A radar-guided missile announces itself. The radar that guides it lights up the target aircraft, and a radar warning receiver hears it. A heat-seeking missile gives no such courtesy. It homes passively on the heat of an engine, and the first sign of trouble may be the missile itself. That is the gap a missile approach warning system, or MAWS, is built to close.

These sensors watch for the flash and plume of a missile in flight and warn the crew in time to dump flares, turn hard or let an automatic jammer go to work. They are standard on transport aircraft and helicopters, built into the F-22 and F-35, and, as the US Air Force admitted this week, still missing from its newest F-15EX.

Why Radar Warning Is Not Enough

A radar warning receiver listens for radar emissions. It is excellent against radar-guided surface-to-air and air-to-air missiles, but useless against an infrared-guided missile, which emits nothing. Shoulder-fired missiles such as the Stinger and Igla, and many short-range air-to-air missiles, fall into that category.

A missile warner instead looks for the missile. Most systems detect the rocket motor's exhaust as it burns, then track its angle and growth to decide whether it is heading for the aircraft. The ideal is near-perfect detection with a reaction time under a second, without crying wolf every time the sun glints off a lake.

Three Ways to See a Missile

Ultraviolet sensors work in the solar-blind part of the ultraviolet spectrum, where sunlight is largely absorbed by the atmosphere. A burning rocket motor stands out clearly against an almost black background, so false alarms from the sun are rare. UV systems are simple and all-weather, but they only see a missile while its motor is burning.

Infrared sensors can detect missiles at longer range, especially at altitude where there is no ground clutter, and they give very accurate direction. They struggle in cloud and rain, because water and ice block infrared, and they need cooling and heavy processing to filter out false targets.

Pulse-Doppler radar warners measure a missile's range and speed directly, so they can calculate time to impact even after the motor burns out. The downside is that they transmit, which can give away the aircraft's position.

Who Carries Them

Transports and helicopters were first to get missile warners as standard, because they fly low and slow where shoulder-fired missiles are most dangerous. The ultraviolet AN/AAR-47 protects many US transports and tiltrotors, BAE Systems' AN/AAR-57 Common Missile Warning System is standard on US Army helicopters, and Hensoldt's AN/AAR-60 MILDS equips German and other European aircraft.

Fighters followed. The F-22 Raptor carries the AN/AAR-56 infrared missile launch detector, and the F-35's AN/AAQ-37 Distributed Aperture System uses six infrared cameras around the airframe to give 360-degree missile warning and situational awareness. The F-15EX was designed with six sensor positions for a warner, but the US Air Force bought it without one and is only now asking industry for a system.

Defense Updates explains the F-35's AN/AAQ-37 Distributed Aperture System, which combines missile warning with 360-degree vision.

From Warning to Defeat

A warning is only useful if it triggers a response. In most aircraft, the missile warner is linked to the countermeasures dispenser, which can fire flares automatically. On larger aircraft it may cue a directional infrared countermeasures system, a turret that points a laser at the missile's seeker to blind or confuse it.

The best defence still combines all three: a sensor that sees the missile early, decoys that give it a better target, and a pilot who turns hard at the right moment. Without the first, the other two often start too late.

Sources: Wikipedia (Missile approach warning system), TWZ, BAE Systems, Hensoldt, Defense Updates