RADAR

RADAR, an acronym for "radio detection and ranging," is a method of locating distant targets by sending bursts of electromagnetic radiation and measuring their reflections. In the most common method, ultrashort radio waves are beamed toward the target by a scanning antenna. The resulting echoes are then displayed on a cathode-ray tube by means of a scanning signal synchronized with the antenna, so that the echo from each target appears as an illuminated dot, in the appropriate direction and at a proportional distance, on a map of the entire area being scanned. In other versions, continuous waves are used, and, in some, only moving targets are revealed (for example, in police sets used to detect speeding vehicles).

The science behind radar dates to the 1920s, when radio operators noticed perturbations caused by obstacles moving in a radio field. Such effects were familiar to both amateur and professional radio enthusiasts in many countries and were at first freely discussed in engineering journals. As the military significance of these observations dawned on researchers in government laboratories in the 1930s, such references grew rarer. Two American reports, in particular, helped shape the nascent science of radio detection: a 1933 report (by C. R. Englund and others in the Proceedings of the Institute of Radio Engineers) describing a systematic investigation of the interferences caused by overflying aircraft and a 1936 report (by C. W. Rice in the General Electric Review) on the uses of ultrahigh-frequency

equipment, among which was listed "radio-echo location for navigation."

The first innovations came from the commercial sector. Radio altimeters were developed to gauge the altitude of planes; experimental equipment intended to prevent collisions was installed on the French Line's giant ship Normandie, producing considerable publicity but only moderate success. Scientists, as well, found applications for these early forms of radar technology. They used radio detection to locate storms, measure the height of the ionosphere, and survey rugged terrain. Essential technologies evolved from these experiments, such as ultrahighfrequency (microwave) tubes, circuits, and antennas; cathode-ray (picture) display tubes; and wide-band receivers capable of amplifying and resolving extremely short pulses of one-millionth of one second (microsecond) or less.

As WORLD WAR II approached, military laboratories in several countries rushed to develop systems capable of locating unseen enemy ships and aircraft. Such a capability, military planners knew, would provide enormous tactical advantages on sea and in the air. Six countries led the race—the United States, Great Britain, France, Germany, Italy, and Japan—but there were doubtless others, including Canada, the Netherlands, and the Soviet Union. Great Britain made the swiftest progress before the outbreak of the war. A team assembled by the engineer Robert Watson-Watt devised a system of radar stations and backup information-processing centers. This complex was partly in place when war broke out in September 1939 and was rapidly extended to cover most of the eastern and southern coasts of England. By the time of the air Battle of Britain a year later, the system was fully operational. The British radar system is credited with swinging the balance in the defenders' favor by enabling them to optimize their dwindling air reserves.

American military developments had started even earlier, in the early 1930s, and were carried on at fairly low priority at the Naval Research Laboratory under R. M. Page and at the army's Signal Corps laboratories under W. D. Hershberger. By the time the United States entered the war, radar had been installed on several capital warships and in a number of critical shore installations. Indeed, a radar post in the hills above Pearl Harbor spotted the Japanese attack in December 1941, but the backup system was not in place and the warning did not reach the main forces in time. American forces in the Pacific quickly corrected this situation, and radar played a significant role six months later in the pivotal victory over a Japanese naval force at Midway Island.

British researchers had not been idle in the meantime. Great Britain made a great step forward with the invention of a high-power magnetron, a vacuum tube that, by enabling the use of even shorter centimetric wavelengths, improved resolution and reduced the size of the equipment. Even before the attack on Pearl Harbor, a British delegation led by Sir Henry Tizard had brought a number of devices, including the centimetric magnetron, to the United States in an effort to enroll U.S. industry in the war effort, since British industry was already strained to full capacity. The resulting agreement was not entirely one-sided, since it placed some American developments at the Allies' disposal: for instance, the transmit-receive (TR) tube, a switching device that made it possible for a single antenna to be used alternately for radar transmission and reception. From then on until the end of the war, British and U.S. radar developments were joined, and the resulting equipment was largely interchangeable between the forces of the two nations.

The principal U.S. radar research laboratories were the Radiation Laboratory at the Massachusetts Institute of Technology (MIT), directed by Lee Du Bridge, where major contributions to the development of centimetric radar (including sophisticated airborne equipment) were made; and the smaller Radio Research Laboratory at Harvard University, directed by F. E. Terman, which specialized in electronic countermeasures (i.e., methods of rendering enemy's radar ineffective and overcoming its countermeasures). The MIT group produced an elaborate and detailed twenty-eight-volume series of books during the late 1940s that established a solid foundation for worldwide radar developments for several decades.

Wartime industrial advances gave U.S. manufacturers a head start over foreign competitors, notably in the defeated nations, where war-related industries remained shut down for several years. Postwar developments were enhanced by commercial demand—there was soon scarcely an airport or harbor any where that was not equipped with radar—and by the exigencies of the space age, including astrophysics. Many of the basic inventions of World War II remained fundamental to new developments, but additional refinements were introduced by researchers in many countries. Among them, the contributions of Americans were perhaps the most numerous and ensured that American-made radar equipment could compete in world markets despite high production costs.

RADAR

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