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Systems typically employ Anti-ballistic Missile s; non-missile defences such as Airborne Lasers are under active development. In the United States, missile defense was originally the responsibility of the Army , but in recent years the Navy and Air Force have developed their own antimissile systems. MISSILE DEFENSE CATEGORIES Missile Defense can be divided into categories based on various characteristics: type/range of missile intercepted, the trajectory phase where the intercept occurs, and whether intercepted inside or outside earth's atmosphere: Classified by type/range of missile intercepted The types/ranges are strategic, theater and tactical. Each entails unique requirements for intercept, and a defensive system capable of intercepting one missile type frequently cannot intercept others; however there is sometimes overlap in capability.
Classified by trajectory phase Ballistic missiles can be intercepted in Three Regions Of Their Trajectory : boost phase, midcourse phase or terminal phase.
Classified by intercept location relative to the atmosphere Missile defense can take place either inside (endoatmospheric) or outside (exoatmospheric) the earth's atmosphere. The trajectory of most ballistic missiles takes them inside and outside the earth's atmosphere, and they can be intercepted either place. There are advantages and disadvantages to either intercept technique.
Some missiles such as THAAD can intercept both inside and outside the earth's atmosphere, giving two intercept opportunities. HISTORY In the 1950s and 1960s , the term meant defense against strategic (usually nuclear-armed) missiles. The technology mostly centered around detecting offensive launch events and tracking in-bound ballistic missiles, but with limited ability to actually defend against the missile. The Soviet Union achieved the first nonnuclear intercept of a ballistic missile warhead by a missile at the Saryshagan antiballistic missile defense test range on 4 March 1961.[http://astronauticsnow.com/history/saryshagan/ In the late 1960s, technology had matured to the point to allow limited rudimentary missile defense by using Anti-ballistic Missile s. At first, these anti-ballistic missiles were armed with their own Nuclear Weapons , since precision technology (to "hit-a-bullet-with-a-bullet") did not exist. Laser s were considered for shooting down the warheads, but various problems including atmospheric interference and warhead hardness impeded this effort. Throughout the 1950s and 1960s, the United States Project Nike air defense program focused initially on bombers, then ballistic missiles. In the 1950s, the first United States anti-ballistic missile system was the Nike Hercules , which had a limited ability to intercept incoming ballistic missiles, although not ICBMs. This was followed by Nike Zeus , which using a nuclear warhead could intercept ICBMs. However Nike Zeus had other limitations which prevented it being deployed. In any case, by the early 1960s the Nike Zeus was the first anti-ballistic missile to achieve hit-to-kill (physically colliding with the incoming warhead). The Zeus missile was enhanced, and the shorter range Sprint Missile was added to the Nike defense system, then called Nike-X. The system included large powerful radars and a computer complex. Eventually, the Nike-X program was realigned and renamed Sentinel . This program's goal was to protect major U.S. cities from a limited ICBM attack, especially focusing on China . This in turn reduced tensions with the Soviet Union, which retained the offensive capability to overwhelm any U.S. defense. The Soviet Union deployed the A-35 Anti-ballistic Missile System around Moscow in 1966, which also defended nearby ICBM sites. That system has been upgraded several times and is still operational. The United States announced an ABM program to protect twelve ICBM sites in 1967. In 1967, then-Secretary of Defense Robert McNamara stated: ''"Let me emphasize -- and I cannot do so too strongly -- that our decision to go ahead with a limited ABM deployment in no way indicates that we feel an agreement with the Soviet Union on the limitation of strategic nuclear offensive and defensive forces is in any way less urgent or desirable."'' The SALT I talks began in 1969, and led to the Anti-Ballistic Missile Treaty in 1972, which ultimately limited the U.S. and U.S.S.R. to one defensive missile site each, with no more than 100 missiles per site. As a result of the treaty and of technical limitations, along with public opposition to nearby nuclear-armed defensive missiles, the U.S. Sentinel program was redesignated the Safeguard Program , with the new goal of defending U.S. ICBM sites, not cities. The U.S. Safeguard system was deployed to defend the Minuteman ICBMs near Grand Forks, North Dakota. It was deactivated in 1976 after being operational for less than four months due to a changing political climate plus concern over limited effectiveness, low strategic value, and high operational cost. The end of the cold war In the early 1980s , technology had matured to consider space based missile defense options. Precision hit-to-kill systems more reliable than the early Nike Zeus were thought possible. With these improvements, the Reagan Administration promoted the Strategic Defense Initiative , an ambitious plan to provide a comprehensive defense against an all-out ICBM attack. See National Missile Defense for additional details. In the early 1990s, missile defense expanded to include tactical missile defense, as seen in the first Gulf War . Although not designed from the outset to intercept tactical missiles, upgrades gave the Patriot system a limited missile defense capability. The effectiveness of Patriot in disabling or destroying incoming Scud s was the subject of Congressional hearings and reports in 1992 {Link without Title} . In the late 1990s, and early 2000s, the issue of defense against Cruise Missile s became more prominent with the new Bush Administration . In 2002, President George W. Bush withdrew the US from the Anti-Ballistic Missile Treaty , allowing further development and testing of ABMs under the Missile Defense Agency , and allowing for deployment of interceptor vehicles beyond the single site allowed under the treaty. There are still technological hurdles to an effective defense against ballistic missile attack. The United States 's and Theodore Postol 's criticisms about the technical feasibility of these sensors have lead to an ongoing investigation of research misconduct and fraud at the Massachusetts Institute Of Technology {Link without Title} . As of February 2007, the U.S. missile defense system consists of 13 ground-based interceptors at Ft Greely in Alaska, plus two interceptors at Vandenberg AFB, California. The U.S. plans to have 21 interceptor missiles by the end of 2007 {Link without Title} . The system was initially called National Missile Defense (NMD), but in 2003 the ground-based component was renamed Ground-Based Midcourse Defense (GMD). Defending against Cruise Missile s is similar to defending against hostile, low-flying manned aircraft. As with aircraft defense, countermeasures such as Chaff , flares, and low altitude can complicate targeting and missile interception. High-flying radar aircraft such as AWACS can often identify low flying threats by using Doppler Radar . Another possible method is using specialized satellites to track these targets. By coupling a target's kinetic inputs with Infrared and radar signatures it may be possible to overcome the countermeasures. WELL-KNOWN DEFENSE SYSTEMS AND INITIATIVES
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