Infrared homing refers to a passive weapon guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track and follow it (imaging infrared IIR). Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared (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 infrared wavelengths of light compared to objects in the background.
The infrared sensor package on the tip or head of a heat-seeking missile is known as the seeker head. The NATO brevity code for an air-to-air infrared-guided missile launch is Fox Two. 90% of all United States air combat losses over the past 25 years have been due to infrared-homing missiles.
The three main materials used in the infrared sensor are lead(II) sulfide (PbS), indium antimonide (InSb) and mercury cadmium 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.
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 micrometer 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.
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 so-called reticle seekers where the transparent plate with a sequence of transparent and opaque segments was placed in front of the IR detector element. First reticles used a simple spokewheel pattern but later more complex patterns were used to improve accuracy, resolution and seeker's immunity to background IR radiation and countermeasures. The purpose of this reticle is a modulation of the scene image to determine how far off-center the target is. The position and line of sight rate of change is then used by the seeker to align itself with the target. The same signal is also amplified and used to steer the missile towards the target.
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. By using the advanced image processing techniques, the target shape can be used to find its most vulnerable part toward which the missile is then steered. All western Short-range air-to-air missile such as AIM-9X Sidewinder and ASRAAM, Chinese PL-10 SRAAM and Israeli Python-5 use imaging infrared seekers, while Russian R-73 still uses infrared seeker.
Before imaging infrared sensors there was also the question of sensor modulation. Earlier seekers used spin scanning with the rotating reticle which produced amplitude modulation (AM) of the incoming signal 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). This modulation method is also very susceptible to IR decoys. Switching to conical scanning which led to frequency modulation (FM) of the signal solved this problem, which is better able to discriminate the distance without being further confused by the image size. A variant of this scanning technique is so-called crossed array seeker where a linear array of detectors alements are arranged in a cross formation while the target scene is scanned conically across the detector arrays. There have also been missiles built using so-called rosette scanning method which didn't use the reticle but used the optical arrangement to scan the field of view in a rosette pattern. This method is a step between earlier reticle seekers and modern imaging seekers and is still widely used by some of the MANPADS.
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 like argon to cool their sensors in order to lock onto the target at longer ranges and all aspects. (Some such as the AIM-9J and early-model R-60 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 cannot 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 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 will not 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 is known as proportional guidance in order to avoid oscillation and to fly an efficient intercept path.
- Infrared countermeasures
- Directional Infrared Counter Measures
- infra-red search and track
- AIM-9 Sidewinder One of the earliest of this type and originally quite simple
- MULTISERVICE AIR-AIR, AIR-SURFACE, SURFACE-AIR BREVITY CODES (PDF), Air Land Sea Application (ALSA) Center, 1997, p. 6, retrieved 2008-02-23
- $96M to DS2 for LAIRCM Aircraft Defense System Support