Infra-red search and track
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An infra-red search and track (IRST) system (sometimes known as infra-red sighting and tracking) is a method for detecting and tracking objects which give off infrared radiation such as jet aircraft and helicopters. Such systems are passive, 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 does attenuate infra-red light 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), the range compared to a radar is limited. Angular resolution at short ranges is better than radar due to the shorter wavelength.
Popularized by modern interceptors like the Russian Su-27 Flanker and American F-14 Tomcat, IRST was first introduced during the 1960s on American F-101 Voodoo and F-102 Delta Dagger fighters. These aircraft carry the IRST systems for use in lieu of their radars when the situation warrants it, such as when shadowing other aircraft or under the control of AWACS or GCI, where an external radar is being used to help vector them onto a target and the IRST is used to pick up and track the target once they are in range. With infra-red homing or fire-and-forget missiles, the aircraft may be able to fire upon the targets without having to turn their radar sets on at all. Otherwise, they can turn the radar on and achieve a lock immediately before firing if desired. They could also close to within cannon range and engage that way. Whether or not they use their radar, the IRST system can still allow them to launch a surprise attack.
An IRST system may also have a regular magnified optical sight slaved to it, to help the IRST-equipped aircraft identify the target at long range. As opposed to an ordinary forward looking infrared system, an IRST system will actually scan the space around the aircraft similarly to the way in which mechanically (or even electronically) steered radars work. When they find one or more potential targets they will alert the pilot(s) and display the location of each target relative to the aircraft on a screen, much like a radar. Again similarly to the way a radar works, the operator can tell the IRST to track a particular target of interest, once it has been identified, or scan in a particular direction if a target is believed to be there (for example, because of an advisory from AWACS or another aircraft).
Note that, like infra-red homing seekers, an IRST is more likely to detect a target with its engine exhaust pointed towards the detector than away from it. This means that many jet aircraft will be detected at longer ranges if they are flying away from the IRST-equipped aircraft rather than towards it. However, most IRST systems are sensitive enough to detect the heat of a jet from head-on as well, either infra-red energy generated from the hot air coming out of the engines, from air friction heating the airframe, or both.
Infra-red search and track systems often incorporate laser range-finders in order to provide full fire-control solutions for cannon fire or launching missiles. Without knowing the range (which could be estimated using the infra-red signature of the target or other factors, but would be very inaccurate), the IRST can only provide direction and thus not enough information for aiming weapons.
Infra-red search and track and forward-looking infra-red are sometimes used interchangeably but a system which has automated searching and tracking functions, rather than being fixed looking forward or manually steerable, is more specifically known as IRST whereas an FLIR is often little more than an infra-red camera hooked up to a monitor. IRST is more often used against aircraft, but can find and track ground targets, whereas FLIRs are more often found on ground attack aircraft.
See also: forward-looking infra-red
