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). Principles 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 a source of infrared radiation with a higher intensity than the target. When this is received by a missile, it may overwhelm the original infrared signal from the aircraft and provide incorrect steering cues to the missile. The missile may then deviate from the target, 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). Drawbacks One of the drawbacks of standard IRCM systems is that they broadcast a bright source of infrared. If the modulation of the signal is not effective against a particular seeker system, the IRCM will enhance the ability of the missile to track the aircraft. The aircrews typically brief about potential threats and choose an IRCM modulation that will be effective against likely threats. Directional IRCM DIRCM, or Directional Infrared Countermeasures, avoid this potential drawback by mounting the energy source on a movable turret (much like a FLIR turret). They only operate when cued by a missile