Defense, Air, measures taken to guard against airborne enemy attack. The main elements in air defense are (1) detection, (2) identification, (3) interception, and (4) destruction.

Defense of the North American continent against air attack is handled by two agencies—the U.S. Space Command (USSC), a unified command of the United States armed forces, and the North American Aerospace Defense Command (NORAD), operated jointly by the United States and Canada.

The USSC provides a single operational structure for strategic defense against longrange missile attack aimed at the United States. It has headquarters at Peterson Air Force Base, near Colorado Springs, Colorado.

NORAD is designed to provide a joint defense by the United States and Canada against air attack from both intercontinental missiles and bombers directed against either nation or both nations. NORAD's headquarters are at Colorado Springs, and its command center is in a complex of buildings located inside nearby Cheyenne Mountain.

The United States and Canada operate a number of surveillance systems designed to provide early warning of an incoming air attack. The most important are as follows:

Airborne Warning and Control System

(AWACS) consists of a force of radar-equipped aircraft that patrol off the coasts of North America. The system is designed both to provide early warning of the presence of hostile aircraft and missiles and to direct defensive weapons against them.

Ballistic Missile Early Warning System

(BMEWS) is composed of huge radar sites at Thule, Greenland; Clear, Alaska; and Fylingdales Moor, England. They can detect a land-based intercontinental missile as far as 3,000 miles (4,800 km) away. This system provides a minimum warning time of about 15 minutes.

Joint Surveillance System

consists of about 50 radar installations operated by the United States and about 25 operated by Canada. The installations detect all airborne man-made objects within 200 miles (320 km) of the coast of North America.

North Warning System

(NWS) consists of 15 radar stations along a 5,000-mile (8,000-km) span from Alaska to Greenland roughly along the 70th parallel. It is designed to detect approaching, possibly hostile aircraft.

Pave Paws

consists of four phased-array radar sites—in Massachusetts, Georgia, Texas, and California—for the detection and tracking of submarine-launched ballistic missiles.

Satellite Early-warning System

(SES) consists of three satellites, one that surveys the Eastern Hemisphere and two that survey the Western Hemisphere. Their purpose is to detect launchings of strategic ballistic missiles from submarines or land bases.

Space Surveillance Network

(SSN) uses a worldwide network of sensors to relay information on artificial satellites to the Space Defense Center at NORAD headquarters. The center tracks, identifies, and catalogs the satellites.

Until after World War II, the United States had little need to develop an air defense system. The Cold War between the Western powers and the Soviet Union, which had long-range bombers and nuclear weapons, brought a demand for development of an air defense system in North America.

To meet the threat of Soviet bombers, the United States and Canada began coordinating their air force interceptor units and built three lines of radar stations across Canada—the Pinetree Line in 1954, the Mid-Canada Line in 1957, and the Distant Early Warning (DEW) Line, also in 1957. (These have since been discontinued.) NORAD was established in 1957.

BMEWS, completed in 1963, was built to provide early warning of Soviet nuclear ballistic missiles. Other warning systems went into operation during the next three decades. In 1991, the same year that the North Warning System replaced the DEW Line, the Soviet Union collapsed. Air defense was deemphasized and planned new systems were cancelled or scaled down.