Infrared countermeasures

Infrared countermeasures are means for defending aircraft against "heat-seeking" missiles that use the difference between their target's thermal radiation and that of the background. The countermeasures include evasive flying techniques, "jamming" techniques (e.g., flares) to make the target hard to find against the background, and systems that directly attack the infrared seeker.

While tactical fighters were the first candidates for such systems, followed by bombers, the advent of man-portable air defense systems (MANPADS) surface-to-air missiles made infrared countermeasures important for much slower aircraft more exposed to the ground, such as helicopters (especially special operations helicopters such as the MH-53 PAVE LOW), gunships such as the AC-130, and airliners threatened by terrorists. Presidential and other VIP transports were reputed to be some of the first aircraft, after fighters, to get infrared countermeasures.

Evasive techniques

The first heat-seeking air-to-air missiles had restricted "engagement geometry" -- the relative angles between the firing aircraft and the target. While more recent missiles are "all-aspect", not needing a clear view of the hottest part of the aircraft, the engine, a defending pilot may still improve his chances by presenting the smallest possible infrared target to the missile. See air-to-air missiles for how the geometries have evolved.

Low infrared observability

"Stealth" aircraft do not only try to be low-observability with respect to radar, but to other potential sensors including infrared. They may use techniques such as mixing the exhaust with cooler air before it exits, shrouding the engine exhaust nozzle so it is as small a point as possible, and perhaps releasing some of the exhaust gas along edges of the aircraft.

Broadband infrared energy sources

The most basic countermeasure to a heat-seeking missile is to launch as many flares as possible into the air between the missile and target, hoping that the missile will lock onto a flare that is hotter than the aircraft engine or skin, or at least the missile cannot find the aircraft in what seems a wall of fire. Originally, the flare launchers were no more than magazine-fed chutes on the tail, and perhaps near engines or other hot parts of the aircraft.

More advanced countermeasures, however, such as the AN/ALE-47 Countermeasures Dispenser System [CMDS], is a "smart" dispenser that can integrate with defensive avionics such as radar warning receivers, radar reflector (i.e., chaff) dispensers, radar jammers, as well as helping the pilot with situational awareness of the threat.

With very intelligent infrared seekers, broadband may not be the ideal solution. If the seeker, for example, looked for the emission spectral lines of burning magnesium, it might reject a flare and continue searching. It might also look for a temperature more typical of an engine than a flare. As with most sensors, there is a never-ending battle of measure-countermeasure-countercountermeasure.

Directed energy countermeasures

If focused infrared energy can be directed straight into the thermal seeker, it may overload it as dows a flare, or, given a sufficiently tight beam of the correct wavelength, it may even damage the sensor. On the MH-53 PAVE LOW and AC-130, such directed energy comes from the AN/ALQ-157 system. Even civilian airliners are being equipped with lasers to damage the seeker of man-portable air defense systems, or shoulder-fired missiles considered to be a significant threat from terrorists.