Infrared countermeasure

An infrared countermeasure (IRCM) is a device designed to protect aircraft from infrared homing ("heat seeking") missiles by confusing the missiles' infrared guidance system so that they will miss their target (Electronic countermeasure).

First deployed during the Vietnam War, they have been enhanced over the years to be lighter, more portable, and more reliable, but the basic principle is the same.

Infrared missile seekers of the first generation typically used a spinning reticle with a pattern on it that modulates infrared energy before it falls on a detector (A mode of operation called Spin scan). The patterns used differ from seeker to seeker, but the principle is the same. By modulating the signal, the steering logic can tell where the infrared source of energy is relative to the missile direction of flight. In more recent designs the missile optics will rotate and the rotating image is projected on a stationary reticle (a mode called Conical scan) or stationary set of detectors which generates a pulsed signal which is processed by the tracking logic. Most shoulder-launched (MANPADS) systems use this type of seeker, as do many air defense systems and air-to-air missiles (for example the AIM-9L).

Infrared seekers are designed to track a strong source of infrared radiation (usually a jet engine in modern military aircraft). IRCM systems are based on modulated source of infrared radiation with a higher intensity than the target. When this modulated radiation is seen by a missile seeker, it overwhelms the modulated signal from the aircraft and provides incorrect steering cues to the missile. The missile will begin to deviate (wobble) from the target, rapidly breaking lock. Once an infrared seeker breaks lock (they typically have a field of view of 1 - 2 degrees), they rarely reacquire the target. By using flares, the target can cause the confused seeker to lock onto a new infrared source that is rapidly moving away from the true target.

The modulated radiation from the IRCM generates a false tracking command in the seeker tracking logic. The effectiveness of the IRCM is determined by the ratio of jamming intensity to the target (or signal) intensity. This ratio is usually called the J/S ratio. Another important factor is the modulation frequencies which should be close to the actual missile frequencies. For spin scan missiles the required J/S is quite low but for newer missiles the required J/S is quite high requiring a directional source of radiation (DIRCM).

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