An Infrared Search and Track (IRST) system (sometimes called infrared sighting and tracking) detects and tracks objects that emit infrared radiation, such as the infrared signatures of jet aircraft and helicopters. Some naval surface vessels, such as the Istanbul-class frigate, are also equipped with IRST. A generalized case of forward-looking infrared (FLIR) systems, IRST systems provide all-around situation awareness. Their thermographic cameras are passive: unlike radar, they do not emit radiation and therefore do not add to an aircraft's emissions signature. Within range, an IRST's angular resolution is better than radar because infrared has a shorter wavelength than radar emissions. But an IRST's range is less than radar because infrared emissions are attenuated by the atmosphere and by poor weather (although less so than visible light).
History
Early systems
IRSTs first appeared in 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. An IRST was added to the F-8 Crusader (F-8E variant). A similar Texas Instruments AN/AAA-4 was installed under the nose of early production aircraft F-4 Phantom B and C models. It was not installed on later F-4Ds due to limited capabilities, but retained the bulge; some F-4Ds 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 a Eurasian country was the Mikoyan-Gurevich MiG-23, which used the (TP-23ML) IRST; 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) and J 35J Draken also used IRST units, 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. A cued search is a search performed in a relatively small volume to acquire a target whose position is approximately known. The target´s position can have been approximately obtained by other sensors or supplied from an external source. 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 IRST pod, which uses a sensor by Leonardo. The United States Air Force is currently incorporating IRST systems for its fighter aircraft fleet, including the F-15, F-16, and F-22.
While IRST systems are most common amongst aircraft, land-based, ship and submarine systems are available.
Distributed Aperture Systems The F-35 is equipped with 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 is assumed to share the similar design concept with their system. 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 for 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. The Eurofighter Typhoon's PIRATE IRST can detect subsonic fighters from 50 km from the front and 90 km from the rear - the larger value being the consequence of directly observing the engine exhaust, with an even greater increase being possible if the target uses afterburners. The range at which a target can be identified with sufficient confidence to decide on weapon release is significantly inferior to the detection range - manufacturers have claimed it is about 65% of the detection range.
Tactics
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