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The history of radar began in the early 1900s when German engineer Christian Huelsmeyer invented a simple omni-directional detecting device (Reichspatent Nr. 165546). However, it implied the prior knowledge of electromagnetism given by the 19 century's discoveries of Maxwell and Hertz. The technique developed through the 1920s and 1930s, leading to the introduction of the first early warning radar systems just before the opening of World War II.

The name radar comes from the acronym RADAR, coined in 1940 by the U.S. Navy for public reference to their highly classified work in RAdio Detection And Ranging. Thus, a true radar system must both detect and provide range (distance) information for a target. Before 1934, no single system gave this performance.

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 sharred their basic information with four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries had also developed indigidous 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.


The place of radar in the larger story of science and technology is argued differently by different authors. Radar, far more than the atomic bomb, contributed to Allied victory in World War II. Robert Buderi states that it was also the precursor of much modern technology. From a review of his book:
. radar has been the root of a wide range of achievements since the war, producing a veritable family tree of modern technologies.
Because of radar, astronomers can map the contours of far-off planets, physicians can see images of internal organs, meteorologists can measure rain falling in distant places, air travel is hundreds of times safer than travel by road, long-distance telephone calls are cheaper than postage, computers have become ubiquitous and ordinary people can cook their daily dinners in the time between sitcoms, with what used to be called a radar range.

But others think that radar is not so important, since the principles were not new:

Le principe fondamental du radar appartient au patrimoine commun des physiciens : ce qui demeure en fin de compte au crédit réel des techniciens se mesure à la réalisation effective de matériels opérationnels., or roughly

The fundamental principle of the radar belongs to the common patrimony of the physicists : after all, what is left to the real credit of the technicians is measured by the effective realisation of operational materials. — Maurice Ponte L'histoire du "radar ", les faits, or The history of the "radar", the facts

Early Developments

In 1887 the German physicist Heinrich Hertz began experimenting with electromagnetic waves in his laboratory. He found that these waves could be transmitted through different types of materials, and were reflected by others, such as conductors and dielectrics. The existence of electromagnetic waves was predicted earlier by the Scottish physicist James Clerk Maxwell, but it was Hertz who first succeeded in generating and detecting what were soon called radio waves.

Early Contributors

Between the time of Hertz's first demonstrations of radio waves, and the first application of these waves in an apparatus -- a radar system -- for detecting and locating distance objects, a number of persons made important contributions to this evolution.

Guglielmo Marconi

The development of the wireless or radio is often attributed to Guglielmo Marconi. Although he was not the first to "invent" this technology, it might be said that he was the greatest early promoter of practical radio systems and their applications. In a paper read before the Institution of Electrical Engineers in London on March 3, 1899, Marconi described radio beacon experiments he had conducted in Salisbury Plain. Concerning this lecture, in a 1922 paper he wrote:

I also described tests carried out in transmitting a beam of reflected waves across country . . . and pointed out the possibility of the utility of such a system if applied to lighthouses and lightships, so as to enable vessels in foggy weather to locate dangerous points around the coasts. . . .

It [now] seems to me that it should be possible to design [an] apparatus by means of which a ship could radiate or project a divergent beam of these rays in any desired direction, which rays, if coming across a metallic object, such as another steamer or ship, would be reflected back to a receiver screened from the local transmitter on the sending ship, and thereby immediately reveal the presence and bearing of the other ship in fog or thick weather.

This paper and a speech presenting the paper to a joint meeting of the Institute of Radio Engineers and the American Institute of Electrical Engineers in New York City on June 20, 1922, is often cited as the seminal event which began widespread interest in the development of radar.

Christian Huelsmeyer

In 1904 Christian Huelsmeyer gave public demonstrations in Germanymarker and the Netherlandsmarker of the use of radio echoes to detect ships so that collisions could be avoided. His device consisted of a simple spark gap used to generate a signal that was aimed using a dipole antenna with a cylindrical parabolic reflector. When a signal reflected from a ship was picked up by a similar antenna attached to the separate coherer receiver, a bell sounded. During bad weather or fog, the device would be periodically "spun" to check for nearby ships. The apparatus detected presence of ships up to 3 km, and Huelsmeyer planned to extend its capability to 10 km. It did not provide range (distance) information, only warning of a nearby object. He patented the device, called the telemobiloscope, but due to lack of interest by the naval authorities the invention was not put into production.

Huelsmeyer also received a patent amendment for estimating the range to the ship. Using a vertical scan of the horizon with the telemobiloscope mounted on a tower, the operator would find the angle at which the return was the most intense and deduce, by simple triangulation, the approximate distance. This is in contrast to the later development of pulsed radar, which determines distance directly.

Nikola Tesla

One of the hundreds of concepts generated by Nikola Tesla included principles regarding frequency and power levels for primitive radio-location units. In an interview published in The Electrical Experimenter, August 1917, Tesla gave the following:

For instance, by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed.

Tesla also proposed the use of these standing electromagnetic waves along with pulsed reflected surface waves to determine the relative position, speed, and course of a moving object and other modern concepts of radar.

Tesla had first proposed that radio location techniques might help find submarines (for which it is not well-suited) with a fluorescent screen indicator.

Origins in America

In the United States, both the Navy and Army needed means of remotely locating enemy ships and aircraft. In 1930, both services initiated the development of radio equipment that could meet this need. Unfortunately, there was little coordination of these efforts; thus, they will be described separately.

The U.S. Navy

In the autumn of 1922, Albert H. Taylor and Leo C. Young at the U.S. Naval Aircraft Radio Laboratory were conducting communication experiments when they noticed that a wooden ship in the Potomac River was interferring with their signals; in effect, they had demonstrated the first continuous wave (CW) interferrence-type radio-detection system with separated transmitting and receiving antennas. In 1930, Lawrence A. Hyland working with Taylor and Young, now at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., used a similar arrangement of radio equipment to detect a passing aircraft. This led to a proposal by Taylor for using this technique for detecting ships and aircraft.

A simple wave-interference apparatus can detect the presence of an object, but it cannot determine its location or velocity. That had to await the invention of pulsed radar, and later, additional encoding techniques to extract this information from a CW signal. When Taylor's group at the NRL were unsuccessful in getting interferrence radio accepted as a detection means, Young suggested trying pulsing techniques. This would also allow the direct determination of range to the target. The British and the American research groups were independently aware of the advantages of such an approach, but the problem was to develop the timing equipment to make it feasible.

Robert M. Page was assigned by Taylor to implement Young's suggestion. Page designed a transmitter operating at 60 MHz and pulsed 10 μs in duration and 90 μs wait time. In December 1934, the apparatus was used to detected a plane at a distance of one mile flying up and down the Potomac.

Although the detection range was small and the indications on the oscilloscope monitor were almost indistinct, it demonstrated the basic concept of a pulsed radar system. Based on this, Page, Taylor, and Young are usually credited with building and demonstrating the world’s first true radar.

An important subsequent development by Young and Page was the duplexer, a device that allowed the transmitter and receiver to use the same antenna without over-whelming or destroying the sensitive receiver circuitry. This also solved the problem associated with synchronization of separate transmitter and receiver antennas which is critical to accurate position deterination on long-range targets.

The experiments with pulsed radar were continued, primarily in improving the receiver for handling the short pulses. In June 1936, the NRL's first prototype radar system, now operating at 28.6 MHz, was demonstrated to government officials, successfully tracking an aircraft at distances up to 25 miles. Their radar was based on low frequency signals, at least by today's standards, and thus required large antennas, making it impractical for ship or aircraft mounting.

Antenna size is inversely proportional to the operating frequency; therefore, the operating frequency of the system was increased to 200 MHz, allowing much smaller antennas. (The frequency of 200 MHz was then the highest possible with existing transmitter tubes and other components.) The new system was successfully tested at the NRL in April 1937, That same month, the first sea-borne testing was conducted. The equipment was temporarily installed on the USS Leary, with a Yagi antenna mounted on a gun barrel for sweeping the field of view.

The U.S. Army

As the Great Depression started, economic conditions led the U.S. Army Signal Corps to consolidated its widespread laboratory operations to Fort Monmouth, New Jerseymarker. On June 30, 1930, these were designated the Signal Corps Laboratories (SCL) and Lt. Colonel (Dr.) William R. Blair was appointed the SCL Director.

Among other activities, the SCL was made responsible for research in the detection of aircraft by acoustical and electromagnetic radiation means. Blair had performed his doctoral research in the interaction of electromagnet waves with solid materials, and naturally gave attention to this type of detection. Initially, attempts were made to detect infrared radiation, either from the heat of aircraft engines or as reflected from large searchlights with infrared filters, as well as from radio signals generated by the engine ignition.

Some success was made in the infrared detection, but little was accomplished using radio. In 1932, progress at the Naval Research Laboratory (NRL) on radio interference for aircraft detection was passed on to the Army. While it does not appear that any of this information was used by Blair, the SCL did undertake a systematic survey of what was then known throughout the world about the methods of generating, modulating, and detecting radio signals in the microwave region.

The SCL's first definitive efforts in radio-based target detection started in 1934 when the Chief of the Army Signal Corps, after seeing a microwave demonstration by RCA, suggested that radio-echo techniques be investigated. The SCL called this technique radio position-finding (RPF). Based on the previous investigations, the SCL first tried microwaves.

During 1934 and 1935, tests of microwave RPF equipment resulted in Doppler-shifted signals being obtained, initially at only a few hundred feet distance and later greater than a mile. These tests involved a bi-static arrangement, with the transmitter at one end of the signal path and the receiver at the other, and the reflecting target passing through or near the path.

Blair was evidently not aware of the success of a pulsed system at the NRL in December 1934. In an internal 1935 note, Blair had commented:
Consideration is now being given to the scheme of projecting an interrupted sequence of trains of oscillations against the target and attempting to detect the echoes during the interstices between the projections.

In 1936, W. Delmar Hershberger, SCL’s Chief Engineer at that time, started a modest project in pulsed microwave transmission. Lacking success with microwaves, Hershberger visited the NRL (where he had earlier worked) and saw a demonstration of their pulsed set. Back at the SCL, he and Robert H. Noyes built an experimental apparatus using a 75-watt, 110-MHz (2.73-m) transmitter with pulse modulation and a receiver patterned on the one at the NRL. A request for project funding was turned down by the War Department, but $75,000 for support was diverted from a previous appropriation for a communication project.

In October 1936, Paul E. Watson (later Lt. Colonel) became the SCL Chief Engineer and led the project. A field setup near the coast was made with the transmitter and receiver separated by a mile. On December 14, the experimental set detected at up to 7 miles range aircraft flying in and out of New York Citymarker.

Work then began on a prototype system. Ralph I. Cole headed receiver work and William S. Marks lead transmitter improvements. Separate receivers and antennas were used for azimuth and elevation detection. Both receiving and the transmitting antennas used large arrays of dipole wires on wooden frames. The system output was intended to aim a searchlight.

The first demonstration of the full set was made on the night of May 26, 1937. An unlighted bomber was detected and then illuminated by the searchlight. The observers included the Secretary of War, Henry A. Woodring; he was so impressed that the next day orders were given for the full development of the system. Congress gave an appropriation of $250,000.

The frequency was increased to 200 MHz (1.5 m). The transmitter used 16 tubes in a ring oscillator circuit(developed at the NRL), producing about 75-kW peak power. Major James C. Moore was assigned to head the complex electrical and mechanical design of lobe switching antennas. Engineers from Western Electric and Westinghouse were brought in to assist in the overall development.

Origins in Great Britain

In 1915 Robert Watson-Watt joined the Meteorological Office as a meteorologist. Working at an outstation at Aldershotmarker, in Hampshire, Britainmarker, he developed the use of radio signals generated by lightning strikes to map out the position of thunderstorms. The difficulty in pinpointing the direction of these fleeting signals led to the use of rotating directional antennas, and in 1923 the use of oscilloscopes in order to display them. An operator would periodically rotate the antenna and look for "spikes" on the oscilloscope to find the direction of a storm.

By 1934, Watson Watt was well established in the area of radio as Superintendent of the Radio Research Station at Ditton Parkmarker near Sloughmarker. He was approached by H.E. Wimperis from the Air Ministry, who asked about the use of radio to produce a 'death ray', after reading of Nikola Tesli's claims to have built such a device. Watt quickly wrote back that this was unlikely, and he pointed out that in the absence of progress, "attention is being turned to the still difficult, but less unpromising, problem of radio detection and numerical considerations on the method of detection by reflected radio waves will be submitted when required." Watson Watt's scientific assistant, Arnold Frederic Wilkins, outlined a technique and made detailed calculations. This information was sent by Watson Watt to the Air Ministry on February 12, 1935, in a secret report titled "The Detection of Aircraft by Radio Methods."

To test if radio signals reflected from an aircraft could be detected, Wilkins set up receiving equipment in a field near Upper Stowe, Northamptonshiremarker. On February 26,1935, a Handley Page Heyford bomber flew along a path between the receiving station and the transmitting towers of a BBC shortwave station in nearby Daventrymarker. The aircraft reflected the 6-MHz (49-m) BBC signal, and this was readily detected at ranges up to 8 mi (132 km). This convincing test, known as the Daventry Experiment, was witnessed by a representative from the Air Ministry, and led to the immediate authorization to build a full demonstration system.

Origins in Germany

In Germany, Hans Hollmann had been working for some time in the field of microwaves, which were to later become the basis of most radar systems. In 1935 he published Physics and Technique of Ultrashort Waves, which was picked up by researchers around the world. At the time, he had been most interested in microwaves for use for communications, but he and his partner Hans-Karl von Willisen had also worked on radar-like systems.

In the autumn of 1934 their company, GEMA, built the first commercial radar system for detecting ships. Operating in the 50 cm range it could detect ships up to 10 km away. This device was similar in operation to Huelsmeyer's earlier system, and like it, did not provide range information.

In the summer of 1935 a pulse radar was developed at GEMA with which they detected a light cruiser, the Königsberg, 8 km away, with an accuracy of up to 50 m, enough for gun-laying. The same system could also detect an aircraft at 500 m altitude at a distance of 28 km. The military implications were not lost this time around, and construction of land and sea-based versions soon took place as Freya and Seetakt.

Origins in the USSR

On January 3, 1934 Soviet scientists, led by Pavel Oshchepkov and A.F. Ioffe, successfully tested experimental radar, developed by joint project between Central artillery Board of Soviet Army (GAU) and Central radio Laboratory (TsRL). A plane, flying at an altitude of about 150 meters, was able to be detected at a distance of around 600/ 700 meters of a radar facility.

Later in the same 1934 year radar devices for AA (anti-aircraft artillery) were created under contract between Central artillery Board of Soviet Army (GAU) and LEFI (Electrical and Physical Institute of Leningrad).

On July, 1934 an experimental meter-band radar station called "Rapid" (Рапид) was tested near by Leningrad by engineers of LEFI (Electrical and Physical Institute of Leningrad) under contract with AD Board of Soviet Army.

Origins in Japan

  Under Development

Origins in Holland

Several years before Watson-Watt, Dutch scientists Weiler and Gratema were inspired to start developing radar by queries about "death rays" from their military. They were well advanced by May 1940, and had built four working prototypes of centrimetric gunlaying radar operating at a wavelength of 50 cm and a practical range of 20 km. While these devices were far more technically sophisticated than British early warning radar of the time, they were not operationally integrated into the armed forces. As the Germans advanced their offensive through the Netherlands, radar operators could only track their planes. Says Max Staal: "Frustratingly, we had nothing to shoot at them with." Some scientists escaped to Britain before the Dutch capitulation on May 14, 1940, taking with them prototypes that aided the development of the British-American centrimetric radar.

Origins in France

In 1927, French physicists Camille Gutton and Emile Pierret experimented with magnetrons and other devices generating wavelengths going down to 16 cm. Camille's son, Henri Gutton, was with the Compagnie Générale de Télégraphie Sans Fil (CSF0 where he improved his father's magnetrons. Following reports made by the U.S. Naval Research Laboratory concerning detection by interferrence methods,in 1934Henri Gutton started the development of a radio-detection apparatus using the short-wavelength tubes.

Emile Girardeau [101120], the head of the CSF, recalled in testimony that they were at the time intending to build radio-detection systems "conceived according to the principles stated by Tesla". The CSF submitted the French patent (no. 788.795, "New system of location of obstacles and its applications") on July 20 1934, for a device detecting obstacles (icebergs, ships, planes) using continuous radiation of ultra-short wavelengths produced by a magnetron. This is the first patent of an operational radio-detection apparatus using centimetric wavelengths.

The system was tested from November to December 1934 aboard the cargo ship Oregon, with two transmitters working at 80 cm and 16 cm wavelengths. Coastlines were detected from a range of 10-12 nautical miles. The shortest wavelength was chosen for the final design, which equipped the liner Normandie as early as mid-1935 for operational use. These were still CW systems and depended on Doppler interference for detection.

In late 1937, SFR developed a means of pulse-modulating transmitter tubes. This led to a nrw 16-cm system with a peak power near 10 W and a pulse width of 6-ms. The Navy set up tests in early 1939 and detected large vessels at 10 km, but it could not detect aircraft and was thus not accepted by the military. The system was planned to be sea-tested aboard the Normandie, but this was cancelled at the outbreak of war.

In the late 1920s, Pierre David at the Laboratori National de Radioelectricite (National Laboratory of Radioelectricity, LNR) experimented with reflected radio signals at about a meter wavelength. Starting in 1931, he observed that aircraft caused interferrence to the signals. The LNR initiated research on a detection technique called barrage électromagnétique (electromagnetic curtain). While this could indicate the general location of penetration, precise determination of direction and speed was not possible.

In 1936, the Défense Aérienne de Territoire (Defence of Air Territory, ran tests on David’s electromagnetic curtain. In the tests, the system detected most of the entering aircraft, but too many were missed. As the war grew closer, the need for an aircraft detection was critical. David realized the advantages of a pulsed system, and in OIctober 1938 he designed a 50 MHz, pukse-modulated system with a peak-pulse power of 12 kW.

France declared war on Germany on September 1, 1939. Although work on these systems continued for some time, France was eventually occupied by the Germans and work on both types of pulsed detection systems stopped.

Origins in Italy

  Under Development

World War II

At the start of World War II both the United Kingdommarker and Germanymarker knew of each other's ongoing efforts in their "battle of the beams". Both nations were intensely interested in the other's developments in the field, and engaged in an active campaign of espionage and false leaks about their respective equipment. By the time of the Battle of Britain, both sides were deploying radar units and control stations as part of integrated air defense capability. However, German radars could not assist in offensive role and the Luftwaffe did not sufficiently appreciate the importance of British radar stations as part of RAF's air defense capability, contributing to their failure.

Research had been initiated by Sir Henry Tizard's Aeronautical Research Committee in 1935 and, from 1940, was based at the Telecommunications Research Establishmentmarker (TRE). But much of the credit belongs to Watson-Watt, head of the team working at Bawdsey Manor in Suffolk, who turned from the technical side of radar to building up a usable network of machines and the people to run them. After watching a demonstration in which his radar operators were attempting to locate an "attacking" bomber, he noticed that the primary problem was not technological, but worker overload. By 1940 Watt had built up a layered organization that efficiently passed information along the chain of command, and was able to track large numbers of aircraft and direct defenses to them.


Chain Home

Shortly before the outbreak of World War II several radar stations known as Chain Home (or CH) were constructed along the South and East coasts of Britain, based on the successful model at Bawdsey. As one might expect from the first radar to be deployed, CH was a simple system. The broadcast side was formed from two 300 ft (100 ;m) tall steel towers strung with a series of antennas between them. A second set of 240 ft (73 m) tall wooden towers were used for reception, with a series of crossed antennas at various heights up to 215 ft (65 m). Most stations had more than one set of each antenna, tuned to operate at different frequencies.

Typical operating conditions were:
  • FREQUENCY: 20 to 30 MHz (15 to 10 metres).
  • PEAK POWER: 350 kW (later 750 kW).

  • PULSE REPETITION FREQUENCY: 25 and 12.5 pps.
  • PULSE LENGTH: 20 μs.

The CH radar was read with an oscilloscope. When a pulse was sent out into the broadcast towers, the scope was triggered to start its beam moving horizontally across the screen very rapidly. The output from the receiver was amplified and fed into the vertical axis of the scope, so a return from an aircraft would deflect the beam upward. This formed a spike on the display, and the distance from the left side –measured with a small scale on the bottom of the screen– would give the distance to the target. By rotating the receiver goniometer connected to the antennas to make the display disappear, the operator could determine the direction to the target (this is the reason for the cross shaped antennas), while the size of the vertical displacement indicated something of the number of aircraft involved. By comparing the strengths returned from the various antennas up the tower, the altitude could be determined to some degree of accuracy.

CH proved highly effective during the Battle of Britain, and is often credited with allowing the RAF to defeat the much larger Luftwaffe forces. Whereas the Luftwaffe had to hunt all over to find the RAF fighters, the RAF knew exactly where the Luftwaffe bombers were, and could converge all of their fighters on them. In modern terminology, CH was a force multiplier, allowing the RAF fighters to operate more effectively as if they were a much larger force operating at the same effectiveness as the Germans. In addition, the CH system allowed pilots to rest on the ground instead of flying continuous 'standing patrols', and only needing to 'scramble' (take off) when the air threat was imminent. This not only reduced pilot's workloads, but also reduced wear on engines, as well as reducing unnecessary petrol consumption.

Very early in the battle the Luftwaffe made a series of small raids on a few of the stations, including the Bawdsey research and training station, but they were returned to operation in a few days. In the meantime the operators took to broadcasting radar-like signals from other systems in order to fool the Germans into believing that the systems were still operating. Eventually the Germans gave up trying to bomb them. The Luftwaffe apparently never understood the importance of radar to the RAF's efforts, or they would have assigned them a much higher priority – even a concerted effort would not have had much effect on the transmitters as their structure made them very resistant to blast which passed through the spaces in the metal lattice.

In order to avoid the CH system the Luftwaffe adopted other tactics. One was to approach Britain at very low levels, below the sight line of the radar stations. This was countered to some degree with a series of shorter range stations built right on the coast, known as Chain Home Low (CHL). These radars had originally been intended to use for naval gun-laying and known as Coastal Defence (CD), but their narrow beams also meant they could sweep an area much closer to the ground without seeing the reflection of the ground (or water) –known as clutter. Unlike the larger CH systems, CHL had to have the broadcast antenna itself turned, as opposed to just the receiver. This was done manually on a pedal-crank system run by Women's Auxiliary Air Force until more reliable motorized movements were installed in 1941.

Ground Controlled Intercept

Similar systems were later adapted with a new display to produce the Ground Controlled Intercept stations in January 1941. In these systems the antenna was rotated mechanically, followed by the display on the operator's console. That is, instead of a single line across the bottom of the display from left to right, the line was rotated around the screen at the same speed as the antenna was turning.

The result was a 2-D display of the air around the station with the operator in the middle, with all the aircraft appearing as dots in the proper location in space. These so-called Plan Position Indicators (PPI) dramatically simplified the amount of work needed to track a target on the operator's part. Such a system with a rotating, or sweeping, line is what most people continue to associate with a radar display. Philo Taylor Farnsworth, the American inventor of all-electronic television in September 1927, has contributed to this in an important way. Farnsworth refined a version of his picture tube (cathode ray tube, or CRT) and called it an "Iatron." It could store an image for milliseconds to minutes (even hours). One version that kept an image alive about a second before fading, proved to be a useful addition to the evolution of radar. This slow-to-fade display tube was used by air traffic controllers from the very beginning of radar.

Airborne Intercept

Rather than avoid the radars, the Luftwaffe took to avoiding the fighters by flying at night and in bad weather. Although the RAF was aware of the location of the bombers, there was little they could do about them unless the fighter pilots could see the opposing planes.

This eventuality had already been foreseen, and a successful programme by Edward George Bowen in 1936 (likely at the urging of Tizard) developed a miniaturized radar system suitable for aircraft, the so-called Airborne Interception (AI) set. At the same time Bowen developed radar sets for aircraft to detect submarines, the Air to Surface Vessel (ASV) set, making a significant contribution to the defeat of the German U-boats.

Initial AI sets were available in 1939 and fitted to Bristol Blenheim aircraft, replaced quickly with the better performing Bristol Beaufighter. These quickly put an end to German night- and bad-weather bombing over Britain. Mosquito night intruders were fitted with AI Mk VIII and later derivatives which, along with a device called "Serrate" to allow them to track down German night fighters from their Lichtenstein B/C and SN2 radar emissions, as well as a device named "Perfectos" that tracked German IFF, allowed the Mosquito to find and destroy German night fighters. As a countermeasure the German night fighters employed Naxos ZR radar detectors.

Centimetric radar

The next major development in the history of radar was the invention of the cavity magnetron by John Randall and Harry Boot of Birmingham Universitymarker in early 1940. This was a small device which generated microwave frequencies much more efficiently than previous devices, allowing the development of practical centimetric radar, which operates in the radio frequency band from 3 to 30 GHz. Centimetric radar allowed for the detection of much smaller objects and the use of much smaller antennas than the earlier lower frequency radars, and the cavity magnetron is the single most important invention in the history of radar and played a major part in the Allies' victory. It was given free as a gift to the US in 1940 together with several other inventions such as jet technology, so that the British could use American R&D and production facilities. The British need to produce the magnetron in large quantities was great. Consequently that Edward George Bowen was sent as the radar expert in the Tizard Mission to the USA in 1940, which resulted in the creation of the MITmarker Radiation Lab to develop the device further. Half of the radar deployed during World War II were designed at the RadLab, including over 100 different radar systems costing $1.5 billion.

At about the same time Robert M. Page invented the duplexer switch at the U.S. Naval Research Laboratory, allowing a pulse transmitter and receiver to share the same antenna without destabilizing the sensitive receiver.

The combination of the magnetron, the duplexer switch, small antennas and high resolution allowed small high quality radars to be installed in aircraft. They could be used by maritime patrol aircraft to detect objects as small as a submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric contour mapping radars like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign. Centimetric gun laying radars were much more accurate than the older technology. They made the big gunned Allied battleships more deadly and along with the newly developed proximity fuze made anti-aircraft guns much more dangerous to attacking aircraft. The two coupled together and used by anti-aircraft batteries, placed along on the German V-1 flying bomb flight paths to Londonmarker, are credited with destroying many of the flying bombs before they reached their target.


German developments mirrored those in the United Kingdom, but it appears radar received a much lower priority until later in the war. The Freya radar was much more sophisticated than its CH counterpart, and by operating in the 1.2–m wavelength (as opposed to ten times that for the CH) around 250 MHz the Freya was able to be much smaller and yet offer better resolution. Yet by the start of the war only eight of these units were in operation, offering much less coverage.

Compared to the British PPI systems, the German system was far more labour intensive. This problem was compounded by the lackadaisical approach to command staffing. It was some time before the Luftwaffe had a command and control system nearly as sophisticated as the one set up by Watson-Watt before the war.

This state of affairs did not last long. By 1940 the RAF's night raids were becoming a nuisance, and action was finally taken to address the problem. Josef Kammhuber was promoted to become the General of the Night Fighters and set about creating a network of Freya radar stations in a chain of "cells" through Holland, Belgium and France. Known as the Kammhuber Line, each cell of the network contained a radar and a number of searchlights, as well as one primary and one backup night fighter. When a bomber was detected flying into the cell the searchlights were directed by the radar to pick it up, at which point the night fighter could see the now-lit bomber.

While somewhat effective, the system was useless during bad weather or other times where the light would be blocked. In order to address this problem, the Würzburg radarmarker was developed. Würzburg was a short-range radar mounted on a highly directional parabolic antenna that was sensitive in only one direction. This made it useless for finding the targets, but once guided to one by an associated Freya it could track it with extreme accuracy: later models were accurate to 0.2 degrees or less.

Two Würzburgs were assigned to each cell, one to track the target bomber, and another the night fighter. By plotting the location of both aircraft on a common plotting table, radio operators could direct the fighter manually to the target. The downfall of the Kammhuber Line was that it could only track a single target per Würzburg. When the British learned of this, they directed operations such that all their bombers concentrated on crossing the line en masse over as few cells as possible. This bomber stream introduced in mid 1942 meant that as a raid developed, only a few night fighters could be directed into the raid at any one time, and bomber losses dropped to a handful per raid.

Airborne radars

The use of the accurate Freya and Würzburg allowed the Germans to have a somewhat more lackadaisical approach to the development of an airborne radar. Unlike the British, whose inaccurate CH systems demanded some sort of system in the aircraft, the Würzburg was accurate enough to allow them to leave the radar on the ground. This came back to haunt them when the British figured out their system, and the development of an airborne system became much more important.

Early Lichtenstein BC units were not deployed until 1942, and as they operated on the 2–m wavelength (150–MHz) they required large antennas. By this point in the war the British had become experts on jamming German radars, and when a BC-equipped Ju 88 night fighter landed in Britain one foggy night, it was only a few weeks before the system was rendered completely useless. By late 1943 the Luftwaffe was starting to deploy the greatly improved SN-2, but this required huge antennas that slowed the planes as much as 50–km/h. Jamming the SN-2 took longer, but was accomplished. A 9–cm wavelength system known as Berlin was eventually developed, but only in the very last months of the war.


After early U.S.marker work on radar conducted in the twenties at the Naval Research Laboratories, the success of Robert Page's pulsed radar experiment in 1934 redirected the attention of the Signal Corps, who had been focusing more on use of sound and heat to detect aircraft. Expertise in radio equipment design by the signal corps led to rapid development of an early type of VHF radar at Fort Monmouthmarker and Camp Evansmarker in New Jerseymarker for use with coastal artillery .

Radar arrangement on the aircraft carrier Lexington, 1944
By 1940 when the British and US began technology exchanges, the British were surprised to learn they were not unique in their possession of practical pulse radar technology. The U.S. Navy's pulse radar system, the CXAM radar was found to be very similar in capability to their Chain Home technology. The British were much further ahead on microwave research necessary for the second generation of military radars. Although the US Navy had produced by 1940 an experimental 10–cm radar, they were stymied by the problem of insufficient transmitter power. On entry to World War II, the army and navy had working first generation radar units in front line units, and this technology was relied on throughout the war. The army's type SCR-270 radar detected the Japanese planes attacking Pearl Harbor at a range of , although this information was not used effectively at the command level. After the war this unit was employed in the first application of radar in astronomy by bouncing radio waves off the Moon in 1946.

Although the US had developed pulsed radar systems independent of the British as had the Germans, there were serious weaknesses in their efforts - the greatest of which was the lack of integration of radar into unified air defense system. Here the British were without peer. The result of the Tizard Mission in 1940 was a major step forward for utilization of radar technology, both in the transfer of the organizational knowledge that Watson-Watt had worked out as well as the British microwave technology. In particular, the cavity magnetron was the answer the US was looking for, and it led to the creation of the MIT Radiation Lab, a major center for research employing almost 4,000 people at its peak during the Second World War.


Nakajima J1N night fighter with FD-2 nose radar
Well prior to World War II, Japan had knowledgeable researchers in the technologies necessary for radar but due to lack of appreciation of radar's potential, and rivalry between army, navy and civilian research groups, Japanese technology was 3 to 5 years behind that of the US during the war. The Japanese captured a British type gun laying radar in Singapore as well as an American SCR-268 and SCR-270 when they overran the Philippines. In August 1942, US marines captured a Japanese Navy Type 1 model 1 radar, and though judged to be crude even by the standards of early US radars, the fact the Japanese had any radar capability came as a surprise.

One leader in radar technology was Hidetsugu Yagi, a researcher of international stature who was working on applications of power transmission via microwave in the early 1930s. Though his project was overly ambitious, the work he did was directly applicable to advanced microwave radars. The papers he delivered in the late 20s in the US on antennas and magnetron design were closely studied by US researchers. His work was given so little attention by Japanese military researchers that when the Japanese captured the British radar unit in Singapore, at first they were unaware that the "Yagi" antenna mentioned in captured manuals referred to a Japanese invention. Although progress was rapid after the value of radar was better appreciated, research continued to be impeded by inter-service rivalry and new units, though capable, were too late to influence the outcome of the war. Radar was used by the army for gun laying and aircraft detection, by the navy for detection of air and sea threats on all major capital ships, including use of centimetric units in 1944. Towards the end of the war, units were sufficiently miniaturized for airborne intercept (FD-2) radar on J1N1-S Gekko night fighters and airborne ship detection radar in G4M2 "Betty" bombers and Kawanishi H8K patrol planes.


Little radar research was done in Canada prior to the start of WW2. However, in 1939 the National Research Council of Canada was tasked with developing a Canadian designed radar system. After the fall of France in June 1940, radar research was given the highest possible priority, leading to the development and deployment of a series of radar systems, including the CSC type and SW1C naval radars, which were operationally deployed on RCN ships in 1941, placing Canada into the forefront of naval radar deployment.

Cold War

After World War II the primary "axis" of combat shifted to lie between the United States and the Soviet Unionmarker. In order to provide early warning of an attack, both sides deployed huge radar networks of increasing sophistication at ever-more remote locations. The first such system was the Pinetree Line deployed across Canadamarker in the 1950s, backed up with radars on ships and oil platforms off the east and west coasts. The Pinetree Line was a simple system and was vulnerable to jamming, so the more sophisticated Mid-Canada Line (MCL) was set up to supplant it. However, the MCL was not considered to be militarily very useful, and the DEW Line started construction soon after, in the high Arctic. Construction of the DEW line is still considered one of the great logistics and civil engineering projects of the 20th century. In the late 1950s, the Ballistic Missile Early Warning System was added to warn of ICBM launches.

Planetary radar

The moon is comparatively close, and was detected by radar soon after its invention, in 1946. Measurements included surface roughness and later mapping of shadowed regions near the poles.

The next easiest target is Venus. This was a target of great scientific value, since it could provide an unambiguous way to measure the size of the astronomical unit, which was needed for the nascent field of interplanetary spacecraft. After several false starts, the first un-ambiguous detection of Venus by radar was made by JPL on 10 March 1961. Other planets were then observed by radar, using both powerful Earth-based systems, and radars carried by spacecraft to the vicinity of the planet. The following planetary bodies have been observed by this means:
Mars - Mapping of surface roughness from Arecibo Observatorymarker. The Mars Express mission carries a ground-penetrating radar.
Mercury - Improved value for the distance from the earth observed (GR test). Radar has meassured the rotational period, libration, and surface mapping, especially of the polar regions.
Venus - Radar originally measured the rotation period and gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.
Jupiter - Ground based radars have measured the system, including the Galilean satellites.
Saturn - Radar has observed the rings and Titan from Arecibo Observatorymarker, mapping of Titan's surface and observations of other moons from the Cassini spacecraft.
Earth - numerous airborne and spacecraft radars have mapped the entire planet, for various purposes. One example is the Shuttle Radar Topography Mission, which mapped the entire Earth at 30 m resolution.


See also

Further reading

  • ES310 " Introduction to Naval Weapons Engineering.". (Radar fundamentals section)
  • Barrett, Dick, " All you ever wanted to know about British air defence radar". The Radar Pages. (History and details of various British radar systems)
  • Blanchard, Yves, Le radar. 1904-2004 : Histoire d'un siècle d'innovations techniques et opérationnelles, éditions Ellipses,(in French)
  • Bowen, E.G., Radar Days, Institute of Physics Publishing, Bristol, 1987., ISBN 0-7503-0586-X
  • Bragg, Michael., RDF1 The Location of Aircraft by Radio Methods 1935-1945, Hawkhead Publishing, 1988, ISBN 0-9531544-0-8
  • Brown, Jim, Radar - how it all began, Janus Publishing, 1996, ISBN 1-85756-212-7
  • Brown, Louis, A Radar History of World War 2 - Technical and Military Imperatives, Institute of Physics Publishing, 1999, ISBN 0-7503-0659-9
  • Robert Buderi: The invention that changed the world: the story of radar from war to peace, Simon & Schuster, 1996. ISBN 0-349-11068-9
  • Clark, Ronald, Tizard (London, 1965). An authorized biography of radar's champion in the 1930s.
  • G W A Dummer, Electronic Inventions and Discoveries
  • Frank, Sir Charles, Operation Epsilon: The Farm Hall Transcripts
  • Hanbury Brown, Robert, Boffin: A Personal Story of the early Days of Radar and Radio Astronomy and Quantum Optics, Taylor and Francis, 1991
  • Howse, Derek, Radar At Sea The Royal Navy in World War 2, Naval Institute Press, Annapolis, Maryland, USA, 1993, ISBN 1-55750-704-X
  • Jones, R. V., Most Secret War, Hamish Hamilton, 1978. R.V. Jones's account of his part in British Scientific Intelligence between 1939 and 1945, working to anticipate the German's radar, radio navigation and V1/V2 developments.
  • Kroge, Harry von, GEMA: Birthplace of German Radar and Sonar
  • Latham, Colin & Stobbs, Anne., Radar A Wartime Miracle, Sutton Publishing Ltd, Stroud 1996 ISBN 0-7509-1643-5 A history of radar in the UK during WWII told by the men and women who worked on it.
  • Latham, Colin, and Anne Stobbs, The Birth of British Radar: The Memoirs of Arnold 'Skip' Wilkins, Speedwell for the Defence Electronics History Society 2006, ISBN 0953716627
  • Lovell, Sir Bernard Lovel, Echoes of War - The History of H2S
  • Pritchard, David., The Radar War Germany's Pioneering Achievement 1904-1945 Patrick Stephens Ltd, Wellingborough 1989., ISBN 1852602465
  • Rawnsley, C. F., and Robert Wright, Night Fighter
  • Swords, Seán S., Technical History of the Beginnings of Radar. London: P. Peregrinus on behalf of the Institution of Electrical Engineers, IEE History of Technology Series, 1986
  • Watson, Raymond C., Jr. Radar Origins Worldwide: History of Its Evolution in 13 Nations Through World War II. Trafford Publishing, 2009 ISBN 978-1-4269-2111-7
  • Zimmerman, David., Britain's Shield Radar and the Defeat of the Luftwaffe, Sutton Publishing, 2001., ISBN 0-7509-1799-7

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