AIM-9M conscan seeker with MgF 2 nose window ( © 1999 Carlo Kopp).


Of all the air to air weaponry developed in the latter half of the 20th century, the air to air guided missile has probably had the greatest impact, affecting the design of weapon systems, airframes, propulsion and often leading to a complete reassessment of combat tactics. Air to air missiles (AAMs) differ principally in guidance, the two broad groups being radar guided and heat-seeking or infra-red (IR) missiles. Of the two categories, the second, by virtue of it's simplicity and lower demands on launch aircraft complexity, has become the numerically superior, arming high performance fighters like the F-15 or F-14 operated by frontline air forces, yet also equipping vintage 1950's fighters, defending Third World countries. The initial deployment of heat seeking missiles began in the late 1950's, with the USAF acquiring it's first AIM-4 Falcons and AIM-9 Sidewinders, however, it was not until the Viet-Nam war that the AIM-9 saw widespread use. The weapon was not as successful as expected, reliability was a particular problem, especially with the USN (repeated carrier launches and recoveries - quote ''banging them on the deck after every flight''), but when the missile did work, it was effective, which may be confirmed by a number of NVAF pilots who had the unique experience of a 'Winder entering the tailpipe of their sturdy Mikoyan, resulting in it's subsequent bisection. Current versions of the AIM-9 are a vast improvement, though they are to be replaced by the end of this decade by the ASRAAM - solid state electronics allow for higher reliability and enable the guidance to be ''smarter'' in between discriminating targets and resisting jamming. As their name implies, heat-seeking missiles home on to the hot areas of a target. The target will usually both reflect and emit infra-red radiation, which propagates through the atmosphere, losing it's intensity due to number of effects. This radiation is detected by the missile's seeker head, which, if the conditions are right, will then provide the guidance with the relative position of the target enabling the weapon to home in and destroy the target. In order to fully appreciate the problems involved in creating an effective weapon of this kind, we must examine the behaviour of infra-red radiation, the characteristics of an aircraft as a source of IR energy, the manner in which this energy travels through the atmosphere and finally, how the missile seeker processes it to gain information as to the target's position. Infra-Red Radiation The infra-red is a term used to describe a particular group of electromagnetic waves, those which are longer than visible wavelengths and shorter than microwaves, numerically the band between 0.8 and 1000.0 micrometres. This means that infra-red radiation has very many properties similar to visible light, it can be focussed or diffused, absorbed or reflected. The reason why the IR is so important is that it may be closely associated with heat and it's transfer from bodies. When we heat an object, or a volume of material, we are feeding energy into it - at an atomic level this energy is in the form of crystal lattice vibrations (probably the best way of visualizing the process is to picture an infinite lattice of tiny balls, each connected to it's neighbours by a spring, imagine then what occurs if we tap one of the balls) every atom vibrating about some equilibrium position in the lattice. To make things more complicated, in the physical world, the amount of energy each atom can transfer to it's neighbour is limited to an amount hf (h is Planck's constant and f is the frequency of vibration) which leads to an interesting result - a heated body emits radiation throughout a continuous band of wavelengths, the relative amplitudes (levels) of each wavelength depending on the body's temperature. Graph 1. illustrates this relationship. This means that every object radiates energy, the dominant wavelengths depending on the temperature. As it turns out, objects at temperatures around and below 1200 C radiate mainly in the infra-red band, e in the higher the temperature, the shorter the dominant wavelengths. (note: IR radiation can also be generated by exciting a molecular gas, as the frequencies at which the molecules rotate and vibrate fall into the IR band - this type of radiation forms narrow bands in the spectrum -see TE Dec.1981, Lasers). Sources of IR energy The easiest answer to that question is - any warm or hot object- however, the subject deserves a little more attention. The single greatest source of IR radiation is the sun, however only part of this energy would be available to illuminate the Earth's surface, as large amounts are absorbed and scattered in the atmosphere, particularly by clouds and moisture. The Earth's surface becomes a secondary source of IR, as it is being bombarded