The Improved Stratospheric and Mesospheric Sounder (ISAMS) on NASA's Upper Atmosphere Research Satellite (UARS) measured the global temperature and composition profiles in the stratosphere and mesosphere. The main objectives are to obtain atmospheric temperature measurements as a function of pressure, to study water vapor variability in ... the middle atmosphere, and to measure the distribution of oxides of nitrogen in order to study their origins and how they affect ozone concentrations in the stratosphere.
The ISAMS instrument is an infrared radiometer that observes thermal emission from the Earth's limb. Measurements are in the 4 to 17 micron range using gas correlation spectroscopy and solid-state detectors cooled to 80 degrees Kelvin by closed cycle refrigerators. Observations are made on either side of the spacecraft (although on the -Y side only when the satellite is shielded from the sun by the earth), approximately normal to the direction of flight. ISAMS has a total of 32 detector elements. The detectors are grouped into 4-element arrays at the end of one of 8 optical paths. Each path passes through a different pressure modulator cell: CO2 (two), CO, CH4, NO, N2O, NO2, and H2O. The two CO2 cells each contain filter wheels with 4 selections, allowing for a total of 14 spectral channels. These are multiplied by the two signal channels (wide-band, and pressure-modulated) for a total of 28 radiance channels, of which 16 may be sampled in any measurement period.
Each detector element is rectangular with a nominal width:height ratio of 7:1, giving a field of view (FOV) of angular dimensions 0.35 deg x 0.05 deg, corresponding to approximately 16.6 km x 2.36 km at the tangent point. The eight 4-element arrays are arranged so that ISAMS can view +35 to -35 degrees relative to the boresight, which corresponds to a vertical range of about 35 km between the outermost detectors. To increase vertical coverage, the ISAMS mirror can be scanned in elevation.
The ISAMS Level 3A data products consist of profiles that have been inter- polated both vertically and horizontally from the original profiles to standard surfaces and locations defined by the UARS project for all limb-viewing instruments. The Level 3A format is common to all UARS instruments to allow for intercomparison. The ISAMS Level 3A data products consist of two product types: 1) ISAMS_L3AT_DAILY = time-ordered daily data, and 2) ISAMS_L3AL_DAILY = latitude-ordered daily data. Associated with each Level 3A product is a parameter file (3TP with 3AT files and 3LP with 3AL files), containing information on ISAMS viewing side and other relevant details.
Level 3AT/3TP data products consist of time-ordered profiles and are spaced at intervals of 1 UARS minute, or approximately 495 km along the tangent track. Level 3AL/3LP data products consist of profiles at the intersection of the tangent track spaced at intervals of 4 degrees latitude.
The ISAMS Level 3AT and 3AL data products consist of 10 data files per day, and 12 corresponding parameter files (one parameter file for each data file). The following species are measured: 1) AERO12P1 - aerosol at 12.1 microns, 2) AERO6P23 - aerosol at 6.23 microns, 3) CH4 - methane, 4) CO - carbon monoxide, 5) H2O - water vapor, 6) N2O - nitrous oxide, 7) N2O5 - nitrogen pentoxide, 8) NO2 - nitrogen dioxide, 9) O3 - ozone, and 10) TEMP - atmospheric temperature. Measurement units are in parts per million by volume (ppmv) for chemical species, and degrees Kelvin for atmospheric temperature.
The horizontal resolution is 495 km along track for the Level 3AT/TP files, and 4 degrees latitude for level 3AL/LP files. The UARS standard data array is the common data structure used for all Level 3 data subtypes. For ISAMS the index into the data array corresponds to standard pressure levels given in millibars, with each level at approximately 2.5 km increments.
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