HIRS is a 20-channel infrared scanning radiometer that performs operational atmospheric sounding.
HIRS has 19 infrared channels (3.8-15 µm) and one visible channel. The swath width is 2160 km, with a 10 km resolution at nadir.
IR calibration of the HIRS/4 is provided by programmed views of two radiometric targets mdash; the warm target, mounted on the instrument baseplate, and a view of deep space. Data from these views provides sensitivity calibrations for each channel at 256 second intervals, if commanded. Internally generated electronic signals provide calibration and stability monitoring of the detector amplifier and signal processing electronics.
HIRS uses CO2 absorption bands for temperature sounding (CO2 is uniformly mixed in the atmosphere). HIRS also measures water vapour, ozone, N2O and cloud and surface temperatures.