A homing guidance system requires that a missile contain such electronic sensing and control devices that the missile will seek a target on its own without the need for command signals from outside the missile. Homing systems are classified as active, semiactive, and passive, depending upon the nature of the sensing and controlling mechanisms.

An active homing system generates and transmits a signal which is sent to the target and reflected back to the missile from the target. The guidance system utilizes this reflected signal as a beam to guide the missile to the target; it is common practice to employ a radar signal for this purpose.

In a semiactive homing system the target is "illuminated" by a signal from a source outside the missile. The illuminating signal may be transmitted from a ground station or a mother ship in flight. In any case, the missile system receives a reflected signal from the target and rides the reflected signal to the target.

The passive homing system employs some form of energy radiation from the target to serve as a beam for guidance; the energy may be in the form of heat, light, or sound.

Infrared Homing System
Among the most effective homing systems for guided missiles is the infrared sensing system used on many airborne missiles. The infrared homing system is passive, because the energy for guidance of the missile is supplied by the target.

The missile guidance system uses this energy to provide the directional signals needed to home on the target. The heat from the exhausts of a flying aircraft provides an excellent form of radiated energy which is easily detected by an infrared sensor not too distant from the source.

Infrared energy, although often called heat, is not actually heat. It would be better to state that heat causes the radiation of infrared energy and that infrared radiation will cause heat in an object exposed to it. Infrared radiation is actually an electromagnetic wave of the same type as radio waves and light
waves; it lies in the frequency range between the two.

The full range of wavelengths for infrared radiation is 0.7 to 30 microns (1 micron is one-millionth of a meter), and the range employed for infrared homing is about 1.5 to 6 microns. This is a frequency range of 200 to 50 million MHz, which may also be expressed 2 x 10' to 5 x 10' MHz.

The principal device in an infrared detector is usually a parabolic mirror which collects the infrared radiation and focuses it upon a heat-sensitive device such as a thermopile or bolometer. A thermopile consists of a group of thermocouples connected in series to increase the total voltage output, which is then amplified and used to actuate the control system of the missile.

A bolometer consists of thin metal strips (usually nickel) connected in series and arranged so so that they may be exposed to infrared radiation. The metal strips change resistance whenever there is
a change in temperature, and hence a current flowing through the unit will be affected by temperature
change. In an infrared homing system, the radiation received by the infrared detector is focused by the
parabolic mirror onto the bolometer, and the resultant signal is employed for the operation of the
guidance system.

In order to provide a directional reference, the mirror which picks up the infrared radiation must be rotated eccentrically to produce a conical scan. If the signal is equal throughout a complete rotation of the mirror, the target must be centered in the scan area.

If the target is not centered, there will be a stronger signal on one side of the scanned circle than the other. This signal will be employed in the missile computer to indicate the position of the target with respect to the missile heading and will also cause guidance commands to be sent to the missile autopilot.

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