The Nimbus 4 infrared spectroscopy experiment: 2. Comparison of observed and theoretical radiances from 425–1450 cm−1

The Nimbus 4 infrared interferometer spectrometer (IRIS) measured the thermal emission of the earth's atmosphere and surface from 400–1600 cm−1 with an apodized spectral resolution of 2.8 cm−1. A comparison of theoretical radiances, computed from in situ measurements and using a direct integration slant path atmospheric transmittance model, with the observed IRIS radiances has been made to verify the radiometric and spectral performance of the instrument and to assess the accuracy of the atmospheric transmittances. The radiance comparison has indicated a relatively constant difference of less than 5% in the water vapor continuum in the 425- to 550-cm−1 and 750- to 1200-cm−1 atmospheric ‘window’ regions, whereas in the 667-cm−1 CO2 band the difference was 5–10%. The absolute accuracy was found to be ∼5–10% for each of the parameters, measured radiances, in situ measurements, and the atmospheric transmittances, and thus it is not possible to uniquely specify the degree of error arising from each parameter in the total resultant difference. Because of the magnitude of the errors in the measured radiances and in the in situ measurements and because of the insensitivity of the radiances to the atmospheric transmittances it is very difficult to improve atmospheric transmittances through the radiance comparison technique.