Infrared homing
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Infra-red homing refers to a guidance system which uses the infra-red light emission from a target to track it. Missiles which use infra-red seeking are often referred to as "heat-seekers". Infra-red (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infra-red wavelengths of light compared to objects in the background.
Seeker types
Not all "infra-red" seekers stick strictly to this part of the electromagnetic spectrum. For example, aircraft engines tend to emit a lot of radiation in the infra-red band but do not emit much ultraviolet radiation unlike countermeasure flares that strongly emit both. Newer "infra-red" seekers in air-to-air missiles and surface-to-air missiles tend to be the "two-color" variety which use both the infra-red and ultra-violet radiation, greatly increasing their resilience to countermeasures such as flares, which tend to only produce primarily IR light, and infra-red jammers. These types are also more effective at locking on to the signature of an aircraft while it is pointing towards the missile, giving them all-aspect capability.
Because flares decelerate rapidly, modern missiles are able detect and avoid them, as well as using more advanced seekers like the two-colour type mentioned above to defeat them, because they emit mostly in the infra-red part of the spectrum.
The three main materials used in the infra-red sensor are lead sulfide (PbS), Indium Antimonide (InSb) and MerCad telluride (HgCdTe). Older sensors tend to use PbS, newer sensors tend to use InSb or HgCdTe. All perform better when cooled, as they are both more sensitive and able to detect cooler objects.
Scanning patterns and modulation
A missile's resistance to decoys can also be determined by the method in which the space in front of itself is scanned for targets. Early missiles used spin scanning while newer seekers use conical scanning which gives them superior decoy discrimination as well as overall increased sensitivity for longer range tracking. There have also been missiles built using so-called "rosette" scanning methods. Very modern heat-seeking missiles utilise imaging infra-red (IIR), where the IR/UV sensor is actually a focal planar array sensor which is able to "see" in infra-red, much like the CCD in a digital camera. This requires much more signal processing but can be much more accurate and harder to fool with decoys. In addition to being more flare-resistant, newer seekers are also less likely to be fooled into locking onto the sun, another common trick for avoiding heat-seeking missiles.
Before imaging infra-red sensors there was also the question of sensor modulation; earlier seekers used amplitude modulation (AM) to determine how far off-center the target was and thus how hard the missile had to turn to center it, but this led to increased error as the missile approached the target and the target's image became relatively larger (creating an artificially stronger signal). Switching to frequency modulation (FM) solved this problem, which is better able to discriminate the distance without being further confused by the image size.
Cooling
All-aspect seekers also tend to require cooling to give them the high degree of sensitivity required to lock onto the lower level signals coming from the front and sides of an aircraft. Background heat from inside the sensor, or the aerodynamically heated sensor window, can overpower the weak signal entering the sensor from the target. (CCDs in cameras have similar problems; they have much more "noise" at higher temperatures.) Modern all-aspect missiles like the AIM-9M Sidewinder and FIM-92 Stinger use compressed gas or liquid nitrogen to cool their sensors in order to lock onto the target at longer ranges and all aspects. (The AIM-9J used a peltier thermoelectric cooler).
Tracking
Most infra-red guided missiles have their seekers mounted on a gimbal. This allows the sensor to be pointed at the target when the missile is not. This is important for two main reasons. One is that before and during launch, the missile can't always be pointed at the target. Rather, the pilot or operator points the seeker at the target using radar, a helmet-mounted sight, an optical sight or possibly by pointing the nose of the aircraft or missile launcher directly at the target. Once the seeker sees and recognises the target, it indicates this to the operator who then typically "uncages" the seeker (which is then allowed to follow the target). After this point the seeker remains locked on the target, even if the aircraft or launching platform moves. When the weapon is launched, it may not be able to control the direction it points until the motor fires and it reaches a high enough speed for its fins to control its direction of travel. Until then, the gimballed seeker needs to be able to track the target independently.
Finally, even while it is under positive control and on its way to intercept the target, it probably won't be pointing directly at it; unless the target is moving directly toward or away from the launching platform, the shortest path to intercept the target will not be the path taken while pointing straight at it, since it is moving laterally with respect to the missile's view. The original heat-seeking missiles would simply point towards the target and chase it; this was inefficient. Newer missiles are smarter and use the gimballed seeker head combined with what's known as "proportional guidance" in order to avoid oscillation and to fly an efficient intercept path.
See also: infra-red search and track
References
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