an acronym for RAdio Detection And Ranging, is based on German scientist Heinrich Hertz's 1880s discovery that a beam of radio energy that strikes an object of sufficient density will be reflected by it. If that reflected energy is then captured by a receiver at the beam's origin it can be analyzed. Another German scientist, Christian Hulsmeyer, patented the first radio echo device in 1904. Because radio energy travels at a constant speed (the speed of light) the length of time between sending and receiving the energy can thus be used to calculate the object's distance. The direction from which the energy is received can be used to determine the object's bearing. Combining distance and bearing indicates the object's location on/above the surface of the Earth. Modern radars belong to one of two general types. Pulse radars emit a short, intense burst of radio energy, while continuous‐wave radars emit a steady signal. The latter, often called Doppler radar, cannot track the range to the object but instead measures the Doppler shift caused by the object's movement, from which the direction and speed of its movement can be determined. There are several other specific types of radars, such as Synthetic Aperture Radar, which electronically focus or shape the radar beam.

The Italian Guglielmo Marconi first demonstrated radio reflection for detection in the 1920s. In the United States, Gregory Breit and Merle A. Tuve discovered the principle of pulse ranging in 1925. Research and development was underway simultaneously in Germany, Great Britain, and the United States by the early 1930s. The Germans initially had better equipment aboard warships that began radar‐aided commerce raiding in September 1939. In 1937 the British began deploying the Chain Home early warning network along the Channel coast, which would provide the decisive advantage in the Battle of Britain. Early World War II radars used radio pulses of low frequency and long (a meter or more) wavelength, but these required large antennas, suitable only for large ships or ground stations and were imprecise compared to the next generation radars. With the invention in Great Britain of the cavity magnetron in 1940, however, much smaller sets employing centimeter wavelengths capable of much greater precision were possible. In 1940 Henry Tizard led a mission to the United States that successfully enlisted American industrial aid, and the Germans fell behind, never to regain parity. In the Pacific, the Japanese never even came close to it, and most Japanese radar systems were based on early ones captured from the British and Americans in 1942.

At sea, Allied naval radar was key in the defeat of the U‐boat threat in 1943, and radar‐directed naval gunfire was decisive in several sea battles, including the Battle of Leyte Gulf in October 1944, in which US battleships in the Surigao Straits using radar‐directed gunfire at night destroyed a Japanese fleet. In the air, the radar struggle between countermeasure and counter‐countermeasure was dynamic, deadly, and decisive. In July 1943 the Royal Air Force first used “window” (American term: “chaff”), small strips of reflective tinfoil, to negate German air defenses of Hamburg (Operation “Gomorrah”) in a raid that killed approximately 40,000 inhabitants. American bombers equipped with radar jamming transmitters (called “Carpet”) blocked German “Wurzburg” anti‐aircraft gun‐ laying radars and assisted in a deceptive spoof on the night of the Normandy landings. Offensively, American and British aircraft carried increasingly sophisticated navigational radars, such as the H2S and H2X (“Mickey”) sets that portrayed ground features with greater and greater detail and enabled bombing at night or through cloud cover. Night fighters equipped with small radar sets such as the German “Lichtenstein” hunted enemy aircraft in the darkness and located them entirely by radar. Specialized aircraft (“ferrets”) gathered radar intelligence while electronic warfare operators (“ravens”) waged an invisible but critical war in what was then called “the ether,” and might today be called “cyberspace.”

During the Cold War both the U.S. and Russians erected radar networks such as the Distant Early Warning or “DEW” line across Canada to warn of enemy aircraft. Strategic Air Command (SAC) warplans from the 1950s through the 1980s depended on radar to accurately navigate to and identify targets, and electronic countermeasures (ECM) such as radar jamming and chaff were the key to negating enemy defenses. Intercontinental ballistic missiles forced both sides in the 1960s to develop even more sophisticated radar nets such as the Ballistic Missile Early Warning System (BMEWS) to warn of missile attack. Perhaps the ultimate were radars devised to support anti‐ missile defenses, capable of not only detecting enemy missiles in space but also of tracking them for interception and destruction by defensive missiles. Radars belonging to the Space Detection and Tracking System (SPADATS) keep constant track of the thousands of objects orbiting the earth.

The air war over Vietnam was dominated by radar controlled air defenses, as North Vietnam successfully employed Russian radar‐guided surface‐to‐air missiles (SAMs) against American air operations. American countermeasures included not only traditional ECM, but also direct attacks on radar control systems. This technique, called “Wild Weasel”, had been tried in WW II, but not until the 1960s were detection and homing systems sufficiently advanced to be successful. Anti‐radar electronic warfare EW) was so important by the Persian Gulf War of 1991 that virtually no Coalition aerial attacks were mounted without EW support. Since the 1940s, designers have sought aircraft undetectable by enemy radars. This effort came to fruition with the F‐117 “Stealth Fighter” and B‐2 “Stealth Bomber”, both of which used Low Observable technology to make them almost invisible to enemy radars.

Modern military radars have become increasingly sophisticated, and those mounted in surveillance aircraft such as the Airborne Warning and Control System (“AWACS”) or Joint Surveillance and Tracking Radar System (“JSTARS”) provide virtually a three‐dimensional portrayal of a battlespace the size of a small country. Radar has also had an enormous effect in the civilian world. From radar astronomy, to traffic control, to weather and storm warning, to air and maritime navigation, radar has become an indispensable facet of modern life.

Bibliography

• Alfred Price, Instruments of Darkness (1977).
• Alfred Price, The History of US Electronic Warfare, Volumes I and II (1984, 1989).
• Henry E. Guerlac, Radar in World War II (1987).
• David Pritchard, The Radar War (1989).
• Robert Buderi, The Invention That Changed the World (1996).
• Alan Beyerchen, From Radio to Radar: Interwar Military Adaptation to Technological Change in Germany, the UK, and the US, in Alan R. Millet and Williamson Murray, editors, Military Innovation in the Interwar Period, (1996)