An infrared search and track (IRST) system (sometimes known as infrared sighting and tracking) is a method for detecting and tracking objects which give off infrared radiation, such as the infrared signatures of jet aircraft and helicopters.
IRST is a generalized case of forward looking infrared (FLIR), i.e. from forward-looking to all-round situation awareness. Such systems are passive (thermographic camera), meaning they do not give out any radiation of their own, unlike radar. This gives them the advantage that they are difficult to detect.
However, because the atmosphere attenuates infrared to some extent (although not as much as visible light) and because adverse weather can attenuate it also (again, not as badly as visible systems), their range compared to a radar is limited. Within range, an IRST's angular resolution is better than radar due to the shorter wavelength.
History
Early systems
The first use of an IRST system appears to be the F-101 Voodoo, F-102 Delta Dagger and F-106 Delta Dart interceptors. The F-106 had an early IRST mounting replaced in 1963 with a production retractable mount. The IRST was also incorporated into the Vought F-8 Crusader (F-8E variant) which allowed passive tracking of heat emissions and was similar to the later Texas Instruments AAA-4 installed on early F-4 Phantoms.
The F-4 Phantom had a Texas Instruments AAA-4 infrared seeker under the nose of early production aircraft F-4B's and F-4C's and not installed on later F-4-D's due to limited capabilities, but retained the bulge and indeed some F-4D's had the IRST receiver retrofitted in a modified form.
The F-4E eliminated the AAA-4 IRST bulge and received an internal gun mount which took up the area under the nose. The F-4J which had a pulse-Doppler radar also eliminated the AAA-4 IRST receiver and bulge under the nose.
The first use of IRST in an Eastern European country was the Mikoyan-Gurevich MiG-23 The MiG-23 used the (TP-23ML) IRST and later versions used the (26SH1) IRST. The Mikoyan-Gurevich MiG-25PD was also equipped with a small IRST under the nose.
The Swedish Saab J-35F2 Draken (1965) also used an IRST, a Hughes Aircraft Company N71.
Later systems
IRST systems re-appeared on more modern designs starting in the 1980s with the introduction of 2-D sensors, which cued both horizontal and vertical angle. Sensitivities were also greatly improved, leading to better resolution and range. In more recent years, new systems have entered the market. In 2015, Northrop Grumman introduced its OpenPod(TM) IRST pod, which uses a sensor by Leonardo.
While IRST systems are most common amongst aircraft, land-based, ship and submarine systems are available.
Distributed Aperture Systems
The F-35 equipped infrared search and track system AN/AAQ-37 Distributed Aperture System (DAS), which consists of six IR sensors around the aircraft for full spherical coverage, providing day/night imaging and acting as an IRST and missile approach warning system.
Chengdu J-20 and Shenyang FC-31 shares the similar design concept with their EORD-31 system that provides 360 degree IRST coverage. IRST systems can also be used to detect stealth aircraft, in some cases, outperforming traditional radar.
Technology
These were fairly simple systems consisting of an infra-red sensor with a horizontally rotating shutter in front of it. The shutter was slaved to a display under the main interception radar display in the cockpit. Any IR light falling on the sensor would generate a "pip" on the display, in a fashion similar to the B-scopes used on early radars.
The display was primarily intended to allow the radar operator to manually turn the radar to the approximate angle of the target, in an era when radar systems had to be "locked on" by hand. The system was considered to be of limited utility, and with the introduction of more automated radars they disappeared from fighter designs for some time.
Performance
Detection range varies with external factors such as
clouds
altitude
air temperature
target's attitude
target's speed
The higher the altitude, the less dense the atmosphere and the less infrared radiation it absorbs - especially at longer wavelengths. The effect of reduction in friction between air and aircraft does not compensate the better transmission of infrared radiation. Therefore, infrared detection ranges are longer at high altitudes.
At high altitudes, temperatures range from −30 to −50 °C - which provide better contrast between aircraft temperature and background temperature.