The High-resolution Infrared Radiation Sounder (HIRS/4) is a 20-channel infrared scanning radiometer that will perform operational atmospheric sounding to provide calibrated vertical profiles of temperature and humidity, information on cloud cover, cloud top height, cloud top temperature and cloud phase, as well as surface albedo. HIRS has 19 infrared channels (3.8-15 μm) and 1 visible channel. The swath width is 2160 km with a 10 km resolution at nadir.

The HIRS/4 instrument provides multispectral data from one visible channel (0.69 μm), seven shortwave channels (3.7 to 4.6 μm) and twelve longwave channels (6.7 to 15 μm) using a single telescope and a rotating filter wheel containing twenty individual spectral filters. A rotating scan mirror provides cross-track scanning of 56 steps in increments of 1.8 degrees. The mirror steps rapidly (<35 msec), then holds at each position while the optical radiation, passing through the 20 spectral filters, is sampled. This action takes place each 0.1 second. The instantaneous field of view for each channel is approximately 0.7 degrees which, from a spacecraft altitude of 837 km, encompasses a circular area of 10 km at nadir on the earth.

Three detectors are used to sense the optical radiation. A silicon photodiode, at nominal instrument temperature (15ºC), detects the visible radiation. An Indium Antimonide detector and Mercury Cadmium Telluride detector (mounted on a passive radiator and operating at 95 K) sense the shortwave and longwave IR radiation, respectively. The shortwave and visible optical paths have a common field stop, while the longwave path has an identical but separate field stop. Size and registration of the optical fields of view in all channels is determined by these stops.

HIRS/4 Summary Budgets

HIRS/4 is manufactured by ITT/Aerospace/Defense Communications in Fort Wayne, Indiana under contract to NASA.

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