Profilers/Sounders
type: Concept

HIRS/3
in scheme: instruments
definition: The High Resolution Infrared Radiation Sounder (HIRS/3) is a discrete stepping, line-scan instrument designed to measure scene radiance in 20 spectral bands to permit the calculation of the vertical temperature profile from Earth's surface to about 40 km. Multispectral data from one visible channel (0.69 micrometers), seven shortwave channels (3.7 to 4.6 micrometers) and twelve longwave channels (6.5 to 15 micrometers) are obtained from a single telescope and a rotating filter wheel containing twenty individual filters. An elliptical scan mirror provides cross-track scanning of 56 increments of 1.8 degrees. The mirror steps rapidly (<35 msec), then holds at each position while the 20 filter segments are sampled. This action takes place each 100 msec. The instantaneous FOV for each channel is approximately 1.4 degrees in the visible and shortwave IR and 1.3 degrees in the longwave IR band which, from an altitude of 833 kilometers, encompasses an area of 20.3 kilometers and 18.9 kilometers in diameter, respectively, at nadir on the Earth. Three detectors are used to sense the radiation. A silicon photodiode at the instrument temperature (nominally 15 degrees C) detects the visible energy. An Indium Antimonide detector and a Mercury Cadmium Telluride detector (mounted on a passive radiator and operating at 100K) sense the shortwave and longwave IR energy. The shortwave and visible optical paths have a common field stop, while the longwave path has an identical but separate field stop. Registration of the fields of view in all channels is largely determined by these stops with secondary effects from detector position. IR Calibration of the HIRS/3 is provided by programmed views of two radiometric targets: a warm target mounted to the instrument base and a view of space. Data from these views provides sensitivity calibrations for each channel every 40 lines (256 seconds), if commanded. Internally generated electronic signals provide calibration and stability monitoring o…
type: Concept