Active Defense Systems
As the threat from Soviet manned bombers steadily declined, U.S. air defenses were correspondingly reduced. Almost all surface-to-air missile batteries have been disbanded or relocated abroad, and the U.S. interceptor force fell from about 40 squadrons in 1964 to 5 in the mid-1980s (augmented by Air National Guard and tactical air units as available). Belts of radar stations, such as the Distant Early Warning (DEW) Line in northern Canada, still provide notice of a conventional air attack, while the newer Joint Surveillance System (JSS) coordinates the surveillance and tracking of all objects in North American airspace through eight regional control centers. The JSS is operated jointly by the U.S. Air Force and the Federal Aviation Administration for both air traffic control and air defense. One significant addition to air defense has been the E-3A Airborne Warning and Control System (AWACS) aircraft, in effect a flying radar station.
The United States has no active antiballistic missile (ABM) system in operation. The ABM treaties of 1972 and 1974 allowed limited deployment of such systems in the United States and the Soviet Union, but neither side took full advantage of those provisions. In fact, the U.S. Safeguard ABM system at Grand Forks, North Dakota, protecting the PARCS radar site, was closed down. The Soviet Golash ABM missiles, which were located around Moscow, had only limited capabilities. While American ABMs increased the protection of radar and ICBM sites, the parallel deployment of ABMs by the Soviet Union tended to reduce any defensive advantage, and in both countries their construction, maintenance, and continued improvement would have proved extremely costly.
In the early 1980s the United States considered reopening its ABM site and employing new, highly accurate impact-type missiles to replace the nuclear-tipped versions used by the Safeguard system; however, development of the MX Peacekeeper and of the Strategic Defense Initiative took precedence. The most active U.S. defensive systems are those employed by the U.S. Navy to detect and destroy ballistic-missile-launching submarines. Ship-mounted sonars on destroyers and frigates are now supplemented by towed, underwater sonar arrays that disregard distorting thermal currents and eliminate false reverberations and other clutter. Shipborne helicopters capable of "dunking" small sonar sets, expendable sonar buoys, and high-speed, sonar-equipped submarines also enhance detecting capabilities.
Sensitive shipborne and airborne radars can locate small objects such as periscopes on the surface of the sea, and aircraft equipped with magnetic anomaly detectors (MAD) are able to sense minute variations in the earth's magnetic field caused by large metal objects such as underwater vessels. Once detected, submarines can be attacked with a variety of weapons, from nuclear depth charges delivered by missiles to wire-guided torpedoes launched from ships or helicopters. Without adequate warning of an impending attack, however, these forces could do little to stop such submarines from launching a first-strike missile attack.