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Radiometrically Accurate FTS For Atmospheric Emission Observations

The calibration procedures and radiometric performance of the High-resolution Interferometer Sounder (HIS) aircraft instrument are discussed to illustrate the superior performance of FTIR instrumentation for achieving accurate emission measurements of the atmosphere. Radiometrically accurate emission spectra of the earth have been observed both from the NASA U2/ER2 aircraft and from the ground in the spectral range from 3.8 to 16.6 Am with resolving powers of 1800-3800. While the HIS was primarily designed for atmospheric temperature and moisture profiling, its broad spectral coverage leads to a wide range of important applications. The HIS was designed to give absolute brightness temperature errors of less than 1.0°C and a reproducibility of 0.1°C. Laboratory tests indicate that these goals were achieved over most of the measurement domain. The single scan detector noise is somewhat larger than the reproducibility. Agreement is remarkably good between HIS spectra and calculations from radiosonde temperature and water vapor profiles using the AFGL FASCOD2 program, a tribute to the current state of line-by-line codes. Brightness temperature differences are, less than 1-2°C over much of the spectral domain, with some important exceptions. The largest discrepancies, currently at the CO2 Q-branches, will be greatly reduced when recent improvements in the treatment of line mixing are released in FASCOD3. Other substantial differences have recently been identified as resulting from trace gas absorption, CO2 and H2O line strength uncertainties, and from uncertainties in the strength of the H2O foreign broadened continuum.