Infrared search and track

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.

Source: Wikipedia — Infrared search and track (CC BY-SA 4.0)