radar is an acronym for Radio Detection And Ranging, now embracing a diverse group of systems which employ radio waves to detect the presence or measure the location of distant objects. There exist two essentially different types of radar system, monostatic and bistatic: the former and more common has its transmitting and receiving antennas at the same location, whereas the latter may work with the entire transmitting and receiving systems widely separated from each other. The frequencies utilized by radar sets during the Second World War ranged from 20 MHz to 10 GHz (see Chart). Pulse radars predominated. In a pulse radar energy is transmitted for brief periods of time (of the order of a millionth of a second) separated by relatively long quiescent intervals (generally greater than one-thousandth of a second), during which the echoes from targets can be received and processed.

Between the years 1934 and 1936, radar emerged independently and for military purposes in eight countries: France, Germany, the Netherlands, Italy, Japan, the UK, the USA and the USSR. In most it was first seen as a means of early warning against air attack and as an anti-aircraft gun-laying device (see Table for some of the more notable radar sets).

Three related events, which were very significant for the Allies, were the British discovery of the cavity magnetron in July 1940, which made centimetric radar possible (see electronic navigation systems and H2S); the Tizard mission to the USA two months later, when the cavity magnetron was disclosed to the Americans; and the setting up of the Radiation Laboratory at the Massachusetts Institute of Technology in November 1940. From an objective global viewpoint, the impact of the work of the Radiation Laboratory, both on the operational uses of radar during the latter half of the war and thereafter on its immediate post-war usage, can hardly be over-emphasized.

The radar programmes in the countries listed above will receive brief mention. These programmes were extensive in the case of the UK (and its Dominions), the United States, and Germany and, to a lesser extent, Japan.

France

A bistatic early-warning system using continuous waves and beat-frequency technique was proposed by Pierre David in 1934 and operationally tested in 1937. In 1939 fixed David radar chains protected naval bases along the English Channel, the Atlantic Ocean, and the Mediterranean, and also the north-eastern approach to Paris. A limited number of early-warning pulse radar equipments were manufactured in 1939 by commercial companies. Mobile early-warning and gun-laying radar sets were employed in France by the British Expeditionary Force from October 1939 to May 1940.

Germany

Radar in Germany had its origins in the foresight and determination of Dr Rudolf Kuhnold of the German Navy's Signals Research Establishment. In 1934 he set up the company GEMA, which in 1936 produced prototypes of both the Seetakt series of ship-based search and gun-ranging radars operating at 370 MHz and the Freya early-warning sets operating at 125 MHz. Radar development in Germany, while technically successful, did not follow a clear co-ordinated programme. There is no doubt that much work, particularly in microwaves, was curtailed or delayed because of Hitler's edict prohibiting research which could not guarantee early success.

Radar was used for aircraft reporting purposes as early as December 1939 with radar plots being passed directly to fighter and Flak units. The first night-bombing of Germany by the RAF took place on 15 May 1940. It was a prelude to a sustained air defence campaign of almost five years by the Germans in which searchlights, night fighters, radar—and, for a while, sound detectors—and the Luftwaffe's flak arm played their various roles. General Joseph Kammhuber and the Kammhuber Line figured prominently in this. The area bombing raids by the RAF which began in the spring of 1943 radically affected the efficiency of the Kammhuber Line and this ushered in new methods of fighter control and a re-allocation of the areas of control. This period also coincided with the widespread use of radar counter-measures as part of the electronic warfare being waged by both sides.

Italy

Following observations made in 1933, Guglielmo Marconi built a radar system which he successfully demonstrated to Mussolini and members of the general staff on 14 May 1935. The military authorities enlisted the services of Professor Ugo Tiberio who produced, practically single-handed, a series of experimental sets; the last, EC-3 ter, was completed in 1941 and went into production with the SAFAR company. There appear to have been more than 100 Italian naval radar sets available at the time of the armistice in September 1943: their potential had not, however, been realized.

Japan

Microwave and magnetron research was carried out from 1933 onwards at the Japan Radio Company and the Naval Technical Research Institute, and experiments were also conducted by Professor Okabe Kinjiro of Osaka University in 1936. The first sets, which went into operation in 1941, were continuous wave, as opposed to pulse, bistatic systems operating in the 40–80 MHz band. Serious work on pulse radars began in 1941. A complete lack of liaison between the army and navy led to great inefficiency and duplication of effort (see rivalries). The navy, from 1941 until the end of hostilities, carried out an extensive programme of design and manufacture of successful land-based, shipboard, and airborne metric and centimetric wave sets. One cannot say that the Japanese radar programme was not good: it just could not equal that of the USA and UK.

Netherlands

In 1936, work on the design of a radar began independently in the Philips Physics Laboratory at Eindhoven and in the physics laboratory of the Dutch armed forces. A few prototypes were built. One of these was operating in The Hague, when the Germans invaded in May 1940.

UK

The unease felt by A. P. Rowe in 1934, when, as assistant to the director of scientific research at the air ministry, he examined the state of Britain's preparedness against air attack, set a chain of events in motion. By 31 May 1935 a basic radar transmitter and receiver, at that time called Radio Direction Finding (RDF), a name the British retained until the middle of the war, were operating at Orford Ness on the east coast on a frequency of 6 MHz. In August and September 1935 the early-warning chain of radar stations which became known as the Chain Home (CH) was planned. In the organization of all this, Robert Watson-Watt played a leading part. At the outbreak of war, on 3 September 1939, eighteen CH stations were operational and connected to the Stanmore filter room in north London; two more in Scotland were operating locally. This result was achieved after much effort and the benefit of air exercises. The effectiveness of the CH system throughout the war, but especially during the battle of Britain, lay not only in the performance and reliability of the radars but in the efficiency of the personnel in the radar stations, the filter rooms (where information on a raid was evaluated before being passed to the operations room), and the operations rooms.

From 1940 to 1943, the Chain Home coverage was extended to the south-west and west coasts and Northern Ireland. Its capability was added to by the employment of 200 MHz CHL (Chain Home Low) and 3 GHz CHEL (Chain Home Extra Low) stations which were effective against low-flying aircraft: the latter also reported the movements of shipping.

The setting up of the CH network was purposely given priority of resources. Nevertheless, even as early as 1935, most of the later radar developments were given consideration. In 1937, Dr E. G. Bowen carried out successful AI (aircraft interception) tests using a radar fitted in a Heyford bomber and a co-operating transmitter on the ground. This work eventually led to the development of a number of successful ASV (aircraft to surface vessel) and AI radars.

Naval set development began in 1935 and by 1939 an air-warning set, Type 79, operating at 43MHz, was fitted to some of the larger vessels. The fitting of microwave radar to escort vessels in 1942 was an important factor in the war against the U-boats during the battle of the Atlantic. Serious work on army radars began at the end of 1936. Before the end of the war some 80 types of sets and their variants had been designed.

Radar: Some notable radars

USA

The development of radar in the USA from its origins to the end of the war can be viewed in two stages. It was born in the USA in the Naval Research Laboratory from observations made in June 1930 by Leo Young and Laurence Pat Hyland which eventually led in 1934 to Robert Page's building of a 60 MHz pulse radar set. A development of this, the CXAM, became available in November 1939. Twenty sets were installed on battleships, aircraft carriers, and cruisers in 1940.

Major William Blair, the director of the Signal Corps Laboratories at Fort Monmouth, New Jersey, promoted radar experiments from 1933 onwards. A simple pulse radar was demonstrated in December 1936. By May 1937, a prototype of the first US Army radar, the SCR-268, was built. A long-range radar, operating at 106 MHz, the mobile SCR 270 and its fixed counterpart the SCR-271, went into service in 1940. About 800 were produced between 1939 and 1944. By early 1942 the Aircraft Warning Service had a chain of SCR-270 and SCR-271 radars protecting the east coast from Maine to Key West and the west coast from Washington to San Diego.

At the time of the Tizard mission and the exchange of information with the UK in 1940, the USA possessed a very solid and developing radar programme though it lacked, perhaps, the urgency engendered by a country threatened by war. The second stage of development was initiated by the setting up of the Radiation Laboratory in November 1940, a direct result of the mission.

USSR

The air defence command, dissatisfied with sound-location, contracted with the Leningrad Electro-Physics Institute to undertake research into the radio detection of objects. The result was the experimental set ‘RAPID’, a bistatic continuous-wave system, which in August 1934 detected the presence of aircraft up to a distance of 75 km. (53 mi.). Acute political changes occurred during 1937–8 which undoubtedly affected the radar programme, but from February 1942 onwards the state defence committee actively promoted radar. By the end of the war successful naval surface-search and airborne AI sets were being produced.

Radar as a weapon

Radar systems enhanced the capability of air, land, and sea forces in both defensive and offensive roles. The element of surprise in attack became more difficult to achieve. In some operations or battles the use of radar or a particular type of radar was of decisive importance.

The use of the Chain Home in the battle of Britain was an essential factor in the RAF's victory. It was also the first time in history that early warning and control were used in an air battle. In the assault phase of the Normandy landings (see OVERLORD), the chain again played an essential part: its radar coverage was extended by three specially equipped fighter director tenders, which finally hove to off the coast, and by the GCI (ground control of interception) and other sets which were landed on the bridgehead. The landings, involving accurate blind-bombing and naval bombardment of shore targets, and the expeditious movement of a huge air and sea armada, would have been impossible without a heavy involvement of radar.

The Allied strategic air offensive against Germany went through many phases. Overall, it was by no means a one-sided victory for RAF Bomber Command and the Eighth US Army Air Force. The German radar early-warning system was very effective so that the defences were hardly ever surprised. In the biggest night air battle of the war, on 30 March, 1944, a force of 782 Halifaxes and Lancasters, carrying out a raid on Nuremberg, suffered 13.6% losses due in large part to the effectiveness of the German night fighters' Lichtenstein SN2 AI sets.

The Pacific war, which lasted almost four years, was essentially a naval war. American submarines and radar-equipped aircraft inflicted heavy losses on Japanese merchant shipping and tankers. The balance of naval battles, in both defensive and offensive phases of the campaign, was determined largely by the effectiveness of the American Task Forces' radar directed fighters. The ‘Great Marianas Turkey Shoot’ of 19 June 1944, during the battle of the Philippine Sea, when nearly 300 Japanese aircraft were destroyed for a loss of 30 American aircraft, was an example of the potency of well-organized radar fighter-control.

One instance where the usage of radar was critical for the Allies and in which the outcome of battle was vital to the whole conduct of the war was the battle of the Atlantic. The submarine war on Allied merchant shipping lasted from the outbreak of hostilities until the defeat of Germany. Losses such as those of June 1942 when 141 ships were sunk, could not have been sustained for long. Many factors apart from radar, including code-breaking (see ULTRA, 1), ship-building potential, anti-submarine weapon development, and convoy procedures, denied ultimate victory to the U-boats. The use of radar, particularly the British naval Type 271 microwave set used in escort vessels, proved very successful. At the end of 1940, the ASV MkII was fitted to a variety of aircraft including Wellingtons, Whitleys, Sunderlands, and Catalinas. Later, American long-range aircraft, such as the Liberator, fitted with radar were used in the western Atlantic. At night, the combination of ASV radar which could pick up a surfaced submarine and the Leigh Light (see searchlights) which could illuminate it as the aircraft made its final run in before dropping depth charges, proved a deadly weapon. The 200 MHz ASV MkII lost much of its potency due to the Germans' introduction of listening receivers on submarines. However, the advent in March 1943 of the 3GHz ASV MkIII set, which could not be detected, heralded the defeat of the submarines.

The graph starkly and simply illustrates the flow of the U-boat war, focusing on the ratio of Allied vessels sunk to U-boats lost. When the USA entered the war the U-boats moved into American waters and for many months convoy losses rose dangerously high: the peak in the curve for February 1942 illustrates this.

Sean Swords

Bibliography

Guerlac, H. E. , Radar in World War II (New York, 1987).
Burns, R. W. (ed.), Radar Development to 1945 (London, 1988).

RADAR

█ LARRY GILMAN

RADAR—an acronym for RAdio Detection And Ranging— is the use of electromagnetic waves at sub-optical frequencies (i.e., less than about 10¹² Hz) to sense objects at a distance. Hundreds of different RADAR systems have been designed for various purposes, military and other. RADAR systems are essential to the navigation and tracking of craft at sea and in the air, weather prediction, and scientific research of many kinds.

Principles. In basic RADAR, radio waves are transmitted from an antenna. These outgoing waves eventually bounce off some distant object and return an echo to the sender, where they are received, amplified, and processed electronically to yield an image showing the object's location. The waves sent out may be either short oscillatory bursts (pulses) or continuous sinusoidal waves. If a RADAR transmits pulses it is termed a pulse RADAR, whereas if it transmits a continuous sinusoidal wave it is termed a continuous-wave RADAR.

On closer examination, the RADAR process is seen to be more complex. For example, reflection of an echo by the object one wishes to sense is anything but straightforward. Upon leaving a transmitting antenna, a radio wave propagates in a widening beam at the speed of light (> 186,000 miles per hour [3 × 10⁸ m/sec]); if it encounters an obstacle (i.e., a medium whose characteristic impedance differs from that of air and vacuum [> 377 Ω), it splits into two parts. One part passes into the obstacle and is (generally) absorbed, and the other is reflected. Where the reflected wave goes depends on the shape of the obstacle. Roundish or irregular obstacles tend to scatter energy through a wide angle, while flat or facet-like surfaces tend to send it off in a single direction, just as a flat mirror reflects light. If any part of the outgoing wave is reflected at 180° (which is not guaranteed) it will return to the transmitter. This returned or backscattered signal is usually detected by the same antenna that sent the outgoing pulse; this antenna alternates rapidly between transmitting pulses and listening for echoes, thus building a realtime picture of the reflecting targets in range of its beam. The energy the echoes receive is a small fraction of that in the pulses transmitted, so the strength of the transmitted pulse and the sensitivity of the receiver determines a RADAR's range. By systematically sweeping the direction in which its antenna is pointed, a RADAR system can scan a much larger volume of space than its beam can interrogate at any one moment; this is why many RADAR antennas, on ships or atop air-traffic control towers, are seen to rotate while in operation.

Radio waves are not the only form of energy that can be used to derive echoes from distant targets. Sound waves may also be used. Indeed, because radio waves are rapidly absorbed in water, sonar (SOund Navigation and Ranging) is essential to underwater operations of all sorts, including sea-floor mapping and anti-submarine warfare.

Applications. Since World War II RADAR has been deployed in many forms and has found a wide application in scientific, commercial, and military operations. RADAR signals have been bounced off targets ranging in size from dust specks to other planets. RADAR is essential to rocketry and early-warning detection of missiles, air traffic control, navigation at sea, automatic control of weapons such as antiaircraft guns, aircraft detection and tracking, mapping of the ground from the air, weather prediction, intruder detection, and numerous other tasks. Few craft, military or civilian, put to sea or take to the air without carrying some form of RADAR.

In recent decades, development of the basic RADAR principle—send pulse, listen for echo—has proceeded along a number of interesting paths. By exploiting the Doppler effect, which causes frequency shifts in echoes reflected from moving objects, modern RADARs can tell not only where an object is but what direction it is moving in and how quickly. The Doppler effect also allows for the precision mapping of landscapes from moving aircraft through the synthetic-aperture technique. Synthetic-aperture systems exploit the fact that stationary objects being swept by a RADAR beam projected from a moving source have, depending on their location, slightly different absolute velocities with respect to that source. By detecting these velocity differences using the Doppler effect, synthetic aperture type RADAR greatly permits the generation of high-resolution ground maps from small, airborne RADARs.

In many modern RADAR systems the need for a mechanically moving antenna has been obviated by phased arrays. A phased array consists of a large number of small, computer-controlled antennas termed elements. These elements, of which there are usually thousands, are crowded together to form a flat surface. In transmit mode, the elements are all instructed to emit a RADAR pulse at approximately the same time; the thousands of outbound waves produced by the elements merge into a single powerful wave as they spread outward. By timing, or phasing, the elements in the array so that, for example, elements along the left-hand edge of the array fire first while those farther to the right fire progressively later, the composite wave formed by the merging of the elements' lesser outputs can be steered in any desired direction within a wide cone (in this example, to the right). Beam steering can be accomplished by such a system millions of times more rapidly than would be possible with mechanical methods. Phased-array systems are used for a number of applications; including the 71.5-foot (21.8-m) tall AN/FPS-115 PAVE PAWS Early Warning RADAR Array Antennas, which provide early warning of ballistic-missile attack; shipboard systems such as the AN/SPY-1D, which is about 15 feet (3 m) across and is mounted flush with the upper hull of some warships; the Hughes AN/TPQ-37 Firefinder, a trailer-mounted system designed for tracking incoming artillery and missiles and calculating their point of origin; and many other real-world systems.

RADAR is a powerful weapon of war, but has its weaknesses. For example, numerous missiles have been developed to home in on the radio pulses emitted by RADARs, making it very dangerous to turn on a RADAR in a modern battlefield situation. Further, jamming and spoofing ("electronic warfare") have evolved rapidly alongside RADAR itself. For example, an aircraft that finds itself interrogated by a RADAR pulse can emit blasts of noise or false echoes, or request that a drone or other unit emit them, in order to confuse enemy RADAR. Finally, aircraft have been built that are "low observable," that is, which scatter very little energy back toward any RADAR that illuminates them. Low-observable or "stealth" aircraft are built of radio-absorbent materials and shaped to present little or no surface area perpendicular to RADAR pulses approaching from most angles (except directly above and directly below, the two least likely places for an enemy RADAR to be at any given moment). What RADAR they do reflect is deflected at low angles rather than returned to the RADAR transmitter. The U.S. B-2 bomber and F-117A and F-22 fighters are working examples of low-observable aircraft.

█ FURTHER READING:

BOOKS:

Edde, Byron. RADAR: Principles, Technology, Applications. Englewood Cliffs, NJ: PTR Hall, 1993.

Skolnik, Merrill I. Introduction to RADAR Systems. New York: McGraw Hills, 2001.

SEE ALSO

Stealth Technology
RADAR, Synthetic Aperture

RADAR, 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).

Daniel T. Kuehl

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.

BIBLIOGRAPHY

Buderi, Robert. The Invention that Changed the World. New York: Simon and Schuster, 1996.

Burns, Russell, ed. Radar Development to 1945. London: Institution of Electrical Engineers, 1988.

Fisher, David E. A Race on the Edge of Time. New York: McGrawHill, 1988.

Page, Robert M. The Origin of Radar. Garden City, N.Y.: Anchor Books, 1962.

CharlesSiisskind/a. r.

See alsoAir Defense ; Aircraft Industry ; Signal Corps, U.S. Army ; Weather Service, National .

RADAR

Discovery of Radar

Though the principles of radar—an acronym for radio detection and ranging—were understood in the early 1930s, its application in the United States was slow to develop. In 1930 Lawrence Hyland of the Naval Research Laboratory observed that radio signals transmitted from the ground were reflected back by passing airplanes and showed up on a radio-wave detection screen. This discovery opened up the possibility of developing a system to detect and locate aircraft. The U.S. Army did not recognize the importance of radar until 1936, when it established a radar research unit at the U.S. Army Signal Corps laboratory in Fort Monmouth, New Jersey. Research moved slowly because the navy classified pulse-radar research as secret, preventing the navy and army from exchanging information.

The Tizard Mission

After the outbreak of World War II Great Britain helped to strengthen U.S. radar operations. In summer 1940 Prime Minister Winston Churchill sent a technical mission to the United States. Its director, Sir Henry Tizard, a defense scientist who believed in the full exchange of technical information, brought Britain's most vital secrets of military technology. Led by Alfred Loomis, head of the radar section of the National Defense Research Committee (NDRC), American scientists were in the throes of developing microwave radar, which had the advantages of using small antennae, accurately locating low-flying aircraft, and discriminating between adjacent targets. The Tizard Mission brought to the United States the resonant cavity magnetron, which emitted radiation at an intensity thousands of times greater than the most advanced American tube. Loomis claimed that the British magnetron advanced the U.S. radar program by two years. The British desperately needed an airborne radar-intercept system to help them cope with German night bombing. Loomis agreed to develop the magnetron into an airborne-intercept system and established a project code-named AI-10.

The Rad Lab

Under contract from the NDRC, project AI-10 was established at the Massachusetts Institute of Technology, where the MIT Radiation Laboratory, known as the "Rad Lab," was created especially for the project. Physicist Lee DuBridge from the University of Rochester directed the Rad Lab and recruited a staff of nearly forty young nuclear physicists. Quickly building a microwave lab of wood and tar paper on the roof of one of MIT's tall buildings, they succeeded by April 1941 in developing a workable prototype of the microwave AI-10 that detected both aircraft and submarines.

Robert Watson-Watt Reinforces U.S. Radar

In 1941 British radar expert Robert Watson-Watt inspected U.S. aerial defenses and found the American radar barrier filled with holes. Watson-Watt made a series of suggestions—including relocating stations and providing ground control of aerial interceptions—to provide the United States with effective coverage.

The ASV

Following British innovations, Rad Lab scientists made building an airborne submarine-detection unit their top priority, and by August 1941 they had designed the Rad Lab ASV (air-to-surface vessel) radar, which detected ships twenty to thirty miles away and submarines at the surface two to five miles away. In December flight trials of the ASV so impressed the U.S. Army that by January 1942, with the United States at war, the U.S. Army Air Corps requested ten Rad Lab ASV radar units to equip B-18 aircraft for patrol along critical American coastal sea lanes.

Loran

By March 1942, B-18 bombers and naval destroyers equipped with long-wave ASV search radar were able to keep enemy submarines three hundred miles from the coast. During 1942 Rad Lab physicists also developed loran (long-range navigation) radar for in-flight navigation. By the end of 1942 the Rad Lab budget had reached $1,150,000 a month, and by 1945 it employed nearly five hundred physicists. Wartime innovation in radar technology led to the development of the civilian radar systems installed at U.S. airports after the war.

Source:

Henry Guerlac, Radar in World War II (Los Angeles: Tomash / New York: American Institute of Physics, 1987).

Radar

Radar (a contraction of ra dio d etection a nd r anging) is an electronic system that measures the position, speed, or other characteristics of a far-off object by means of radio waves bounced off the surface of that object. It can pierce any atmospheric disturbance, such as a storm, all the way to the horizon. Within its range, radar can reveal clouds, a landmass, or objects such as ships, airplanes, or spacecraft. Radar can measure distance to a target object; for instance, aircraft use it to determine altitude. Radar is also used to monitor atmospheric systems, to track storms, and to help predict the weather. Military applications include weapons ranging (determining the distance from a weapon to a target) and direction in the control of guided missiles.

Basic radar operation

Light waves, radio waves, microwaves, and radar waves are all examples of electromagnetic waves. Unlike water waves, electromagnetic waves do not require a medium such as water or air to travel through. They can travel through a complete vacuum. Similar to light waves, radar waves bounce off some objects and travel through others.

The simplest mode of radar operation is range-finding, or determining how far away an object is. The radar unit sends radar waves out toward the target (radar systems can send out thousands of pulses per second). The waves hit the target and are reflected back. The returning wave is received by the radar unit, and the travel time is registered. According to basic principles of physics, distance is equal to the rate of travel (speed) multiplied by the time of travel. All electromagnetic waves travel at the same speed in a vacuum—the speed of light, which is 186,282 miles (299,727 kilometers) per second. This speed is reduced by a small amount when the waves are traveling through air, but this can be calculated.

Bats and dolphins are able to emit high-frequency sounds and orient (position) themselves by means of reflected sound waves. This ability is known as echolocation.

Radar and World War II

When the 1930s saw the possibility of a German air invasion, the English government accelerated its research into radar. A chain of radar stations was constructed that would throw an invisible net of radio waves over England. These waves could detect the approach and precise location of any aircraft.

When German dictator Adolf Hitler ordered a massive air strike against England in 1940, the radar shield worked. Although the German air force greatly outnumbered the British Royal Air Force, it was soundly beaten because the radar's eye could easily locate German planes, even in darkness and poor weather.

The military use of radar continued throughout World War II (1939–45). Compact transmitters were developed that could be mounted

on the underside of a plane to scan the ground far below for targets. Bombs and shells equipped with radar tracking systems were designed to "look" for their targets, exploding at just the right moment.

Other uses of radar

Radar devices began to trickle into everyday use soon after the end of the war. In 1947, a young engineer named John Barker attempted to use radar to regulate traffic lights. He noticed that a passing automobile would reflect a radio pulse, and that the speed of the vehicle could then be determined by examining the returning signal. Much to the dismay of speeders, Barker had devised the first radar speed-gun, now used by police worldwide.

Marine navigators, surveyors, meteorologists, and astronomers have also found uses for radar technology. A continuous-wave version called Doppler radar is often used to track storms and hurricanes. Probes launched into space have used radar to map the surfaces of other planets.

radar Acronym for radio detection and ranging; the use of electromagnetic energy for the detection of objects which are capable of reflecting it. For example, cloud-detection radars are extensively used in meteorological forecasting and rainfall measurement, while sideways-looking radars are used for topographic mapping. Radar has been used to probe through ice sheets in order to determine ice thickness and to detect internal reflection events; in polar regions the radar equipment has usually been airborne, but the instruments can be mounted on sledges for surface use. Radar can also be used in arid environments to probe through sand in search of water. Ground radar is being developed for use in engineering site investigations, but has very limited depth penetration where the moisture content is high because of the high dielectric loss associated with water.

radar, an acronym for Radio Detection and Ranging, a system for determining the range and bearing of an object by transmitting bursts of electromagnetic energy and timing the intervals between the transmission and return of the echoes. The main purposes of radar at sea are for collision avoidance and navigation in poor visibility. The wavelength of seaborne radar is for the most part 3 cm or 10 cm; some ships carry both. There are several types of display for marine radar, the most common of which is the plan position indicator (PPI) which is either ‘head-up’ where the top of the display corresponds with the ship's heading, or stabilized where true bearings are indicated. On true-motion radar, which is largely used for anti-collision, the origin on the display moves with the vessel.

See also racon; ramark; remote sensing.

Mike Richey

radar (acronym for radio detecting and ranging) Electronic system for determining the direction and distance of objects. Developed during World War II, it works by the transmission of pulses of radio waves to an object. The object reflects the pulses, which are detected by an antenna. By measuring the time it takes for the reflected waves to return, the object's distance may be calculated, and its direction ascertained from the alignment of the receiving radar antenna.

ra·dar / ˈrāˌdär/ • n. a system for detecting the presence, direction, distance, and speed of aircraft, ships, and other objects, by sending out pulses of high-frequency electromagnetic waves that are reflected off the object back to the source. ∎  an apparatus used for this. ORIGIN: 1940s: from ra(dio) d(etection) a(nd) r(anging).

radar n.a radio detection device that provides information on range, azimuth, and/or elevation of objects.

radar •Dada • radar • zamindar • Pindar •chowkidar • havildar • Godard •doodah •purdah, sirdar

radar (ˈreɪdɑː) radio detection and ranging