Radar

V

History

Although British physicist James Clerk Maxwell predicted the existence of radio waves in the 1860s, it wasn’t until the 1890s that British-born American inventor Elihu Thomson and German physicist Heinrich Hertz independently confirmed their existence. Scientists soon realized that radio waves could bounce off of objects, and by 1904 Christian Hülsmeyer, a German inventor, had used radio waves in a collision avoidance device for ships. Hülsmeyer’s system was only effective for a range of about 1.5 km (about 1 mi). The first long-range radar systems were not developed until the 1920s. In 1922 Italian radio pioneer Guglielmo Marconi demonstrated a low-frequency (60 MHz) radar system. In 1924 English physicist Edward Appleton and his graduate student from New Zealand, Miles Barnett, proved the existence of the ionosphere, an electrically charged upper layer of the atmosphere, by reflecting radio waves off of it. Scientists at the U.S. Naval Research Laboratory in Washington, D.C., became the first to use radar to detect aircraft in 1930.

A

Radar in World War II

None of the early demonstrations of radar generated much enthusiasm. The commercial and military value of radar did not become readily apparent until the mid-1930s. Before World War II, the United States, France, and the United Kingdom were all carrying out radar research. Beginning in 1935, the British built a network of ground-based aircraft detection radar, called Chain Home, under the direction of Sir Robert Watson-Watt. Chain Home was fully operational from 1938 until the end of World War II in 1945 and was extremely instrumental in Britain’s defense against German bombers.

The British recognized the value of radar with frequencies much higher than the radio waves used for most systems. A breakthrough in radar technology came in 1939 when two British scientists, physicist Henry Boot and biophysicist John Randall, developed the resonant-cavity magnetron. This device generates high-frequency radio pulses with a large amount of power, and it made the development of microwave radar possible. Also in 1939, the Massachusetts Institute of Technology (MIT) Radiation Laboratory was formed in Cambridge, Massachusetts, bringing together U.S. and British radar research. In March 1942 scientists demonstrated the detection of ships from the air. This technology became the basis of antiship and antisubmarine radar for the U.S. Navy.

The U.S. Army operated air surveillance radar at the start of World War II. The army also used early forms of radar to direct antiaircraft guns. Initially the radar systems were used to aim searchlights so the soldier aiming the gun could see where to fire, but the systems evolved into fire-control radar that aimed the guns automatically.

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B

Radar during the Cold War

With the end of World War II, interest in radar development declined. Some experiments continued, however; for instance, in 1946 the U.S. Army Signal Corps bounced radar signals off of the moon, ushering in the field of radar astronomy. The growing hostility between the United States and the Union of Soviet Socialists Republics—the so-called Cold War—renewed military interest in radar improvements. After the Soviets detonated their first atomic bomb in 1949, interest in radar development, especially for air defense, surged. Major programs included the installation of the Distant Early Warning (DEW) network of long-range radar across the northern reaches of North America to warn against bomber attacks. As the potential threat of attack by ICBMs increased, the United Kingdom, Greenland, and Alaska installed the Ballistic Missile Early Warning System (BMEWS).

C

Modern Radar

Radar found many applications in civilian and military life and became more sophisticated and specialized for each application. The use of radar in air traffic control grew quickly during the Cold War, especially with the jump in air traffic that occurred in the 1960s. Today almost all commercial and private aircraft have transponders. Transponders send out radar signals encoded with information about an aircraft and its flight that other aircraft and air traffic controllers can use. American traffic engineer John Barker discovered in 1947 that moving automobiles would reflect radar waves, which could be analyzed to determine the car’s speed. Police began using traffic radar in the 1950s, and the accuracy of traffic radar has increased markedly since the 1980s.

Doppler radar came into use in the 1960s and was first dedicated to weather forecasting in the 1970s. In the 1990s the United States had a nationwide network of more than 130 Doppler radar stations to help meteorologists track weather patterns.

Earth-observing satellites such as those in the SEASAT program began to use radar to measure the topography of the earth in the late 1970s. The Magellan spacecraft mapped most of the surface of the planet Venus in the 1990s. The Cassini spacecraft, scheduled to reach Saturn in 2004, carries radar instruments for studying the surface of Saturn’s moon Titan.

As radar continues to improve, so does the technology for evading radar. Stealth aircraft feature radar-absorbing coatings and deceptive shapes to reduce the possibility of radar detection. The Lockheed F-117A, first flown in 1981, and the Northrop , first flown in 1989, are two of the latest additions to the U.S. stealth aircraft fleet. In the area of civilian radar avoidance, companies are introducing increasingly sophisticated radar detectors, designed to warn motorists of police using traffic radar.

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