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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.
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Infrared missile seeker technology
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).
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.
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 warning system of a missile launch, and use the missile plume to accurately aim at the missile seeker. The modulated signal can then be directed at the seeker, and the modulation scheme can be cycled to try to defeat a variety of seekers. Countermeasure success depend on threat's tracking techniques and requires threats' analysis capabilities. Defeating advanced tracking systems requires a higher level of DIRCM power. Issues of Laser Safety are also taken into account.
Israel has announced a program to develop a system called Multi Spectral Infrared Countermeasure (MUSIC) that will similarly use active lasers instead of flares to protect civilian aircraft against MANPADs. The US Army is deploying a similar system to protect its helicopters.
Department of the Navy Large Aircraft Countermeasures (DoN LAIRCM) by Northrop Grumman provides infrared threat protection for U.S. Marine Corps CH-53E, CH-46E and CH-53D platforms.
BAE Systems' AN/ALQ-212 advanced threat infrared countermeasures (ATIRCM) - part of a directable infrared countermeasures suite - is fielded on U.S. Army CH-47 Chinook helicopters. The suite provides protection against an array of threats, including all infrared threat bands. The AN/ALQ-212 incorporates one or more infrared jam heads to counter multiple missile attacks.
At IDEX 2013, Finmeccanica Company, Selex ES launched its Miysis DIRCM, suitable for light aircraft, smaller helicopters and UAVs.
CIRCM (Common Infrared Countermeasures)
CIRCM will be a laser based IR countermeasure against current and future IR threat systems for the US Army rotorcraft and fixed wing platforms and US Navy and US Air Force rotorcraft platforms. Currently, systems by BAE Systems, ITT Defense and Information Solutions, Northrop Grumman and Raytheon are under consideration. 
Flares create infrared targets with a much stronger signature than the aircraft's engines. The flares provide false targets that cause the missile to make incorrect steering decisions. The missile will rapidly break off a target lock-on.
Typical IRCM systems are the:
- AN/AAQ-24 by Northrop Grumman - DIRCM.
- AN/ALQ-132 by Sanders/BAE Systems. Used in the 1960s in Vietnam, and was a fuel fired flashlamp system.
- AN/ALQ-144 by BAE Systems, used for helicopter defense.
- AN/ALQ-157 by BAE Systems, used for larger helicopters and aircraft.
- AN/ALQ-212 by BAE Systems, currently fielded on U.S. Army CH-47 Chinook helicopters.
- CAMPS by Saab Avitronics, used for civilian and VIP aircraft.
- CIRCM by Northrop Grumman
- Flight Guard by Israel Aerospace Industries, used in military and civilian aircraft (gain the nickname of "Live Saver" due to history of success in saving air vehicles during battles at several countries), but banned at several European airports. According to defense sources in Israel, the European ban is "odd and based mostly on a misunderstanding
- ITT's CIRCM System
- Selex ES' Miysis System
- "Sukhogruz" - Russian DIRCM (used on Su-25T).
- KT-01 AVE and KT-02 ACE by Adron, used for military aircraft (An-26/32) and helopcters (Mi-8/17/24) defense.
- Infrared signature
- Flare (countermeasure)
- Chaff (radar countermeasure)
- Electronic countermeasures
- Anti-ballistic missile
- The Infrared & Electro-Optical Systems Handbook. Countermeasure Systems, Volume 7
- Evaluating Airliner MANPADS Protection
- "Israel to Fund System to Protect Commercial Aircraft from Missiles". defensetalk.com. 2007-10-12.
- Soldier deploys to Iraq to help field laser defense system for Chinook helicopters
- Northrop Grumman to Provide Infrared Missile Warning Systems and Processors to the U.S. Navy for Heavy Lift Helicopter Fleets
- FedBizOpps - CIRCM RFI
- "Europe objects to El Al's anti-missile shield". Ynetnews.