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.

Channel number Centre wavenumbCener (cm-1) tre wavelength (mm) Specified sensitivity (NEΔN) (mW/
m2 sr
cm-1)
FWHM bandwidth (cm-1)  
1 669 14.95 3.00 3 LW 
2 680 14.71 0.67 10 LW
3 690 14.49 0.50 12 LW
4 703 14.22 0.31 16 LW
5 716 13.97 0.21 16 LW
6 733 13.64 0.24 16 LW
7 749 13.35 0.20 16 LW
8 900 11.11 0.10 35 LW
9 1030 9.71 0.15 25 LW
10 802 12.47 0.15 16 LW
11 1365 7.33 0.20 40 LW
12 1533 6.52 0.20 55 LW

13          2188              4.57           0.006    23             SW
14 2210 4.52 0.003 23 SW
15 2235 4.47 0.004 23 SW
16 2245 4.45 0.004 23 SW
17 2420 4.13 0.002 28 SW
18 2515 4.00 0.002 35 SW
19 2660 3.76 0.001 100 SW
20 14500 0.69 0.10% albedo 1000 VIS

The HIRS/4 instrument is a single package that mounts directly onto the satellite. The major sub-assemblies are:

  • Scan subsystem
  • Filter Wheel Sub-Assembly
  • Detectors
  • Radiant Cooler Assembly
  • Optical Assembly
  • Baseplate

Last update: 28 June 2006

Copyright 2000 - 2014 © European Space Agency. All rights reserved.