NOAA KLM User's Guide
The High Resolution Infrared Radiation Sounder (HIRS/3 and HIRS/4) is a discrete stepping, line-scan instrument. 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 HIRS/3 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. The instantaneous HIRS/4 FOV for each channel is approximately 0.69 degrees in the visible, shortwave and longwave IR bands which, from an altitude of 833 kilometers, encompasses an area of 10.005 kilometers in diameter at nadir on the Earth.
Figure J.2-1 shows the Earth scan pattern and the angular locations of the calibration targets relative to Earth scan for the HIRS/3 and HIRS/4 instrument.
Figures J.2-2 and J.2-3 show the simulated earth-surface footprints for HIRS/3 and HIRS/4; and AMSU-A for a half scan and a full scan, respectively. Note: the scan mirror step size remains at 1.8 degrees per step for HIRS/3 and HIRS/4, so Figures J.2-2 and J.2-3 indicate the correct footprint position. However, the FOV of the HIRS/4 instrument is halved, so the footprint diameter for HIRS/4 will be half that of HIRS/3.
Amended October 23, 2002
Amended December 13, 2005
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