The HIRS/4 is an atmospheric sounding instrument that provides multi-spectral data from one visible channel (0.69 micron), seven shortwave channels (3.7 to4.6 microns), and 12 longwave channels (6.7 to 15 microns) using a single telescope and a rotating filter wheel containing 20 individual spectral filters. The IFOV for each channel is approximately 7.0 ¡ that, from a spacecraft altitude of 870 km (470 nmi), encompasses a circular area of 10 km (6.2 mi) in diameter at nadir on Earth. This is an improvement in resolution over the 20-km (12.4 mi) HIRS/3 instrument that was flown on NOAA-KLM. An elliptical scan mirror provides a cross-track scan of 56 steps of 1.8¡ each. The mirror steps rapidly, then holds at each position while the optical radiation passing through 20 spectral filters is sampled. Each Earth scan takes 6.4 seconds and covers ±49.5¡ from nadir. IR calibration of the HIRS/4 is provided by views of space and the internal warm target, each viewed once per 38 Earth scans.
The instrument measures scene radiance in the IR spectrum. Data from the instrument is used, in conjunction with the Advanced Microwave Sounding Unit (AMSU) instruments, to calculate the atmosphere's vertical temperature profile from the Earth's surface to about 40 km (24.9 mi) altitude. The data is also used to determine ocean surface temperatures, total atmospheric ozone levels, precipitable water, cloud height and coverage, and surface radiance.