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More than three years of data from the Atmospheric InfraRed Sounder (AIRS) have given many new insights into infrared hyperspectral sounding from space. Among other things they have shown that 1) Absolute accuracy at the 200 mK level and stability at the better than 20 mK/year level can be deduced for the AIRS data by comparing the observed brightness temperatures with those predicted by NOAA/NCEP's RTGSST for surface channels and those predicted from the ECMWF temperature and moisture profiles for all channels except those sensitive to stratospheric temperatures or water vapor. While the 2616 cm-1 window channel used with AIRS for the validation of the calibration relative to the RTGSST is superior, the 1231 cm-1 window channel can be used for IASI and CRIS with slightly degraded accuracy. 2) Residual cloud contamination of about 240 mK for tightly cloud-filtered data establishes an effective noise floor for channels with weighting function below the cloud tops. With the AIRS 13.5 km footprints and mean NeDT of 0.2 K, the noise floor is reached by filtering out all but 2% of the spectra. 3) The absolute value of the difference between adjacent clear ocean footprints at night provides an excellent estimate of the NeDT in all spectral regions not affected by water vapor. Water vapor spatial scene inhomogeneity on a 13.5 km scale even in clear footprints acts as a source of noise up to a factor of six larger than the NeDT in water sensitive channels.
The methodology established by AIRS for system performance evaluation and trend analysis, including the use of the RTGSST and the ECMWF profiles, lays the foundation and establishes the benchmark for the validation of the performance of future hyperspectral sounders. AIRS was launched into polar 705 km altitude orbit on the EOS Aqua spacecraft on May 4, 2002. It covers the 3.7 to 15.4 micron region of the thermal infrared spectrum with spectral resolution of 1200. Since the start of routine data gathering in September 2002 AIRS has returned 2.9 million spectra of the upwelling radiance each day.
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