HIRS/4 scanning method

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:

• temperature
• altitude profiles
• moisture content
• cloud height
• surface albedo

The instrument sensitivity (noise equivalent spectral radiance, NEDN) are illustrated in the table below.