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 forward looking infrared (FLIR), i.e. from forward-looking to all-round 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 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.
Read more about Infra-red Search And Track: Early Systems, Later Systems
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