The history of radar starts with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell's seminal work on electromagnetism. However, it was not until the early 20th century that systems were able to use these principles were becoming widely available, and it was German inventor Christian Hülsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog (Reichspatent Nr. 165546). Numerous similar systems were developed over the next two decades.
The term RADAR was coined in 1940 by the United States Navy as an acronym for radio detection and ranging; this was a cover for the highly secret technology. Thus, a true radar system must both detect and provide range (distance) information for a target. Before 1934, no single system gave this performance; some systems were omni-directional and provided ranging information, while others provided rough directional information but not range. A key development was the use of pulses that were timed to provide ranging, which were sent from large antennas that provided accurate directional information. Combining the two allowed for accurate plotting of targets.
In the 1934–1939 period, eight nations developed, independently and in great secrecy, systems of this type: the United States, Great Britain, Germany, the USSR, Japan, the Netherlands, France, and Italy. In addition, Great Britain had shared their basic information with four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries had also developed indigenous radar systems. During the war, Hungary was added to this list.
Progress during the war was rapid and of great importance, probably one of the decisive factors for the victory of the Allies. By the end of hostilities, the United States, Great Britain, Germany, the USSR, and Japan had a wide diversity of land- and sea-based radars as well as small airborne systems. After the war, radar use was widened to numerous fields including: civil aviation, marine navigation, radar guns for police, meteorology and even medicine.
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—Melinda M. Marshall (20th century)