A number of early-twentieth-century scientists and engineers recognized the potential for radio-based detection systems. Radio direction-finding and distance-measuring experiments were conducted during World War I and in the 1920s. From these early efforts, several theoretical radar systems were proposed before World War II. Between 1934 and 1940, practical radar systems were developed independently in several countries as military instruments for detecting aircraft and ships. One of the first practical radar systems was devised in 1935 by Scottish physicist Sir Robert Watson-Watt. His success with this early system can be attributed to the fact that a number of critical technical components became available during the 1930s; it was Watson-Watt who integrated transmitters and receivers, modulators capable of generating microsecond pulses, and high-speed cathode-ray tubes to display search results. Much of this equipment was the by-product of civilian work on broadcast television. By the late 1930s, laboratories in Britain, the United States, Germany, France, Italy, and the Soviet Union had all begun radar experiments on a modest scale. Japan did not take notice until 1941 but then hurried to catch up. Thus, all of the principal belligerents in World War II entered the conflict with some radar technology.
At the insistence of Air Vice Marshal Hugh Dowding, Great Britain adopted radar in the late 1930s to augment the defenses of the home islands. Before the start of the war, Great Britain began construction of the Chain Home (CH) radar network, which was enhanced in 1939 with a number of Chain Home Low (CHL) stations, capable of detecting low-flying aircraft approaching the English coast. The CH and CHL stations were the first integrated radar defense system, staffed by crews who were trained to track incoming aircraft and relay tactical information to air defense control centers and air bases. The CH/CHL system played an important role during the Battle of Britain in 1940, giving British defenders valuable advance warning of the relentless attacks launched by the German Luftwaffe.
In September 1940, Britain provided the United States with examples of key radar components, including a magnetron, with the understanding that cooperation would lead to the further development of radar technology. The Americans moved quickly. The Radiation Laboratory was established in 1940 at the Massachusetts Institute of Technology under the leadership of Lee DuBridge. Other emergency radar research programs were created in 1940 and 1941, and the close cooperation of Bell Laboratories, the Army Signal Corps, and the Naval Research Laboratory led to substantial improvements in the accuracy and range of radar equipment. High-power microwave radar systems were among the important advances made by the United States.
Although Germany had the opportunity to exploit radar technology before World War II began, the indifference of the country's political leadership hindered the development of the technology. Before the war, Adolf Hitler and Hermann Göring, chief of the Luftwaffe, were reportedly suspicious of radar's utility and antagonistic to the idea of adopting the technology as a defensive weapon. Later, interservice rivalries within Germany slowed the development of radar even further as resources for research and development became scarce. Germany did finally put into operation the Kammhuber Line, an interlocking system of radar, aircraft, and ground controllers that ran from the North Sea to southern France and went into full operation in September 1942. It was similar to the Chain Home system in scale and purpose, but the Allied use of long-range fighter escorts for bombers limited its value. Italy had a limited radar capability on land and at sea by the middle of 1942. It utilized its own equipment and that of Germany.
The Japanese were late in developing and adopting radar technology, a fact that greatly advantaged the Allies in the Pacific Theater fighting. However, though U.S. radar identified the Japanese aircraft approaching Pearl Harbor on 7 December 1941, the information was not utilized. Radar provided early warning of subsequent Japanese air strikes as well—for example, during the decisive Battle of Midway in June 1942, where the Japanese lost four aircraft carriers. Throughout World War II, continuous improvements to radar technology increased the accuracy of the U.S. Navy's tracking and intercept capabilities.
By 1943, thanks to aggressive research and development, the Allies had a wide variety of radar systems at their disposal. The technology evolved rapidly during the war, and specialized radar units were developed for early warning, battle management, airborne search, night interception, bombing, and gun aiming. Experiments with terrain-following radar for aircraft presaged the enhanced electronic avionics developed for jets and helicopters after the war. Air defense radar systems, which came to include gun-direction devices, proximity fuses, and increasingly accurate direction-finding and ranging capabilities, had greatly enhanced the accuracy of antiaircraft fire by the end of the war.
In the postwar years, as missiles replaced artillery as the backbone of air defense, radar technology improved still further, and radar systems were adapted for a number of useful purposes. As the systems became even more powerful and sensitive, radar was used in navigation, meteorology, and astronomy (the first radar emissions were reflected back from the moon in 1946). Shannon A. Brown
Brown, Louis. A Radar History of World War II: Technical and Military Imperatives. Philadelphia: Institute of Physics Press, 1999.; Buderi, Robert. The Invention That Changed the World. New York: Simon and Schuster, 1999.; Von Kroge, Harry. Gema: Birthplace of German Radar and Sonar. Philadelphia: Institute of Physics Press, 2000.
Shannon A. Brown