P. G. Weber
Los Alamos National Laboratory
The High Resolution Infrared Radiation Sounder (HIRS) is a 20-spectral band space sensor that measures the earth's emitted radiation, primarily in the infrared
region. The sensor is designed to provide data that will permit radiative transfer calculations of the vertical atmospheric temperature profile from the surface
to 30 km, as well as the vertical water vapor profile and total ozone content.Central wavelengths of the channels range from 15 
m
down to 4 m in the infrared,
plus a visible (red) channel centered at 0.69 mm. The most recent version, HIRS/2, uses a 15-cm optical system; the gathered light is separated by a dichroic
beamsplitter into longwaves (> 6.4 mm) and shortwaves (< 6.4 m) and is passed through field stops, bandpass filters, and relay optics to the detectors. The
visible channel is split off from the shortwave radiation by a second dichroic beamsplitter.
The long wavelength detector is mercury cadmium telluride (HgCdTe), the short wavelength infrared detector is indium antimonide (InSb). Both detectors are operated at 105K using a radiant cooler. A silicon detector operating at ambient temperature is used for the visible channel. The range of signal from each channel is adjusted to conform to the range of input temperature expected in that spectral interval. Each channel output is offset to make full use of the 13-bit digitizer.
The instantaneous field of view (IFOV) of all the channels is stepped across the satellite track by a rotating mirror; 55 steps of 1.85 each constitute a 99
scan, which is completed in 6.4 seconds. The ground IFOV is 17.4 km at nadir and 58.5 km cross-track, 29.9 km along-track at the end of the scan depending,
of course, on satellite altitude.The instrument has flown on Nimbus-6, TIROS-N, and NOAA-9, -10, and -11. Combined with the Stratospheric Sounding Unit (SSU)
and the Microwave Sounding Unit (MSU), the package is referred to as the TIROS Operational Vertical Sounder (TOVS) system. Basic parameters of these instruments
are found in Smith et al. (1979). Smith (1989) provides a survey of related satellite sounding instruments, from the Nimbus-3 (1969) to proposals covering
the rest of the 1990s. Near-term plans for HIRS include a 1993 launch on the NOAA-K; no major modifications are planned to this instrument (Ohring 1989). The
Atmospheric Infrared Sounder (AIRS) package to be flown on EOS is the follow-on instrument, but is designed as a grating spectrometer. The higher spectral
resolution and quasi-complete spectral coverage of AIRS should yield 1 K temperature errors and a 10-20% tropospheric precipitable water vapor profile accuracy
(Smith 1989).
