HIRS/4 description

The High resolution Infrared Radiation Sounder (HIRS/4) is one of the instruments provided by the National Oceanic and Atmospheric Administration (NOAA), being flown on MetOp. It is an atmospheric sounding instrument for measuring temperature profiles, moisture content, cloud height and surface albedo.

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.

Multi-spectral data from 20 channels are obtained from a single scanning telescope. A rotating filter wheel located in the optical train containing 20 individual filters seperates the individual channels. An elliptical scan mirror provides cross-track scanning of 56 steps in increments of 1.8°. The filter wheel makes one revolution for each step of the scan mirror. The mirror rapidly steps (@ 32 msec.), then holds at each position while the 20 filter segments are sampled during the wheel revolution. This action ('stop & stare') takes place each 0.1 sec. As a result, an Earth scan line lasts 6.4 sec. (stops & stares + retrace back).The HIRS/4 optics are designed to operate within its specifications under any Sun angle condition between 0° and 80°. The radiant cooler is designed to operate from 0° to 68° Sun angle and, with spacecraft supplied shading, can operate from 0° to 80° Sun angle. The Sun angle is defined as the angle between the satellite to Sun line and the normal to the orbital plane.HIRS/4 scans the Earth surface in 20 spectral bands in the range of 0.69 - 14.95 microns: one visible channel at 0.69 micron, seven short-wave IR channels (3.7 to 4.6 microns) and 12 long-wave IR channels (6.5 to 15 microns). This provides data for:temperaturealtitude profilesmoisture contentcloud heightsurface albedoThe instrument sensitivity (noise equivalent spectral radiance, NEDN) are illustrated in the table below.

The scan subsystem provides a digitally positioned direct drive mechanism for moving the scan mirror through Earth scanning and radiometric calibration. The scan subsystem includes the:power, control and motor drive ...

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