OVERVIEW: Infrared Search and Track (IRST) is a passive sensor system that detects and tracks objects based on their infrared (heat) emissions. Unlike radar, IRST does not emit signals, making it stealthy and resistant to detection or jamming. IRST is commonly used in air-to-air and surface-based applications to provide long-range surveillance, target detection, and tracking capabilities.

DETAILS: IRST systems utilize infrared sensors, often cooled mid-wave (MWIR: 3–5 µm) or long-wave (LWIR: 8–12 µm), to passively scan the environment for heat sources such as aircraft, missiles, or ground vehicles. Advanced IRST systems use imaging arrays and real-time processing to classify targets and provide angle-of-arrival data, often integrating with fire control systems.
On fighter aircraft, IRST provides an advantage in beyond-visual-range engagements without alerting the enemy through radar emissions. On naval platforms, IRST contributes to anti-air and anti-surface surveillance. The system’s passive nature makes it effective in environments where emissions control (EMCON) is critical.
IRST performance depends on factors such as sensor sensitivity, atmospheric conditions, and target contrast. Modern IRST systems may also integrate laser rangefinders or work in conjunction with radar and electronic warfare systems to enhance tracking and targeting capabilities.

ROLE SUMMARY:
Category: Passive Infrared Target Detection
Primary Use: Detection and tracking of airborne targets using IR emissions
Function: Passive scanning of infrared energy for situational awareness and early warning
Strong Against: Stealth aircraft, non-emitting targets, and targets in radar-denied environments
Weak Against: Low-IR-signature objects, atmospheric obscurants (clouds, rain), and clutter
Platform: Fighter aircraft, maritime patrol aircraft, naval ships
Integration: Often coupled with radar and fire-control systems for target engagement

-First-generation: These early IRST sensors detect changes in infrared radiation to locate targets but lack detailed image formation, limiting identification capabilities. They offer passive long-range detection useful in environments where radar use is restricted or undesirable.

See Specific information under [Sensors/EW] and [Properties] sections.

SOURCE:
Defense Technical Information Center (DTIC)
https://en.wikipedia.org/wiki/Infrared_search_and_track