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. IRST is a generalized case of FLIR, i.e. from Forward-Looking to allround situational awareness. 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 attenuates 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.
The first use of an IRST system appears to be the F-101 Voodoo and F-102 Delta Dagger interceptors. These were fairly simple systems consisting of an IR 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.
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. The best known users of modern IRST systems are the Russian Su-27 Flanker and American F-14 Tomcat. 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.
IRST systems can incorporate laser rangefinders in order to provide full fire-control solutions for cannon fire or launching missiles. Without knowing the range the IRST can only provide direction and thus not enough information for aiming weapons.
Since the 80's development of IRST systems was nearly stagnant. Existing systems were considered sufficient for their tasks until recently. The Russian Mig-35, an upgraded Mig-29 is now available for export has a new IRST system integrated with optical and laser systems, and it poses a significant challenge even to the F/A-18E/F Super Hornet and the Dassault Rafale. This system is unique in the amount of tasks it can perform when compared to older IRST systems. It can provide targeting solutions for ground and air targets at up to 15km. It can detect missiles thanks only to the warmth generated by air resistance on their nose and can provide the pilot with a detailed trajectory and it can do all these tasks across the full 360 degrees of the battlefield compared to its contemporaries that usually only provided a certain amount of coverage in front of the pilot.
See also: forward-looking infra-red