Also aboard Nimbus-3, the Satellite Infrared Spectrometer (SIRS) measured radiances in several broad spectral bands in the carbon dioxide absorption region and thus enabled the first temperature profile retrievals from a satellite (Wark et al. 1970). Comparison with radiosonde observed profiles showed that the satellite-derived temperature profiles were very representative overall, with detailed vertical features smoothed out.

Clouds that usually existed within the instrument's 250-km diameter field of view caused the major limitations with the early SIRS observations. Also the SIRS observed only along the suborbital track, leaving large gaps in data between orbits as illustrated by the boxes shown in the right-hand figure. In spite of these problems, the SIRS data immediately showed promise of benefiting the current weather analysis/forecast operation and was put into operational use on 24 May 1969, barely one month after launch (Smith et al. 1970).