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From my related question, it seems fighter jets do not usually track missiles by radar. Instead, they have a system that warns of radar lock.

So what about infrared missiles (heat-seeking missiles)? Flares are a countermeasure to these, so how does the jet know when one is incoming?

I would be pretty surprised if such a system didn't exist. I want to know more about that system and when it was first implemented.

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Actually most aircraft don't actively know when a missile is being fired at them. I have worked on helicopters and fighters that deploy counter measures.

The first is an IR jammer which we call a disco ball. It is made up of mirrors at different angles that resemble a disco ball. It radiates an IR signal at different angles to confuse the missiles.

The flares or chaff are fired periodically in known hot spots. This creates heat signatures similar to the engines and confuses the missiles.

There are active systems but they don't work well. I have seen a helicopter in testing suspended from a wire with countermeasures turned on. A missile was fired and the countermeasures didn't even get deployed. This is why they usually keep firing the flares or chaff in known hot spots.

Hope that helps.

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    $\begingroup$ From memory on a helicopter, the left side has a self defence suite with 4 rows of 6 tubed. So 24 on each side, up to 48. But don't quote me on the number. If you ever watch the news with a fighter jet in Afghanistan flying over a hot spot they continously fire flares every 2 or 3 seconds. Contrary to what movies display, aircraft don't stay in hot zones for a long time for obvious reasons $\endgroup$ – Mark Aug 11 '15 at 3:50
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    $\begingroup$ @Mark I don't know how you got the idea that disco balls "reflects the missiles IR signal". IR missiles don't transmit anything, therefore there is nothing to reflect. Disco balls work by radiating large amounts of IR pulsed and modulated to confuse a missile. $\endgroup$ – Simon Aug 11 '15 at 5:35
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    $\begingroup$ @DrZ214 Nope. Disco ball jammers (that's a bad term) aim to flood the receiver on the missile making it break lock. The acquisition beam of a most missiles is very narrow, a few degrees, and once lock is broken, it is very unlikely to re-acquire. The missile will simply fall away. It's too complex to detail here but in essence, the missile scanning head sweeps from side to side to track the heat source. Disco balls emit a large source of modulated IR which floods the receiver and prevents it from tracking. Modern missiles render this obsolete and disco balls actually attract them. $\endgroup$ – Simon Aug 11 '15 at 7:06
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    $\begingroup$ @DrZ214 Most IR missiles aim slightly off centre deliberately and explode alongside the target using proximity fuses. Some fire out rods or chains which rip through the aircraft letting holed fuel tanks and engines disintegrating do the real work. The aim of passive IR jammers is to stop lock. $\endgroup$ – Simon Aug 11 '15 at 7:08
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    $\begingroup$ @Simon Typically when countermeasures are fired, they are fired in a 'program' that fires both chaff and flare at the same time in a pre-determined mix. There are two types of countermeasures on the plane, but only one countermeasure system that can be fired. These programs are usually set up before the flight, and optimized against specific threats. $\endgroup$ – Mark Aug 12 '15 at 12:12
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Before continuing, much of the how's and why's of IRCM are going to be either speculation, or classified. Nations will not freely discuss their actual methods of defeating opposing nation's IR capabilities. Therefore, actual answers will be difficult to give. However, here is an excerpt from Wikipedia regarding a specific missile warning system that may be found on military aircraft. This represents one possible method used for certain types of aircraft.

The AN/AAR-47 Missile Warning System is a Missile Approach Warning system used on slow moving aircraft such as helicopters and military transport aircraft to notify the pilot of threats and to trigger the aircraft's countermeasures systems.

The AN/AAR-47 passively detects missiles by their infrared signature, and uses algorithms to differentiate between incoming missiles and false alarms."

https://en.wikipedia.org/wiki/AN/AAR-47_Missile_Approach_Warning_System

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The infrared missiles are usually detected using a Missile Approach Warning Sensor (MAWS).

The Missile Approach Warning Systems are passive detectors, usually detecting either the infrared or ultraviolet rays emitted by the incoming missiles.

The infrared based systems detect the infrared waves emitted by the missile. The ultraviolet based systems detect the ultraviolet rays emitted by the missile's rocket motors and are more suitable for detecting missiles with solid rocket motors.

An example for the infrared based detector is the US-Israel PAWS, while the Swedish MAW-300 uses an ultraviolet detector. Some systems like the MWS-20 uses Doppler Radar to detect the missiles.

All these systems have their advantages and disadvantages, and in relation to your previous question, give a 360 degree coverage based on the location of their installation.

In most of the aircraft, these detection systems, along with the Radar Warning Receiver, which detect the Radar based missiles are integrated into the Self Protection System(SPS), which determines the threat level of the incoming missile and responds accordingly by firing chaff and flares.

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    $\begingroup$ I went to Wikipedia to cite that IR missiles are entirely passive and I found to my surprise that the AIM-9 has four IR emitters! +1 $\endgroup$ – Todd Wilcox Aug 11 '15 at 13:02
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    $\begingroup$ @ToddWilcox Those emitters are only on some models with IR proximity fuses. They have nothing to do with acquisition and tracking and are only used to detonate the warhead when alongside the target. $\endgroup$ – Simon Aug 11 '15 at 14:18
  • $\begingroup$ Equally worth noting, some Air-to-Air IR missiles (like some variants of the AIM-9) can be given their initial steering cues via the FCR on the aircraft, this is active radar illumination on the target and can alert the pilot, causing him to trigger flares manually, even though the rocket and guidance system being used are IR. To say nothing of the fact that sometimes flares are triggered simply because the pilot sees the missile launch. $\endgroup$ – Mark Aug 11 '15 at 15:48
  • $\begingroup$ @Mark How long does a flare usually last? I want to get an idea of how many might really be needed for extended dogfights. $\endgroup$ – DrZ214 Aug 11 '15 at 19:36
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    $\begingroup$ @ToddWilcox: Air friction is going to heat up the nose cone of a missile. It may not be an intentional IR emitter, but it will show up. $\endgroup$ – MSalters Aug 18 '15 at 10:33

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