Improved stratospheric and mesospheric sounder validation: General approach and in‐flight radiometric calibration

Abstract

This paper introduces a series of papers describing the validation of data products from the improved stratospheric ,•t mesospheric sounder (ISAMS) on the Upper Atmosphere Research Satellite. ISAMS is a limb-sounding infrared gascorrelation radiometer, measuring thermal emission from a range of constituents. The constituents measured are ozone, water vapor, methane, nitrous oxide, nitric oxide, nitrogen dioxide, nitrogen pentoxide, nitric acid, carbon monoxide, and aerosol. Atmospheric temperature and composition data were obtained for approximately 180 days between September 26, 1991, and July 29, 1992, with nearglobal coverage. The instrument and the retrieval process are briefly described, together with aspects of the validation process relevant to all data products, including the radiometric calibration and the analysis of the calibrated radiances to produce data on a standard time-altitude grid. stratospheric aerosol in stratospheric chemistry. ISAMS suffered several significant interruptions to its operation but obtained about 180 days of data from which temperature and constituents could be retrieved, between processing and the validation approach and provide validation information about aspects common to all the data products, namely, radiometric calibration and the first stage of the retrieval process, i.e., the transformation of the radiances from the complicated (and variable) scan pattern on which they were measured onto a standard grid used by the constituent retrieval. The remaining papers will describe the validation of individual data products and provide information essential for the proper scientific use of the data. Instrument and Data Analysis The ISAMS InstrumentThe improved stratospheric and mesospheric sounder was designed to make measurements with a vertical spacing of about 2.4 km, a horizontal spacing of about 200 km along the tangent track, and covering a latitude range from 80øN to 80øS. The quantity measured directly by the instrument is thermal emission from the gases involved, both the total radiance in certain spectral intervals and that part of the radiance selected by pressure modulation. The instrument and the measurement concepts are described by Taylor et al. [1993], to which the reader is referred for details not covered here. The Upper Atmosphere Research Satellite on which ISAMS is mounted is in an orbit inclined at 57 ø to the equator and which precesses about 5 ø per day relative to the Sun. For 1 day's data, local solar time around each orbit is an approximately constant function of latitude (differing by about 12 hours at the ascending and descending nodes); but over a period of 72 days, the 9775 9776 RODGERS ET AL' ISAMS VALIDATION AND IN-FLIGHT CALIBRATION orbit precesses through the whole diurnal cycle. Thus near-global coverage is achieved twice per day, on the ascending and descending parts of the orbit. Thermal stability and instrument safety require that one side of the satellite is never illuminated by the Sun, so to maintain this situation the satellite is yawe...

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