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 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.
Read more about this topic: Infrared Homing
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