Countermeasure
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An expendable countermeasure is a body (usually for a military application) designed to prevent sensor-based weapons from acquiring and/or destroying a target. Preemptive release of expendable countermeasures is based on altering the signature of the target by either concealing the platform signature, enhancing the signature of the background and thus minimizing the contrast of the scene.
Reactive release of expendables is based on enhancing the number of likely targets within the field-of-view (FOV).
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[edit] Aerial countermeasures
Generally one has to distinguish between Infrared and Radar countermeasures. As infrared (IR) the wavelength range between 0.8 - 5 µm is considered. As Radar the frequency range between 2 - 18 GHz is considered.
In the wake of missile attacks against civilian passenger and cargo airliners in the early 2000's, various agencies investigated the feasibility of equipping countermeasures chaff and flares. Many commercial carriers found the estimated price of countermeasures to be too costly. However, the Israeli airline El-Al, having been the target of a failed missile attack in Mombassa, Kenya in 2002, began equipping its fleet with radar-based, automated flare release countermeasures from June 2004<ref>Missile defense for El Al fleet, CNN, May 24, 2004. Accessed July 18, 2006.</ref>. This caused concerns in some European countries, which proceeded to ban such aircraft from landing at their airports<ref>Europe objects to El Al's anti-missile shield, Ynetnews, Feb 26, 2006. Accessed July 18, 2006.</ref>.
[edit] IR-decoy flares
IR-decoy flares serve to counter infrared-guided surface-to-air missiles (SAM) or air-to-air missiles (AAM) and can be expelled from a craft according to an anticipated threat in defined sequences.
As stored chemical energy source IR-decoy flares contain either pyrotechnic compositions, liquid or solid pyrophoric substances and/or liquid or solid highly flammable substances.
Upon ignition of the decoy flare a strongly exothermal reaction is started giving rise to release of infrared energy and distinct visible smoke and flame emission being dependent on the chemical nature of the payload used.
[edit] Pyrotechnic Payloads
Certain pyrotechnic compositions as e.g.Magnesium/Teflon/Viton (MTV) upon combustion give a great flame emission and yield a temperature dependent signature and can be understood as Grey bodies of high emissivity (e~0.95). Those payloads are designated blackbody payloads. Other payloads as e.g. iron/potassium perchlorate pellets only yield a low flame emission but show also temperature dependent signature <ref> J. Callaway, Expendable Infrared Radiating means, GB Patent 2 387 430, 2003, GB.</ref>. . Nevertheless the lower combustion temperature as compared to MTV gives rise to a lower amount of energy released in the short wavelength range of the IR.
Now other payloads provide large amounts of hot carbon dioxide upon combustion and thus provide a temperature independent, selective emission in the wavelength range between 3 - 5 µm. Typical pyrotechnic payloads of this type resemble whistling compositions and are made up from potassium perchlorate and hydrogen lean organic fuels <ref>J. Callaway, T. D. Sutlief, Infrared Emitting Decoy Flare, US Patent Application 2004/0011235 A1, 2004, GB. </ref>.
[edit] Pyrophoric Payloads
In contrary to pyrotechnic payloads pyrophoric substances use the oxygen from the environment for oxidation. Hence specific energy density of pyrophorics is always higher as compared to any pyrotechnic. Nevertheless pyrophorics always suffer from low oxygen opartial pressure at great heights. A typical liquid pyrophoric fuels is e.g. triethylaluminum(TEA). Upon combustion of TEA a selective IR spectrum is obtained which is mainly determined from carbon dioxide and water vapour. Any transient or permanent combustion products of aluminum are not IR-active in this region of the electromagnetic spectrum <ref> [1]D. B. Ebeoglu, C. W. Martin, The Infrared Signature of Pyrophorics, AD921319, National Technical Information Service, May 1974. </ref>.
Solid pyrophoric payloads are based on iron platelets coated with a porous aluminium layer. Based on the very high specific surface area of aluminum those platelets instantaneously oxidize upon contact with the air. In contrary to TEA combustion, those platelets yield a temperature dependent signature.
[edit] Highly flammable Payloads
These payloads now contain red phosphorus (RP) als energetic filler. The RP is mixed with organic binders to give brushable pastes that can be coated on thin polyimide platelets. The combustion lof those platelets yields a temperature dependent signature. Endergonic additives such as highly dispersed silica or alkali halides may further lower the combustion temperature<ref> H. Bannasch, M. Wegscheider, M. Fegg, H. Büsel, Spektrale Scheinzielanpassung und dazu verwendbare Flarewirkmasse, WO 95/05572, 1995, D. </ref>.
[edit] Radar-decoys
To counter radar guided missiles chaff is used. These are aluminum coated glass fibers or silver coated nylon fibers having lengths equal to half of the wavelength of the anticipated radar wavelength
[edit] Naval decoys
Land and sea-based forces can also use such countermeasures as well as smoke-screens that can disrupt laser ranging, infrared detection, laser weapons, and visual observation.
[edit] Intercontinental Ballistic Missiles (ICBMS)
Countermeasures are a complicating factor in the development anti-ballistic missile defense systems targeting ICBM's. Like aircraft, ICBM's theoretically could evade such systems by deploying decoys and chaff in the midcourse phase of flight. Novel proposed chaff mechanisms describe the creation of a "threat cloud" by deploying of large alluminized PET film balloon which could conceal a warhead among a large number of inert objects having similar radar profiles.
[edit] References
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