Early infrared seekers were most effective in detecting infrared radiation with shorter wavelengths, such as the 4.2 micrometre emissions of the carbon dioxide efflux of a jet engine. Such seekers, which are most sensitive to the 3 to 5 micrometre range, are now called "single-color" seekers. Modern infrared seekers also operate in the 8 to 13 micrometre wavelength range, which is absorbed least by the atmosphere. Such seekers are called "two-color" systems. Two-color seekers are harder to defeat with countermeasures such as flares and jammers.
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 infrared (IIR), where the IR/UV sensor is a focal plane array 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 infrared 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.
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).
Most infrared 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.
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