging requirements on the test program in terms of proper simulation of the space environment and the logistic issues attendant with testing cryogenic instruments. The AIRS instrument has been fully integrated and thermal vacuum performance testing is underway. This paper provides an overview of the AIRS thermal system design, the test methodologies and the key results from the thermal vacuum tests, which have been completed at the time of this publication.
The Atmospheric Infrared Sounder (AIRS) is a high spectral resolution multispectral sounder providing nearly contiguous coverage between 3.74 and 15.4 micrometers which has been selected as a facility instrument on the Earth Observing System PM series. AIRS is designed to provide data for application in climate studies and weather prediction. The degradation of AIRS by contamination could reduce its performance capabilities, reducing the instrument's expected utility by compromising its accuracy and sensitivity. AIRS contains a number of contamination sensitive subsystems which include an exposed scan mirror, passively cooled optics, mechanically cooled detectors, on board calibrators, and thermal control surfaces. Efforts were undertaken to define the contamination sensitivities of these subsystems based upon system performance goals. A series of analyses have been performed to determine the cleanliness necessary to meet the system performance goals. From this data, a contamination control program and preliminary design guidelines have been implemented. Presented in this paper are an overview of the instrument and its contamination susceptibility, the contamination performance goals for each subsystem area, and the derived cleanliness requirements. The analysis techniques used to derive the subsystem cleanliness requirements from the optical, thermal, and calibration performance goals are included. Also included are the preliminary design concepts for contamination control and contingency decontamination features built into the AIRS design to help assure the contamination requirements are met.
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