HIRS/4 description

The High resolution Infrared Radiation Sounder (HIRS/4) is a 20 channel scanning radiometric sounder. It is essentially the same as its predecessor HIRS/3 only the instrument field of view has been reduced from 18 km to 10 km. This was done to improve the utility of the data near cloud boundaries.

Three detectors are used to sense radiation. A silicon photodiode at the instrument temperature (nominally around 15°C) detects the visible energy. An Indium Antimonide detector and Mercury Telluride detector (mounted on a passive radiator and operated at 95K) sense the short wave IR energy. The short wave and visible paths have a common field stop, while the long wave path has an identical but separate field stop. Registration of the optical fields in all channels is largely determined by these stops with secondary effects from detector position.