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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, so named because a true radar system provides information about both position and distance of an object.

Early Workers

The history of radar started with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell's seminal work on electromagnetism. However, it was not until the early 20th century that systems able to use these principles were becoming widely available, and it was German engineer Christian Huelsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog. Numerous similar systems were developed over the next two decades.

In 1887 the German physicist Heinrich Hertz (1857–1894) 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 (1831–79), but it was Hertz who first succeeded in generating and detecting what were soon called radio waves.

The development of the wireless or radio is often attributed to Guglielmo Marconi (1874–1937). 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 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. See also Marconi's work at the Chelmsford factory.

In 1904 Christian Huelsmeyer (1881–1957) gave public demonstrations in Germany and the Netherlands 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 3Km, and Huelsmeyer planned to extend its capability to 10Km. 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. The telemobiloscope could also be used for estimating the range to the ship. Using a vertical scan of the horizon with the telemobiloscope mounted on a tower, the operator could 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 (1856–1943) published his results regarding frequency and power levels for primitive radio-location units in 1917. He deduced that standing electromagnetic waves were produced by a sending station, from which it was possible to determine information on the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the object, and 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.


The cavity magnetron, which was crucial to the development of radar

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A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. Waves in open space propagate in all directions, as spherical waves. In this way they lose their power proportionally to the square of the distance. A waveguide confines the wave to propagation in one dimension, so that under ideal conditions the wave loses no power while propagating. There are different types of waveguide for each type of wave. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves. Different waveguides are needed to guide different frequencies: an optical fibre guiding light (high frequency) will not guide microwaves (which have a much lower frequency). As a rule of thumb, the width of a waveguide needs to be of the same order of magnitude as the wavelength of the guided wave. Waves are confined inside the waveguide due to total reflection from the waveguide wall, so that the propagation inside the waveguide can be described approximately as a "zigzag" between the walls. The first structure for guiding waves was proposed by J. J. Thomson in 1893, and was first experimentally tested by O. J. Lodge in 1894. The first mathematical analysis of electromagnetic waves in a metal cylinder was performed by Lord Rayleigh in 1897. The study of dielectric waveguides began as early as the 1920s, by several people, most famous of which are Rayleigh, Sommerfeld and Debye. The optical fibre started receiving special attention from the 1960s because of its importance in the communications industry. Specific examples of waveguides are:



The information provided by radar includes the bearing and range (and therefore position) of the object from the radar scanner. It is thus used in many different fields where the need for such positioning is crucial. The first use of radar was for military purposes; to locate air, ground and sea targets. This has evolved in the civilian field into applications for aircraft, ships and roads. In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. They can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot. Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters. Police forces use radar guns to monitor vehicle speeds on the roads. Meteorologists use radar to monitor precipitation. It has become the primary tool for short-term weather forecasting and to watch for severe weather such as thunderstorms, tornados, winter storms or precipitation types. Geologists and archaeologists use specialised ground-penetrating radars to map the composition of the earth's crust or archaeological remains. The list of radar applications is getting longer all the time, and now includes the inventory of forests by aerial scans, and the monitoring of bird migration.

Development in various countries during WW2

Before 1934, no single system gave both detection and ranging; some systems were omni-directional and provided ranging information, while others provided rough directional information but not range. A key development was the use of pulses that were timed to provide ranging, which were then sent from large antennas that provided accurate directional information. Combining the two allowed for accurate plotting of targets. The importance of radar in a military sense ensured that it became an object of international research, particularly in the time of the Second World War. In the 1934–1939 period eight nations developed systems of this type, independently and in great secrecy: the United States, Great Britain, Germany, the USSR, Japan, the Netherlands, France, and Italy. In addition, Great Britain had shared their basic information with four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries had also developed indigenous radar systems. During the war, Hungary was added to this list.

Progress during the war was rapid and of great importance, being 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 speed guns for the police, meteorology and medicine. 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" (microwave oven).

USA

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. There was little coordination of these efforts, however. In the autumn of 1922, the U.S. Naval Aircraft Radio Laboratory noted that a wooden ship in the Potomac River was interfering with their radio signals; in effect, they had demonstrated the first multistatic radar, a system that uses separated transmitting and receiving antennas and detects targets due to changes in the signal. In 1930 the U.S. Naval Research Laboratory (NRL) in Washington, D.C., used a similar arrangement of radio equipment to detect a passing aircraft. Estimation of distance had to wait until pulsed systems became available. In 1934 a transmitter operating at 60 MHz with pulses 10 ?s in duration and 90 ?s between pulses was used to detected a plane at a distance of one mile flying up and down the Potomac. An important subsequent development 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 determination on long-range targets. 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. The equipment operated at a low frequency by modern standards, which required large antennas (antenna size is inversely proportional to the operating frequency), making it impractical for ship or aircraft mounting. The operating frequency of the system was increased to 200 MHz, the highest possible with existing transmitter tubes and other components, allowing much smaller antennas. The new system was successfully tested at the NRL in April 1937, and the first sea-borne testing was conducted on the USS Leary, with a Yagi antenna mounted on a gun barrel for sweeping the field of view. The NRL further improved the system with a ring oscillator, which allowed the use of multiple output tubes, and the pulse-power was increased to 15 kW in 5-µs pulses. A 20ft x 23ft antenna was used, and the system, now designated XAF, detected planes at ranges up to 100 miles. It was installed on the battleship USS New York for sea trials starting in January 1939, and became the first operational radio detection and ranging set in the U.S. fleet. In May 1939, a contract was awarded to RCA for production,



and the equipment, code word CXAM, started in May 1940. Meanwhile, as the Great Depression started, economic conditions led the U.S. Army Signal Corps to consolidate its widespread laboratory operations to Fort Monmouth, New Jersey. On June 30, 1930, these were designated the Signal Corps Laboratories (SCL). SCL was made responsible for research in the detection of aircraft by acoustical and infrared radiation means. Initially, attempts were made to detect infrared radiation, 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. 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 should be investigated. The SCL called this technique radio position-finding (RPF). 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. The frequency was increased to 200 MHz (1.5 m wavelength), and the transmitter used 16 tubes in a ring oscillator circuit (developed at the NRL), producing about 75 kW peak power. Engineers from Western Electric and Westinghouse were brought in to assist in the overall development. Designated SCR-268, a prototype was successfully demonstrated in late 1938 at Fort Monroe, Virginia. Production of SCR-268 sets was started by Western Electric in 1939, and entered service in early 1941. Two new configurations evolved,



the SCR-270 (mobile) and the SCR-271 (fixed-site). Operation at 106 MHz (2.83 m wavelength) was selected, and a single water-cooled tube provided 8 kW (100 kW pulsed) output power. Westinghouse received a production contract, and started deliveries near the end of 1940. The Army deployed five of the first SCR-270 sets around the island of Oahu in Hawaii. At 7:02 on the morning of December 7, 1941, one of these radars detected a flight of aircraft at a range of 136 miles due north. The observation was passed on to aircraft warning control where it was misidentified as a flight of U.S. bombers known to be approaching from the mainland. The alarm went unheeded, and at 7:48, the Japanese aircraft first struck at Pearl Harbor.

Germany

A radio-based device for remotely indicating the presence of ships was built in Germany by Christian Hülsmeyer in 1904 (see above). Often referred to as the first radar system, this did not directly measure the range (distance) to the target, and thus was not a true radar. In the early 1930s, physicist Rudolf Kühnhold, Scientific Director at the Kriegsmarine Nachrichtenmittel-Versuchsanstalt (NVA, the German Navy Experimental Institute of Communication Systems) in Kiel, was attempting to improve the acoustical methods of underwater detection of ships. He concluded that the desired accuracy in measuring distance to targets could be attained only by using pulsed electromagnetic waves. During 1933 Kühnhold first attempted to test this concept with a transmitting and receiving set that operated in the microwave region at 13.5 cm (2.22 GHz). The transmitter used a Barkhausen-Kurz tube (the first microwave generator) that produced only 0.1 watt. Unsuccessful with this, he asked for assistance from Paul-Gunther Erbslöh and Hans-Karl Freiherr von Willisen, amateur radio operators who were developing a VHF system for communications. They enthusiastically agreed, and in January 1934, formed a company, Gesellschaft für Elektroakustische und Mechanische Apparate (GEMA). Work on a Funkmessgerät für Untersuchung (radio measuring device for reconnaissance) began in earnest at GEMA. Hans Hollmann and Theodor Schultes, both affiliated with the prestigious Heinrich Hertz Institute in Berlin, were added as consultants. The first apparatus used a split-anode magnetron purchased from Philips in the Netherlands. This provided about 70 W at 50 cm (600 MHz), but suffered from frequency instability. Hollmann built a regenerative receiver and Schultes developed Yagi antennas for transmitting and receiving. In June 1934 large vessels were detected by Doppler-beat interference at a distance of about 2 km (1.2 miles). In October, strong reflections were observed from an aircraft that happened to fly through the beam; this opened consideration of targets other than ships.Kühnhold then shifted the GEMA work to a pulse-modulated system. A new 50 cm (600 MHz) Philips magnetron with better frequency stability was used. It was modulated with 2?s pulses at a pulse repetition frequency (PRF) of 2000 Hz. The transmitting antenna was an array of 10 pairs of dipoles with a reflecting mesh. The wide-band regenerative receiver used Acorn tubes from RCA, and the receiving antenna had three pairs of dipoles and incorporated lobe switching. A blocking device (a duplexer) shut the receiver input when the transmitter pulsed. A Braun tube (a CRT) was used for displaying the range. The equipment was first tested at a NVA site at Lübeck. During May 1935 the receiver was rebuilt, becoming a set with two intermediate-frequency stages. With this improved receiver, the system readily tracked vessels at up to 8Km range. In September 1935 a demonstration was given to the Commander-in-Chief of the Kriegsmarine. The system performance was excellent, and historically this was the first naval vessel equipped with radar. Although this apparatus was not put into production, GEMA was funded to develop similar systems operating around 50 cm (500 MHz).



These became the Seetakt for the Kriegsmarine (the Admiral Graf Spee used this unit successfully against shipping in the Atlantic)

  

and the Freya for the Luftwaffe (German Air Force, installed in Northern France, e.g. at Auberville). Kühnhold remained with the NVA but also consulted with GEMA. He is considered by many in Germany as the Father of Radar. Between 1933 and 1936 Hollmann wrote the first comprehensive treatise on microwaves, Physik und Technik der ultrakurzen Wellen (Physics and Technique of Ultrashort Waves), published by Springer in 1938.

In 1933, when Kühnhold at the NVA was first experimenting with microwaves, he had sought information from Telefunken on microwave tubes. Wilhelm Tolmé Runge told him that no vacuum tubes were available for these frequencies, but in fact Runge was already experimenting with high-frequency transmitters and had Telefunken’s tube department working on cm-wavelength devices. In the summer of 1935 Runge, now Director of Telefunken’s Radio Research Laboratory, initiated an internally funded project in radio-based detection. Using Barkhausen-Kurz tubes, a 50 cm (600 MHz) receiver and 0.5-W transmitter were built. With the antennas placed flat on the ground some distance apart, Runge arranged for an aircraft to fly overhead and found that the receiver gave a strong Doppler-beat interference signal. Runge, with Hans Hollmann as a consultant, continued in developing a 1.8 m (170 MHz) system using pulse-modulation. Wilhelm Stepp developed a transmit-receive device (a duplexer) for allowing a common antenna. Stepp also code-named the system Darmstadt after his home town, starting the practice in Telefunken of giving the systems names of cities. The system, with only a few watts transmitter power, was first tested in February 1936, detecting an aircraft at about 5Km distance.

  

This led the Luftwaffe to fund the development of a 50 cm (600 kHz) gun-laying system, the Würzburg.

Since before the First World War Standard Elektrik Lorenz had been the main supplier of communication equipment for the German military and was the main rival of Telefunken. In late 1935, when Lorenz found that Runge at Telefunken was doing research in radio-based detection equipment, they started a similar activity under Gottfried Müller. A pulse-modulated set called Einheit für Abfragung (Device for Detection) was built. It used a type DS-310 tube (similar to the Acorn) operating at 70 cm (430 MHz) and about 1KW power. In early 1936 initial experiments gave reflections from large buildings at up to about 7Km. The power was doubled by using two tubes, and in mid-1936 the equipment was set up on cliffs near Kiel, and good detections of ships at 7Km and aircraft at 4Km were obtained. The success of this experimental set was reported to the Kriegsmarine, but they showed no interest, since they were already fully engaged with GEMA for similar equipment. Also, because of extensive agreements between Lorenz and many foreign countries, the naval authorities had reservations concerning the company handling classified work.



The experimental set was then demonstrated to the German Army, and they contracted with Lorenz for developing Kurfürst (Cure prince), a system for supporting Flugzeugabwehrkanone (anti-aircraft guns, abbreviated to the acronym FLAK).

Britain of course wished to obtain as much information about Germany's radar capabilities as possible, and this was the thinking behind Operation Biting, also known as the Bruneval Raid. Operation Biting was the codename given to a British Combined Operations raid on a German radar installation in Bruneval, France that occurred between 27 – 28 February 1942. A number of these installations had been identified from Royal Air Force aerial reconnaissance during 1941, but their exact purpose and the nature of the equipment that they possessed was not known. However, a number of British scientists believed that these stations had something to do with the heavy losses being experienced by RAF bombers conducting bombing raids against targets in occupied Europe. A request was therefore made by these scientists that one of these installations should be raided and the technology it possessed should be studied and, if possible, extracted and taken back to Britain for further study. Due to the extensive coastal defences erected by the Germans to protect the installation from a sea-borne raid, it was believed that a commando raid from the sea would only incur heavy losses on the part of the attackers, and give sufficient time for the garrison at the installation to destroy the radar equipment. It was therefore decided that an airborne assault, followed by sea-borne evacuation would be the ideal way to surprise the garrison of the installation and seize the technology intact, as well as minimize casualties inflicted on the raiding force.

After the end of the Battle of France and the evacuation of British troops from Dunkirk during Operation Dynamo, much of Britain's war production and effort was channelled into RAF Bomber Command and the strategic bombing offensive against Germany. However, bomber losses on each raid began to increase during 1941, which British intelligence concluded was due to German use of advanced radar techniques. British and German radar technology and techniques had been in competition for nearly a decade at this point, with the Germans often either at the same level as Britain, or surpassing Britain due to the heavy German military investment in the fledgling technology. By the beginning of World War II, British radar technology had managed to rise to an effective level, primarily due to the work of Robert Watson-Watt, although much of the technology was still rudimentary in nature and mistakes were being made, such as the inability of Watson-Watt and other scientists to devise an effective night-defence system in time for the German night-time bombing of Britain during 1940. Another British scientist working on radar technology and techniques was R. V. Jones, who had been appointed in 1939 as Britain's first Scientific Intelligence Officer and had spent the first years of the conflict researching how advanced the Germans were in radar technology compared to Britain, and convincing doubters that the Germans actually had radar. By scrutinizing leaked German documents, examining crashed Luftwaffe bombers and Enigma decrypts, as well as interrogating German prisoners of war, Jones had discovered that high-frequency radio signals were being transmitted across Britain from somewhere on the continent which he believed came from a directional radar system. Within a few months of this discovery, Jones had identified several such radar systems, one of which was being used to detect British bombers; this was known as the "Freya-Meldung-Freya" system, named after an ancient Nordic goddess. Jones was finally able to see concrete proof of the presence of the Freya system after being shown several mysterious objects caught in reconnaissance pictures taken by the RAF near The Hague - two circular dishes approximately 20 feet (6.1 m) in diameter which were being rotated. Having found proof of these Freya installations, Jones and the other scientists under his command could begin devising counter-measures against the system, and the RAF could begin to locate and destroy the installations themselves. Jones had also found evidence of a second part of the Freya system, referred to in Enigma decrypts as "Würzburg", but it was not until he was shown another set of RAF reconnaissance photographs in November 1941 that he learnt what Würzburg was. It consisted of a parabolic reflector about 3.0 m in diameter, which worked in conjunction with Freya to locate British bombers and then direct Luftwaffe night-fighters to their position. The two systems complemented each other: Freya was a long-distance radar system, but lacked precision, whereas Würzburg possessed a far shorter range but was far more precise. Würzburg also had the advantage of being much smaller than the Freya system and easier to manufacture in the quantities needed by the Luftwaffe to defend German territory.

In order to effectively neutralize the Würzburg system by developing counter-measures against it, Jones and his team needed to study one of the systems, or at very least the more vital pieces of technology that the system was composed of. Fortunately for Jones, one such site had recently been located by an RAF reconnaissance Spitfire from the Photographic Reconnaissance Unit during a flight over part of the Channel coast near Le Havre.

  

The site was located on a cliff-top immediately north of the village of Bruneval, which was itself twelve miles north of Le Havre, and was the most accessible German radar site that had been located so far by the British; several other installations had been located in France, but were landlocked, and others were as far away as Romania and Bulgaria. A request was therefore passed along to Admiral Lord Louis Mountbatten, the commander of Combined Operations, that a raid be mounted against the Bruneval installation and that a Würzburg radar system be captured and brought back to Britain for study. Mountbatten in turn took the proposal to the Chiefs of Staff Committee, who approved the raid after a brief debate. Having received permission to conduct the raid, Mountbatten and his staff studied the Bruneval installation and its defences, rapidly coming to the conclusion that due to the extensive coastal defences erected in the area around the installation it was too well-guarded to permit a commando raid. They considered that such a raid would be costly in terms of casualties for the commando force and would not be fast enough to capture the Würzburg radar before it was destroyed. Believing that surprise and speed were to be the essential requirements of any raid against the installation to ensure the radar was captured, Mountbatten saw an airborne assault as the only viable method. On 8 January 1942, he therefore contacted the headquarters of 1st Airborne Division and 38 Wing RAF, asking if they could cooperate together to conduct such a raid. Major-General Frederick Browning, commander of 1st Airborne Division, and Group Captain Nigel Norman, commander of 38 Wing, both agreed to conduct the raid; Browning was particularly enthusiastic, as a successful operation would be an excellent morale boost to the airborne troops under his command, as well as a good demonstration of their value.

The two commanders believed that training by both airborne troops and aircrews could be completed by the end of February, when there would be suitable meteorological conditions for such an operation to take place. Training for the raid was begun immediately, but encountered several problems. 38 Wing was a new formation and was unable to provide any aircraft or trained aircrews for the raid, meaning that No. 51 Squadron RAF under Wing Commander Percy Charles Pickard was selected to provide the aircraft and aircrew needed for the operation, although Group Captain Norman would remain in overall command. Another problem encountered was the state of the company of airborne troops chosen to raid the installation. During this period, 1st Airborne Division was composed of only two parachute battalions, of which only one, 1st Parachute Battalion, was fully-trained; Major-General Browning, wishing to keep 1st Parachute Battalion intact for any larger operation the division might be selected for, ordered 2nd Parachute Battalion to provide a company for the operation. The company slected spent a period training on Salisbury Plain in Wiltshire, and then travelled to Inveraray in Scotland where they underwent specialized training on Loch Fyne, practising night embarkations on landing craft. After finishing this training, which was designed to prepare the company for being evacuated by sea after raiding the radar installation, the company returned to Wiltshire and began carrying out practice parachute drops with the aircraft and aircrews of 51 Squadron. Despite the aircrews of the Squadron having no previous experience in dropping parachutists, these exercises proved to be successful. The company's training was aided by the creation of a scale-model of the radar installation and the surrounding buildings being built by the Photographic Interpretation Unit. Information about the Bruneval radar installation was also gathered during the training period, often with the help of the French resistance, without whom detailed knowledge of the disposition of the German forces guarding the installation would have been impossible. This reconnaissance was gathered by Gilbert Renault, known to the British by the code-name 'Remy' and several members of his resistance cell. The installation was composed of two distinct areas; a villa approximately 100 yards (300 ft) from the edge of a cliff which contained the radar station itself, and an enclosure containing a number of smaller buildings which contained a small garrison. The Würzburg apparatus had been erected between the villa and the cliff. The radar station was permanently manned by signallers and was surrounded by a number of guard posts and approximately thirty guards; the buildings in the small enclosure housed approximately 100 German troops, including another detachment of signallers. A platoon of infantry was stationed to the north in Bruneval itself, and was responsible for manning the defences guarding the evacuation beach; these included a strong-point near the beach as well as pillboxes and machine-gun nests on the top of the cliff overlooking the beach. The beach was not mined and had only sporadic barbed-wire defences, but it was patrolled regularly, and a mobile reserve of infantry was believed to be available at one hour's notice and stationed some distance inland.

On the night of 27 February, after a period of intense training and several delays due to poor weather, the small detachment of airborne troops parachuted into France a few miles from the installation. The force then proceeded to assault the villa in which the radar equipment was kept, killing several members of the German garrison and capturing the installation after a brief fire-fight. A technician that had come with the force proceeded to dismantle the Würzburg radar array and to remove several key pieces, placing the pieces on specially-designed trolleys to be taken back to Britain, and the raiding force then retreated to the evacuation beach. The detachment assigned to clear the beach had failed to do so, however, and another brief fire-fight was required to eliminate the Germans guarding the beach. The raiding force was then picked up by a small number of landing craft and transferred to several motor gun boats which took them back to Britain. The journey back to Britain was uneventful, with the force being escorted by four destroyers and a flight of Supermarine Spitfire fighters. The raid was entirely successful. The airborne troops suffered only a few casualties, and the pieces of the radar they brought back, along with a German radar technician, allowed British scientists to understand German advances in radar and to create counter-measures to neutralize those advances.

The success of the raid against the Bruneval installation had two important effects. Firstly, a successful raid against German-occupied territory was a welcome morale boost for the British public, and was featured prominently in the British media for several weeks after the end of the raid. The other important effect was the technical knowledge that British scientists gained. Examination of the components of the radar array showed that it was of a modular design that aided maintenance and made fixing faults far simpler than on similar British radar models. This was confirmed during the interrogation of the captured German technician, who proved to be less well trained than his British counterparts. Examination of the radar array also allowed British scientists to conclude that they would have to deploy a radar countermeasure that had recently been developed, code-named Windows. Examination of the Würzburg array showed that it was impervious to being jammed by conventional means used by the British during the early years of the conflict, and thus Windows would have to be deployed against German radar installations from this point onwards. The effectiveness of Windows against Würzburg radar arrays was confirmed by a raid conducted by RAF Bomber Command on 24 July 1943 against Hamburg (Operation Gomorrah); when the bombers utilized Windows, all the radar arrays in Hamburg were blinded and their operators confused, unable to distinguish between the radar signature of a real bomber and several pieces of Window giving off a similar signature. An unexpected bonus of the raid was the Germans' efforts to improve defences at Würzburg stations and prevent similar commando raids. The radars were surrounded by rings of barbed wire which increased their visibility from the air, making them easier to target prior to Operation Overlord. One final consequence of the raid was that the Telecommunications Research Establishment moved from Swanage further inland to Malvern to ensure that it was not the target of a reprisal raid by German airborne forces.

USSR

In 1895 Alexander Stepanovich Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lighting strikes. The next year, he added a spark-gap transmitter and demonstrated the first radio communication set in Russia. During 1897, while testing this in communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation. In the few years following the 1917 Russian Revolution and the establishment the Union of Soviet Socialist Republics (USSR or Soviet Union) in 1924, Germany’s Luftwaffe had aircraft capable of penetrating deep into Soviet territory. Thus, the detection of aircraft at night or above clouds was of great interest to the Voiska Protivo-vozdushnoi aborony (PVO, Air Defense Forces) of the Raboche-Krest'yanskaya Krasnaya Armiya (RKKA, Workers’–Peasants’ Red Army). The PVO depended on optical devices for locating targets, and had physicist Pavel K. Oshchepkov conducting research in possible improvement of these devices. In June 1933, Oshchepkov changed his research from optics to radio techniques and started the development of a razvedyvlatl’naya elektromagnitnaya stantsiya (reconnaissance electromagnetic station). In a short time, Oshchepkov was made responsible for a PVO experino-tekknicheskii sektor (technical expertise sector) devoted to radiolokatory (radio-location) techniques as well as heading a Special Construction Bureau (SCB) in Leningrad (formerly St. Petersburg). The Glavnoe artilkeriisko upravlenie (GAU, Main Artillery Administration) was the technical branch of the Red Army. It not only had competent engineers and physicists on its central staff, but also had a number of scientific research institutes. Thus, the GAU was also assigned the aircraft detection problem, and Lt. Gen. M. M. Lobanov was placed in charge. After examining existing optical and acoustical equipment, Lobanov also turned to radio-location techniques. For this he approached the Tsentral’naya radiolaboratoriya (TsRL, Central Radio Laboratory) in Leningrad. Here, Yu. K. Korovin was conducting research on VHF communications, and had built a 50 cm (600 MHz), 0.2 W transmitter using a Barkhausen-Kurz tube. For testing the concept, Korovin arranged the transmitting and receiving antennas along the flight path of an aircraft. On January 3, 1934, a Doppler signal was received by reflections from the aircraft at some 600 m range and 100–150 m altitude. For further research in detection methods, a major conference on this subject was arranged for the PVO by the Rossiiskaya Akademiya Nauk (RAN, Russian Academy of Sciences). The conference was held in Leningrad in mid-January, 1934, and chaired by Abram Fedorovich Ioffe, Director of the Leningrad Physical-Technical Institute (LPTI). Ioffe was generally considered the top Russian physicist of his time. All types of detection techniques were discussed, but radio-location received the greatest attention. To distribute the conference findings to a wider audience, the proceedings were published the following month in a journal. This included all of the then-existing information on radio-location in the USSR, available (in Russian language) to researchers in this field throughout the world.

Recognizing the potential value of radio-location to the military, the GAU made a separate agreement with the Leningrad Electro-Physics Institute (LEPI), for a radio-location system. This technical effort was led by B. K. Shembel. The LEPI had built a transmitter and receiver to study the radio-reflection characteristics of various materials and targets. Shembel readily made this into an experimental radio-location system called Bistro (Rapid). The Bistro transmitter, operating at 4.7 m (64 MHz), produced near 200 W and was frequency-modulated by a 1 kHz tone. A fixed transmitting antenna gave a broad coverage of what was called a radiozkzn (radio screen). A receiver, located some distance from the transmitter, had a dipole antenna mounted on a hand-driven reciprocating mechanism. An aircraft passing into the screened zone would reflect the radiation, and the receiver would detect the Doppler-interference beat between the transmitted and reflected signals. Bistro was first tested during the summer of 1934. With the receiver up to 11Km away from the transmitter, the set could only detect an aircraft entering a screen at about 3Km range and at under 1,000m altitude. With improvements, it was believed to have a potential range of 75Km, and five sets were ordered in October for field trials. Bistro is often cited as the USSR’s first radar system; however, it was incapable of directly measuring range and thus was not a true radar.

Research on magnetrons began at Kharkov University in Ukraine during the mid-1920s. Before the end of the decade this had resulted in publications with worldwide distribution, such as the German journal Annalen der Physik (Annals of Physics). Based on this work, Ioffe recommended that a portion of the LEPI be transferred to the city of Kharkov, resulting in the Ukrainian Institute of Physics and Technology (LIPT) being formed in 1930. Within the LIPT, the Laboratory of Electromagnetic Oscillations (LEMO), headed by Abram A. Slutskin, continued with magnetron development. Led by Aleksandr S. Usikov, a number of advanced segmented-anode magnetrons evolved. In 1936, one of Usikov’s magnetrons producing about 7 W at 18 cm (1.7 GHz) was used by Shembel at the NII-9 as a transmitter in a radioiskatel (radio-seeker) called Burya (Storm). Operating similarly to Bistro, the range of detection was about 10 km, and provided azimuth and elevation coordinates estimated to within 4 degrees. No attempts were made to make this intro a pulsed system, thus, it could not provide range and was not qualified to be classified as a radar. It was, however, the first microwave radio-detection system. While work by Shembel and Bonch-Bruyevich on continuous-wave systems was taking place at NII-9, Oshehepkov at the SCB and V. V. Tsimbalin of Ioffe’s LPTI were pursuing a pulsed system. In 1936, they built a radio-location set operating at 4 m (75 MHz) with a peak-power of about 500 W and a 10?s pulse duration. Before the end of the year, tests using separated transmitting and receiving sites resulted in an aircraft being detected at 7Km. In April 1937, with the peak-pulse power increased to 1KW and the antenna separation also increased, test showed a detection range of near 17Km at a height of 1.5Km. Although a pulsed system, it was not capable of directly providing range – the technique of using pulses for determining range had not yet been developed.

In June 1937 all work in Leningrad on radio-location suddenly stopped. The infamous Great Purge of dictator Joseph Stalin swept over the military high commands and its supporting scientific community. The PVO chief was executed. Oshchepkov, charged with “high crime,” was sentenced to 10 years at a Gulag penal labor camp. NII-9 as an organization was saved, but Shenbel was dismissed and Bonch-Bruyevich was named the new director. The Nauchnoissledovatel skii ispytalel nyi institut suyazi RKKA (NIIIS-KA, Scientific Research Institute of Signals of the Red Army), had initially opposed research in radio-location, favouring instead acoustical techniques. However, this portion of the Red Army gained power as a result of the Great Purge, and did an about face, pressing hard for speedy development of radio-location systems. They took over Oshchepkov’s laboratory and were made responsible for all existing and future agreements for research and factory production. This led to the development being placed under a single organization, and the rapid reorganization of the work to accomplish needed results. At Oshchepkov’s former laboratory, work with the 4 m (75 MHz) pulsed-transmission system was continued by A. I. Shestako. Through pulsing, the transmitter produced a peak power of 1 kW, the highest level thus far generated. In July 1938, a fixed-position experimental system detected an aircraft at about 30Km range at heights of 500 m, and at 95Km range, for high-flying targets at 7.5Km altitude. The system was still incapable of directly determining the range.



The project was then taken up by Ioffe’s LPTI, resulting in the development of a mobile system designated Redut (Redoubt). An arrangement of new transmitter tubes was used, giving near 50KW peak-power with a 10?s pulse-duration. Yagi antennas were adopted for both transmitting and receiving. The Redut was first field tested in October 1939, at a site near Sevastopol, a port in Ukraine on the coast of the Black Sea. This testing was in part to show the NKKF (Soviet Navy) the value of early-warning radio-location for protecting strategic ports. With the equipment on a cliff about 160m above sea level, a flying boat was detected at ranges up to 150Km. The Yagi antennas were spaced about 1,000m; thus, close coordination was required to aim them in synchronization.

At the NII-9 under Bonch-Bruyevich, scientists developed two types of very advanced microwave generators. In 1938, a linear-beam, velocity-modulated vacuum tube (a klystron) was developed by N. D. Devyatkov, based on designs from Kharkpv. This device produced about 25 W at 15–18 cm (2.0–1.7 GHz) and was later used in experimental systems. Devyatkov followed this with a simpler, single-resonator device (a reflex klystron). At this same time, D. E. Malyarov and N. F. Alekseyev were building a series of magnetrons, also based on designs from Kharkov; the best of these produced 300 W at 9 cm (3 GHz).

Also at NII-9, D. S. Stogov was placed in charge of the improvements to the Bistro system. Redesignated as Reven (Rhubarb), it was tested in August 1938, but was only marginally better than the predecessor. With additional minor operational improvements, it was made into a mobile system called Radio Ulavlivatel Samoletov (RUS, Radio Catcher of Aircraft), soon designated as RUS-1. This continuous-wave system had a truck-mounted transmitter operating at 4.7 m (64 MHz) and two truck-mounted receivers. Although the RUS-1 transmitter was in a cabin on the rear of a truck, the antenna had to be strung between external poles anchored to the ground. A second truck carrying the electrical generator and other equipment was backed against the transmitter truck. Two receivers were used, each in a truck-mounted cabin with a dipole antenna on a rotatable pole extended overhead. In use, the receiver trucks were placed about 40 km apart; thus, with two positions, it would be possible to make a rough estimate of the range by triangulation on a map. The RUS-1 system was tested and put into production in 1939, then entered service in 1940, becoming the first deployed radio-location system in the Red Army. A total of about 45 RUS-1 systems were built at the Svetlana Factory in Leningrad before the end of 1941, and deployed along the western USSR borders and in the Far East. Without direct ranging capability, however, the military found the RUS-1 to be of little value.

Even before the demise of efforts in Leningrad, the NIIIS-KA had contracted with the UIPT in Kharkov to investigate a pulsed radio-location system for anti-aircraft applications. This led the LEMO, in March 1937, to start an internally funded project with the code name Zenit (a popular football team at the time). The transmitter development was led by Usikov, supplier of the magnetron used earlier in the Burya. For the Zenit, Usikov used a 60 cm (500 MHz) magnetron pulsed at 10–20 ?s duration and providing 3KW pulsed power, later increased to near 10KW. Semion Braude led the development of a superheterodyne receiver using a tunable magnetron as the local oscillator. The system had separate transmitting and receiving antennas set about 65 m apart, built with dipoles backed by 3-meter parabolic reflectors. Zenit was first tested in October 1938. In this, a medium-sized bomber was detected at a range of 3Km. The testing was observed by the NIIIS-KA and found to be sufficient for starting a contracted effort. An agreement was made in May 1939, specifying the required performance and calling for the system to be ready for production by 1941. The transmitter was increased in power, the antennas had selsens added to allow them to track, and the receiver sensitivity was improved by using an RCA 955 acorn triode as the local oscillator. A demonstration of the improved Zenit was given in September 1940. In this, it was shown that the range, altitude, and azimuth of an aircraft flying at heights between 4,000 and 7,000 m could be determined at up to 25Km distance. The time required for these measurements, however, was about 38 seconds, far too long for use by anti-aircraft batteries. Also, with the antennas aimed at a low angle, there was a dead zone of some distance caused by interference from ground-level reflections. While this performance was not satisfactory for immediate gun-laying applications, it was the first full three-coordinate radio-location system in the Soviet Union and showed the way for future systems. Work at the LEMO continued on Zenit, particularly in converting it into a single-antenna system designated Rubin. This effort, however, was disrupted by the invasion of the USSR by Germany in June 1941. In a short while, the development activities at Kharkov were ordered to be evacuated into the Far East. The research efforts in Leningrad were similarly dispersed. Thus, after eight years of effort by highly qualified physicists and engineers, the USSR entered World War II without a fully developed and fielded radar system.

Japan

As a sea-faring nation, Japan had an early interest in wireless (radio) communications. The first known use of wireless telegraphy in warfare at sea was by the Imperial Japanese Navy, in defeating the Russian Imperial Fleet in 1904. There was an early interest in equipment for radio direction-finding, for use in both navigation and military surveillance. The Imperial Navy developed an excellent receiver for this purpose in 1921, and soon most of the Japanese warships had this equipment. In the two decades between the two World Wars, radio technology in Japan made advancements on a par with that in the western nations. There were often impediments, however, in transferring these advancements into the military. For a long time, the Japanese had believed they had the best fighting capability of any military force in the world. The military leaders, who were then also in control of the government, sincerely felt that the weapons, aircraft, and ships that they had built were fully sufficient and, with these as they were, the Japanese Army and Navy were invincible. In 1936 Japan joined Nazi Germany and Fascist Italy in a Tripartite Pact.

Radio engineering was strong in Japan’s higher education institutions, especially the Imperial (government-financed) universities. This included undergraduate and graduate study, as well as academic research in this field. Special relationships were established with foreign universities and institutes, particularly in Germany, with Japanese teachers and researchers often going overseas for advanced study. The academic research tended toward the improvement of basic technologies, rather than their specific applications. There was considerable research in high-frequency and high-power oscillators, such as the magnetron, but the application of these devices was generally left to industrial and military researchers. One of Japan’s best-known radio researchers in the 1920s-30s era was Professor Hidetsugu Yagi. After graduate study in Germany, England, and America, Yagi joined Tohoku University where his research centered on antennas and oscillators for high-frequency communications. A summary of the radio research work at Tohoku University was contained in a 1928 seminal paper by Yagi. Jointly with Shintaro Uda, one Yagi’s first doctoral students, a radically new antenna emerged. It had a number of parasitic elements (directors and reflectors) and would come to be known as the Yagi-Uda, Yagi antenna or Yagi Array. A U.S. patent, issued in May 1932, was assigned to RCA. To this day, this is the most widely used directional antenna worldwide.

The cavity magnetron was also of interest to Yagi. This HF (~10-MHz) device had been invented in 1921 by Albert W. Hull at General Electric, and Yagi was convinced that it could function in the VHF or even the UHF region. In 1927, Kinjiro Okabe, another of Yagi’s early doctoral students, developed a split-anode device that ultimately generated oscillations at wavelengths down to about 12 cm (2.5 GHz). Researchers at other Japanese universities and institutions also started projects in magnetron development, leading to improvements in the split-anode device. These included Kiyoshi Morita at the Tokyo Institute of Technology, and Tsuneo Ito at Tokoku University. Shigeru Nakajima at Japan Radio Company (JRC) saw a commercial potential of these devices and began the further development and subsequent very profitable production of magnetrons for the medical dielectric heating (diathermy) market. The only military interest in magnetrons was shown by Yoji Ito at the Naval Technical Research Institute (NTRI). The NTRI had been formed in 1922, and became fully operational in 1930. Located at Meguro, Tokyo, near the Tokyo Institute of Technology, first-rate scientists, engineers, and technicians were engaged in activities ranging from designing giant submarines to building new radio tubes. Included were all of the precursors of radar, but this did not mean that the heads of the Imperial Navy accepted these accomplishments. In 1936 Tsuneo Ito developed an 8-split-anode magnetron that produced about 10 W at 10 cm (3 GHz). Based on its appearance, it was named Tachibana (or Mandarin, an orange citrus fruit). Tsuneo Ito also joined the NTRI and continued his research on magnetrons in association with Yoji Ito. In 1937, they developed the technique of coupling adjacent segments (called push-pull), resulting in frequency stability, an extremely important magnetron breakthrough. By early 1939 NTRI/JRC had jointly developed a 10cm (3GHz), stable-frequency Mandarin-type magnetron (No. M3) that, with water cooling, could produce 500W power. In the same time period, magnetrons were built with 10 and 12 cavities operating as low as 0.7 cm (40 GHz). The configuration of the M3 magnetron was essentially the same as that used later in the magnetron developed by Boot and Randall at Birmingham University in early 1940, including the improvement of strapped cavities. Unlike the high-power magnetron in Great Britain, however, the initial device from the NTRI generated only a few hundred watts.

In general, there was no lack of scientific and engineering capabilities in Japan; their warships and aircraft clearly showed high levels of technical competency. They were ahead of Great Britain in the development of magnetrons, and their Yagi antenna was the world standard for VHF systems. It was simply that the top military leaders failed to recognize how the application of radio in detection and ranging – what was often called the Radio Range Finder (RRF) – could be of value, particularly in any offensive role; offence, not defence, totally dominated their thinking.

In 1938 engineers from the Research Office of the Nippon Electric Company (NEC) were making coverage tests on high-frequency transmitters when rapid fading of the signal was observed. This occurred whenever an aircraft passed over the line between the transmitter and receiving meter. Masatsugu Kobayashi, the Manager of NEC’s Tube Department, recognized that this was due to the beat-frequency interference of the direct signal and the Doppler-shifted signal reflected from the aircraft. Kobayashi suggested to the Army Science Research Institute that this phenomenon might be used as an aircraft warning method. Although the Army had rejected earlier proposals for using radio-detection techniques, this one had appeal because it was based on an easily understandable method and would require little developmental cost and risk to prove its military value. NEC assigned Kinji Satake of their Research Institute to develop a system called the Bi-static Doppler Interference Detector (BDID). For testing the prototype system it was set up on an area recently occupied by Japan along the coast of China. The system operated between 4.0-7.5 MHz (75–40 m) and involved a number of widely spaced stations; this formed a radio screen that could detect the presence (but nothing more) of an aircraft at distances up to 500Km. The BDID was the Imperial Army’s first deployed radio-based detection system, placed into operation in early 1941. A similar system was developed by Satake for the Japanese homeland. Information centres received oral warnings from the operators at BDID stations, usually spaced between 65 and 240Km. Although originally intended to be temporary until better systems were available, they remained in operation throughout the war. It was not until after the start of war the Imperial Army had equipment that could be called a radar.

In the mid-1930s, some of the technical specialists in the Imperial Navy became interested in the possibility of using radio to detect aircraft. For consultation, they turned to Professor Yagi who, then the Director of the Radio Research Laboratory at Osaka Imperial University. Yagi suggested that this might be done by examining the Doppler frequency-shift in a reflected signal. Funding was provided to the Osaka Laboratory for experimental investigation of this technique. Kinjiro Okabe, the inventor of the split-anode magnetron and who had followed Yagi to Osaka, led the effort. Theoretical analyses indicated that the reflections would be greater if the wavelength was approximately the same as the size of aircraft structures. Thus, a VHF transmitter and receiver with Yagi antennas separated some distance were used for the experiment. In 1936 Okabe successfully detected a passing aircraft by the Doppler-interference method; this was the first recorded demonstration in Japan of aircraft detection by radio. With this success, Okabe’s research interest switched from magnetrons to VHF equipment for target detection. This, however, did not lead to any significant funding. The top levels of the Imperial Navy believed that any advantage of using radio for this purpose were greatly outweighed by enemy intercept and disclosure of the sender’s presence. Historically, warships in formation used lights and horns to avoid collision at night or when in fog. Newer techniques of VHF radio communications and direction-finding might also be used, but all of these methods were highly vulnerable to enemy interception. At the NTRI, Yoji Ito proposed that the UHF signal from a magnetron might be used to generate a very narrow beam that would have a greatly reduced chance of enemy detection. Development of microwave system for collision avoidance started in 1939, when funding was provided by the Imperial Navy to JRC for preliminary experiments. In a cooperative effort involving Yoji Ito of the NTRI and Shigeru Nakajima of JRC, an apparatus using a 3cm (10GHz) magnetron with frequency modulation was designed and built. The equipment was used in an attempt to detect reflections from tall structures a few Km away. This experiment gave poor results, attributed to the very low power from the magnetron. The initial magnetron was replaced by one operating at 16 cm (1.9 GHz) and with considerably higher power. The results were then much better, and in October 1940, the equipment obtained clear echoes from a ship in Tokyo Bay at a distance of about 10Km. There was still no commitment by top Japanese naval officials for using this technology aboard warships. Nothing more was done at this time, but late in 1941 the system was adopted for limited use.

In late 1940, Japan arranged for two technical missions to visit Germany and exchange information about their developments in military technology. Commander Yoji Ito represented the Navy’s interest in radio applications, and Lieutenant Colonel Kinji Satake did the same for the Army. During a visit of several months, they exchanged significant general information, as well as limited secret materials in some technologies, but little directly concerning radio-detection techniques. Neither side even mentioned magnetrons, but the Germans did apparently disclose their use of pulsed techniques. After receiving the reports from the technical exchange in Germany, as well as intelligence reports concerning the success of Great Britain using RDF, the Naval General Staff tentatively accepted pulse-transmission technology.


Radar array on the Japanese flagship Yamato

On 2nd August 1941, even before Yoji Ito returned to Japan, funds were allocated for the initial development of pulse-modulated radars. Commander Chuji Hashimoto of the NTRI was responsible for initiating this activity. A prototype set operating at 4.2 m (71 MHz) and producing about 5KW was completed with great urgency. With the NTRI in the lead, the firm NEC and the Research Laboratory of Japan Broadcasting Corporation (NHK) made major contributions to the effort. Kenjiro Takayanagi, Chief Engineer of NHK’s experimental television station and called “the father of Japanese television,” was especially helpful in rapidly developing the pulse-forming and timing circuits, as well as the receiver display. In early September 1941 the prototype set was first tested; it detected a single bomber at 97Km and a flight of aircraft at 145Km. The system, Japan’s first full Radio Range Finder (RRF – radar), was designated Mark 1 Model 1. Contracts were given to three firms for serial production; NEC built the transmitters and pulse modulators, Japan Victor the receivers and associated displays, and Fuji Electrical the antennas and their servo drives. The system operated at 3.0 m (100 MHz) with a peak-power of 40KW. In November 1941 the first manufactured RRF was placed into service as a land-based early-warning system at Katsuura, Chiba, a town on the Pacific coast about 100Km from Tokyo. The detection range was about 130Km for single aircraft and 250Km for groups.

Netherlands

Early radio-based detection in the Netherlands were along two independent lines: the development of a microwave system at the firm of Philips and of a VHF system at a laboratory of the Armed Forces. The Philips Company in Eindhoven, Netherlands had founded the Natuurkundig Laboratorium (NatLab) for fundamental research related to its products. NatLab researcher Klass Posthumus developed a magnetron, and C.H.J.A. Stall tested it by using parabolic transmitting and receiving antennas set side-by-side, both aimed at a large plate some distance away. To overcome frequency instability of the magnetron, pulse modulation was used. It was found that the plate reflected a strong signal. Recognizing the potential importance of this as a detection device, NatLab arranged a demonstration for the Koninklijke Marine (Royal Netherlands Navy) in 1937, and the Navy was sufficiently impressed to initiate sponsorship of the research. In 1939 an improved set was demonstrated to detect a vessel at a distance of 3.2Km. A prototype system was built by Philips, and plans were started by the firm Nederlandse Seintoestellen Fabriek (a Philips subsidiary) for building a chain of warning stations to protect the primary ports. Some field testing of the prototype was conducted, but the project was discontinued when Germany invaded the Netherlands on 10th May 1940. Within the NatLab, however, the work was continued in great secrecy until 1942.

During the early 1930s there were widespread rumours of a "death ray" being developed. The Dutch Parliament set up a Committee for the Applications of Physics in Weaponry under G.J. Elias to examine this potential, but the Committee quickly discounted the existence of death rays. The Committee did, however, establish the Laboratorium voor Fysieke Ontwikkeling (LFO, Laboratory for Physical Development), dedicated to supporting the Netherlands Armed Forces. Operating in great secrecy, the LFO opened a facility called the Meetgebouw (Measurements Building) located on the Plain of Waalsdorp. In 1934, J.L.W.C. von Weiler joined the LFO and, with S.G. Gratama, began research on a 1.25m (240MHz) communication system to be used in artillery spotting. In 1937, while tests were being conducted on this system, a passing flock of birds disturbed the signal. Realizing that this might be a potential method for detecting aircraft, the Minister of War ordered continuation of the experiments. Weiler and Gratama set about developing a system for directing searchlights and aiming anti-aircraft guns. This experimental "electrical listening device" operated at 70 cm (430 MHz) and used pulsed transmission at a PRF of 10 KHz. A transmit-receive blocking circuit was developed to allow use of a common antenna. The received signal was displayed on a cathode ray tube with a circular time base. This set was demonstrated to the Army in April 1938 and detected an aircraft at a range of 18 Km. The set was rejected, however, because it could not withstand the harsh environment of Army combat conditions.

The Navy was more receptive. Funding was provided for final development, and Max Staal was added to the team. To maintain secrecy, they divided the development into parts. The transmitter was built at the Delft Technical College and the receiver at the University of Leiden. Ten sets were assembled under the personal supervision of J.J.A. Schagen van Leeuwen, head of the firm Hazemeijer Fabriek van Signaalapparaten. The prototype had a peak-power of 1KW with a 10 to 20KHz PRF. The receiver was a super-heterodyne type using Acorn tubes and 6MHz intermediate frequency (IF) stage. The antenna consisted of 4 rows of 16 half-wave dipoles backed by a 3m x 3m mesh screen. The operator used a bicycle-type drive to rotate the antenna, and the elevation could be changed using a hand crank. Several sets were completed, and one was put into operation on the Malievelt in The Hague just before the Netherlands fell to Germany in May 1940. The set worked well, spotting enemy aircraft during the first days of fighting. To prevent capture, operating units and plans for the system were destroyed. Von Weiler and Max Staal fled to England aboard one of the last ships able to leave, carrying two disassembled sets with them. Later, Gratama and van Leeuwen also escaped to England. Thus the Netherlands radar secrets became available to Britain.

By the early 1930s screen grid and pentode valves were available for RF amplification for frequencies up to about 30 MHz, which was adequate for both broadcast and commercial purposes at the time, when radio usage had not extended into the UHF band. At frequencies above 30 MHz the gain available from valves fell very sharply; there were two principal problems: the first was caused by the inductance and capacitance of the internal leads that connected the valve electrodes to the terminating pins; the second was due to the finite transit time that the electrons took to travel between the valve electrodes. The first problem arose through the valve design and manufacturing techniques which had evolved from those used in the electric lamp industry. One particular constructional feature of the valve, copied directly from the lamp industry, was the use of an internal glass stem and pinch that held the support wires to the electrode assembly, and also provided a vacuum seal for the lead-out wires. The problem that arose from this method of construction was that the total length of the connections from the electrodes to their terminating pins was quite long, resulting in significant self-inductance of the wires as well as excessive self-capacitance between them. At frequencies below 30 MHz, these parasitic inductive and capacitive components did not seriously affect the performance of the valve, but their effects became increasingly more serious at frequencies above this. The second problem, the finite transit time for the electrons to move between the electrodes, was very serious for valve circuits operating at frequencies above 30 MHz. For a typical RF valve of conventional construction, the transit time for the electrons to move between the cathode and control grid was about 1ns. At frequencies of a few MHz, this transit time was insignificant compared with the time for one cycle of the signal frequency. At 100 MHz, however, the time was about 10% of one cycle and this was very significant. The phase lag caused by this time delay resulted in a low input resistance at high frequencies, which significantly reduced the amplification available. The problem was one of the invariance of the speed of light (or electricity) and, as with the later development of computers, it was therefore necessary to get smaller to go faster. A great deal of experimental research work was carried out at the RCA laboratories during the early 1930s to investigate the behaviour of radio frequency amplifier valves, where it was found that improved circuit performance could be achieved if the valve dimensions were reduced. With a linear reduction, the mutual conductance and other valve parameters remained almost unchanged, but the lead inductance, interelectrode capacitance and electron transit time all fell in direct proportion to the reduction of dimensions.



The tiny Acorn valves that resulted from this work were capable of providing amplification at frequencies up to about 400 MHz. The first of these valves to go into production was the type 955 triode which was introduced in 1934. This was followed by the 954 pentode in 1935, and by the type 956 in 1936. They all had indirectly heated cathodes, operating at 6.3 V, 0.15 A. The diameter of the heater-cathode assembly was comparable with that of a common household pin and the overall length was less than one half of conventional valves. The capacitance between the control grid and the anode for both the triode and the pentode was again about half that of conventional valves, and all other internal capacitances were also significantly reduced. Before long, acorn valves based on the RCA design, were introduced in Britain by Mazda, Marconi-Osram and Mullard. Initally, all the British acorn valves had 4 V heaters, but 6.3 V versions were introduced in 1940. Mullard was, however, wholly owned by the Netherlands Philips Company, and all the valve R&D; work was carried out at the Philips Eindhoven plant. Suitably robust valves for radar equipment were being developed for the Netherlands government, and the UK government approached the Dutch government to supply samples. The valve in question was the EF50, capable of providing wideband UHF amplification, and at that time all the valves were being manufactured in Holland. With the outbreak of war it was realized that the supply of EF50 valves would dry up and Mullard itself did not have the capability of manufacturing the special glass base with sealed-in pins. Consequently, just before Germany invaded Holland, a lorry was sent from Holland to Britain with one million of these glass bases. Later, huge numbers of the valves were manufactured by Sylvania in the USA.

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, worked for the Compagnie Générale de Télégraphie Sans Fil (CSF) where he and Robert Warneck improved his father's magnetrons. Following reports made by the U.S. Naval Research Laboratory concerning detection by interference methods, Henri Gutton started the development of a radio-detection apparatus using the short-wavelength tubes in 1934. Emile Girardeau,the head of the CSF, has stated that they were at the time intending to build radio-detection systems "conceived according to the principles stated by Tesla". In 1934, CSF submitted a patent application for a device for detecting obstacles using continuous radiation of ultra-short wavelengths produced by a magnetron. Since this device could not directly measure distance, it was not a true radar, but it was the first patent of an operational radio-detection apparatus using centimetric wavelengths. The system was tested in late 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 SS Normandie as early as mid-1935 for operational use. These equipments depended on Doppler interference for detection. In late 1937 Maurice Elie at the Société Française Radio-Éléctrique (SFR) developed a means of pulse-modulating transmitter tubes. This led to a new 16cm system with a peak power near 10 W. The Navy set up tests in early 1939 and detected large vessels at 10Km, but it was unable to detect aircraft and was thus not accepted by the military. French and U.S. patents were filed in December 1939. The system was planned to be sea-tested aboard the SS Normandie, but this was cancelled at the outbreak of war. Pierre David at the Laboratorie National de Radioéléctricitée (National Laboratory of Radioelectricity, LNR) experimented with reflected radio signals at about a m wavelength. Starting in 1931, he observed that aircraft caused interference to the signals. The LNR then 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 system became critical. David realized the advantages of a pulsed system, and in October 1938 he designed a 50MHz, pulse-modulated system with a peak-pulse power of 12 Kw. This was built by the firm SADIR.


SADIR Direction Finding Equipment

France declared war on Germany on 1st September 1939, and there was a great need for an early-warning detection system. The SADIR system was therefore taken to near Tulon, and it detected and measured the range of invading aircraft as far away as 55Km. The SFR pulsed system was also set up near Paris where it detected aircraft at ranges up to 130Km.


1938 Henri Gutton magnetron

To further improve the SFR system, Henri Gutton and Robert Warneck developed in 1938 a 500cm (600MHz) magnetron with an oxide-coated cathode that produced 500 W. In May 1940, just before the Germans arrived, several of the new magnetrons were taken to Britain where their oxide-coated cathods were used to improve the performance of the Boots and Randall magnetron. French designs thus contributed to the British war effort after the occupation of France by the Germans.

Italy

Guglielmo Marconi initiated the research in Italy on radio-based detection technology. In 1933, while participating with his Italian firm in experiments with a 600MHz communications link across Rome, he noted transmission disturbances caused by moving objects adjacent to its path. This led to the development at his laboratory at Cornegliano of a 330MHz (0.91m) Doppler detection system that he called radioecometro. Barkhausen-Kurz tubes were used in both the transmitter and receiver. In May 1935 Marconi demonstrated his system to the Fascist dictator Benito Mussolini and members of the military General Staff; however the output power was insufficient for military use. While Marconi’s demonstration raised considerable interest, little more was done with his apparatus. Mussolini, however, directed that radio-based detection technology be further developed, and it was assigned to the Regio Instituto Electrotecnico e delle Comunicazioni (RIEC, Royal Institute for Electro-technics and Communications). The RIEC had been established in 1916 on the campus of the Italian Naval Academy in Livorno. Lieutenant Ugo Tiberio, a physics and radio-technology instructor at the Academy, was assigned to head the project on a part-time basis. Tiberio prepared a report on developing an experimental apparatus that he called telemetro radiofonico del rivelatore (RDT, Radio-Detector Telemetry). The report was submitted in mid-1936. When the work got underway Nello Carrara, a civilian physics instructor who had been doing research at the RIEC in microwaves, was appointed to be responsible for developing the RDT transmitter. Before the end of 1936 Tiberio and Carrara had demonstrated the EC-1, the first Italian RDT system. This had an FM transmitter operating at 200 MHz (1.5 m) with a single parabolic cylinder antenna. It detected by mixing the transmitted and the Doppler-shifted reflected signals, resulting in an audible tone. The EC-1 did not provide a range measurement; to add this capability, development of a pulsed system was initiated in 1937. Captain Alfeo Brandimarte joined the group and primarily designed the first pulsed system, the EC-2. This operated at 175 MHz (1.7 m) and used a single antenna made with a number of equi-phased dipoles. The detected signal was intended to be displayed on an oscilloscope. There were many problems, and the system never reached the testing stage. Work then turned to developing higher power and operating frequencies. Carrara, in cooperation with the firm FIVRE, developed a magnetron-like device. This was composed of a pair of triodes connected to a resonant cavity and produced 10 kW at 425 MHz (70 cm). It was used in designing two versions of the EC-3, one for shipboard, and the other for coastal defence.


The Italian ship Littorio, with the EC-3 radar antenna mounted above the bridge on the lower rangefinder

Italy, joining Germany, entered WW2 in September 1939 without an operational radar. A breadboard of the EC-3 was built and tested from the top of a building at the Academy, but most radar research work was stopped as direct support of the war took priority.

UK

In 1915 Robert Watson Watt joined the Meteorological Office as a meteorologist, working at an outstation at Aldershot in Hampshire. Over the next 20 years he studied atmospheric phenomena and used the radio signals generated by lightning strikes to map out the position of thunderstorms. The difficulty in pinpointing the direction of these momentary signals led to the use of rotating directional antennas, and in 1923 to the use of oscilloscopes in order to display the signals. An operator would periodically rotate the antenna and look for "spikes" on the oscilloscope to find the direction of a storm. The operation eventually moved to the outskirts of Slough in Berkshire, and in 1927 it became the Radio Research Station (RRS), Slough, established by the Department of Scientific and Industrial Research (DSIR). Watson Watt was appointed the RSS Superintendent.

As the Second World War approached the likelihood of air raids and the threat of invasion by air and sea drove a major effort in applying science and technology to defence. In November 1934 the Air Ministry established the Committee for Scientific Survey of Air Defence (CSSAD) with the official function of considering "how far recent advances in scientific and technical knowledge can be used to strengthen the present methods of defence against hostile aircraft." Commonly called the "Tizard Committee" after its Chairman, Sir Henry Tizard, this group had a profound influence on technical developments in Great Britain.

H.E. Wimperis, Director of Scientific Research at the Air Ministry and a member of the Tizard Committee, had read about Nikola Tesla's claim of inventing a 'death ray'. Watson Watt was now well established as an authority in the field of radio, and in January 1935 Wimperis contacted him asking if radio might be used for such a device. Watson Watt wrote back that this was unlikely, but added the following comment: "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." This radio detection problem was considered and detailed calculations of necessary transmitter power, reflection characteristics of an aircraft, and needed receiver sensitivity were made. Watson Watt sent this information to the Air Ministry on 12 February 1935, in a secret report titled "The Detection of Aircraft by Radio Methods."


Sketch of the Daventry Experiment, 26 February 1935, to detect radio signals reflected from an aircraft.

Reflection of radio signals was critical to the proposed technique, and the Air Ministry asked if this reflection could be proven. To test this, receiving equipment was set up in a field near Upper Stowe, Northamptonshire. On 26 February 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 Daventry. The aircraft reflected the 6 MHz (49m wavelength) BBC signal, and this was readily detected by Doppler-beat interference at ranges up to 13 Km. This convincing test, known as the Daventry Experiment, was witnessed by a representative from the Air Ministry, and led to immediate authorization to build a full demonstration system.

A preliminary system based on pulsed transmission, as used for probing the ionosphere, was designed and built at the RRS. The original transmitter had a peak power of about 1 KW, but a more powerful transmitter was then built, operating at 6 MHz (50m wavelength), with pulse-repetition rate of 25 Hz, pulse width of 25 microseconds, and approaching 100 KW power. Orfordness, a narrow 30 Km peninsula in Suffolk along the coast of the North Sea, was selected as the test site. Here the equipment was operated in the guise of an ionospheric monitoring station from May 1935. Six wooden towers were erected, two for stringing the transmitting antenna, and four for the corners of the crossed receiving antennas. On 17 June 1935 radio-based detection and ranging was first demonstrated in Great Britain, the first target detected being a Supermarine Scapa flying boat at 27 Km range.

In December 1935 the British Treasury appropriated £60,000 for a five-station system called Chain Home (CH), covering approaches to the Thames Estuary. The Tizard Committee coined the acronym RDF as a cover for the work, meaning Range and Direction Finding. By March 1936, the research and development was centered at the Bawdsey Research Station located at Bawdsey Manor in Suffolk. While this operation was under the Air Ministry, both the Army and Navy became involved and soon initiated their own programs.


Bawdsey Manor

At Bawdsey, outstanding engineers and scientists evolved the technology that was code-named Range and Direction Finding (RDF), but much of the credit belongs to Watson-Watt, the head of the team, who turned from the technical side of RDF to building up a usable network of machines and the people to run them. After watching a demonstration in which his equipment operators were attempting to locate a test aircraft, he noticed that the primary problem was not technological, but worker overload. Following Watson-Watt's advice, by early 1940 the RAF had built up a layered control organization that efficiently passed information along the chain of command, and was able to track large numbers of hostile aircraft and direct British fighters to them.

Air Ministry

In March 1936 the work at Orfordness was moved to nearby Bawdsey Manor on the mainland. Until this time, the work had officially still been under the DSIR, but was now transferred to the Air Ministry. At the new Bawdsey Research Station the CH equipment was assembled as a prototype. The Royal Air Force (RAF) first used the prototype station in September 1936. The initial hardware at the CH stations was as follows: 1. A transmitter operating on four pre-selected frequencies between 20 and 55 MHz, adjustable within 15 seconds, and delivering a peak power of 200 KW. The pulse duration was adjustable between 5 and 25 microseconds, with a pulse repetition frequency of either 25 or 50 Hz; for synchronization of all CH transmitters, the pulse generator was locked to the 50 Hz of the British power grid; 2. Four 120m steel towers supporting transmitting antennas; 3. Four 80m wooden towers supporting cross-dipole arrays at three different levels; and 4. a goniometer to improve the directional accuracy from the multiple receiving antennas. The CH output was read with an oscilloscope. When a pulse was sent out into the broadcast towers, the scope was triggered to start its beam scanning horizontally across the screen. 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 on a scale at the bottom of the screen, gave 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, while the size of the vertical displacement provided an estimate 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. Problems arose when the aircraft approached so close that they entered the ground ray.

By the summer of 1937 twenty initial CH stations were being tested. A major RAF exercise was performed before the end of the year, and was such a success that £10 million was appropriated by the Treasury for an eventual full chain of coastal stations. At the start of 1938 the RAF took over control of all CH stations, and the network began regular operations.

From 1938, in addition to the work on CH and its successor systems, there was now major work in airborne RDF equipment. 200MHz (1.5m wavelength) sets were developed, the higher frequency allowing smaller antennas appropriate for aircraft installation.

The British Army

From the initiation of RDF work at Orfordness the Air Ministry had kept the British Army and the Royal Navy generally informed; this led to both of these forces having their own RDF developments.

In 1931 the Woolwich Research Station of the Army’s Signals Experimental Establishment (SEE) had experimented with pulsed 600 MHz (50cm wavelength) signals for the detection of ships. Although a memorandum on this subject was prepared, and preliminary experiments were performed, for undefined reasons the War Office did not adopt the recommendations. As the Air Ministry’s work on RDF progressed, Watson Watt informed the Royal Engineer and Signals Board of the RDF equipment and techniques being developed at Orfordness. His report "The Proposed Method of Aeroplane Detection and Its Prospects” led the SEE to set up an "Army Cell" at Bawdsey in October 1936, which developed two general types of RDF equipment: Gun-Laying (GL) systems for assisting anti-aircraft guns and searchlights, and Coastal-Defence (CD) systems for directing coastal artillery at Army bases overseas. A gun-laying RDF equipment code-named Mobile Radio Unit (MRU) consisted of a small lorry-mounted version of a CH station, operating at 23 MHz (13m wavelength) with a power of 300 KW. A single 35m tower supported a transmitting antenna, as well as two receiving antennas set orthogonally for estimating the signal bearing. In February 1937 a developmental unit detected an aircraft at 96 Km range. The Air Ministry also adopted this system as a mobile auxiliary unit, additional to the CH system.

In early 1938 development of a CD system based on the evolving Air Ministry 200MHz (1.5m wavelength) airborne sets was begun. The transmitter had a 400Hz pulse rate, a 2 microseconds pulse width, and 50KW power (later increased to 150 KW). Since the Army version of the system would not be airborne, there were no limitations on antenna size. By May 1939 the CD RDF could detect aircraft flying as low as 160m and at a range of 40 Km. With an antenna mounted 20m above sea level it could determine the range of a 2,000 ton ship at 38 Km with an angular accuracy of as little as a quarter of a degree.

While at Bawdsey, the Army Cell developed a GL system code-named Transportable Radio Unit (TRU). Operating at 60 MHz (6m wavelength) with 50 KW power, the TRU had two vans for the electronic equipment plus a power van; it used a 35m erectable tower to support a transmitting antenna and two receiving antennas. A prototype was successfully tested in October 1937, detecting aircraft at 96 Km range; production of 400 sets designated GL Mk I started the following June. The Air Ministry also adopted some of these sets as gap-fillers and emergency substitutes in the CH network. As the war started, GL Mk I sets were used overseas by the British Army in Malta and Egypt during 1939 and 1940. Seventeen sets were sent to France with the British Expeditionary Force; most of these were destroyed at the Dunkirk evacuation in late May 1940, but a few were captured and gave the Germans their first full information on British RDF hardware. An improved version, GL Mk II, was used throughout the war; some 1,700 sets were put into service, including over 200 supplied to the Soviet Union. Operational research found that anti-aircraft batteries using the GL averaged 4,100 rounds fired per hit, compared with about 20,000 rounds for unassisted guns.

In early 1938, Alan Butement began the development of a Coastal Defense (CD) system that involved some of the most significant advancements in the early history of radar. The 200 MHz transmitter and receiver already being developed for the AI and ASV sets were used at first, but, since the CD would not be airborne, the use of more power and a much larger antenna became possible. The transmitter power was increased to 150 KW, and a huge dipole array, 3.5m high and 8m wide, was developed, giving much narrower beams and higher gain. This “broadside” array was rotated at 1.5 revolutions per minute, sweeping a field covering 360 degrees. Lobe switching was incorporated in the transmitting array, giving higher directional accuracy. For analysis of the system, Butement formulated the first mathematical relationship used in Britain that would later become the well-known “radar range equation”. Early tests showed that the CD set had much better capabilities for detecting aircraft at low altitudes than the existing CH. Consequently, the CD was also adopted by the Air Defence to augment the CH stations; in this role it was designated the Chain Home Low (CHL).

Royal Navy

Although the Royal Navy maintained close contact with the Air Ministry work at Bawdsey, they chose to establish their own RDF development at the Experimental Department of His Majesty’s Signal School (HMSS) in Portsmouth, Hampshire, on the south coast. The HMSS started RDF work in September 1935. Initial experiments were in wavelengths ranging between 75 MHz (4 m wavelength) and 1.2 GHz (25 cm wavelength). All the work was under the utmost secrecy; it could not even be discussed with other scientists and engineers at Portsmouth. The initial 75MHz set designated Type 79X, capable of range-finding but not direction, proved unsatisfactory, so the frequency was decreased to 43 MHz (7 m wavelength), designated as Type 79Y, with separate, stationary transmitting and receiving antennas. Prototypes of the Type 79Y air-warning system were successfully tested at sea in early 1938. The detection range of aircraft was between 48 and 80 Km, depending on the aircraft height. Type 79Y systems were then placed into service in August 1938 on the cruiser HMS Sheffield and in October on the battleship HMS Rodney. These were the first vessels in the Royal Navy to be fitted with RDF systems.

Valve Developments

From the mid-1930s the UK top-secret work radar work required thermionic valves for amplification which would not fail at high frequencies (small wavelengths). At this time Tom Goldup, a senior director of the Mullard Valve Company, was liaising with the British government and was made aware of this requirement. Mullard was wholly owned by the Dutch Philips Company and all the valve R&D; work was carried out at the Philips Eindhoven plant. Goldup approached Philips asking if there was a valve with the required specification (because RDF could not be mentioned, he probably referred to high-frequency television applications). He was told that a suitable valve was being developed for the Dutch government; samples, therefore, could not be supplied to Mullard. It would appear that the UK government approached the Dutch government directly and samples were then supplied.

The valve in question was the EF50, which became available for television use in 1939. At this time all the valves were being manufactured in Holland. The construction of the Mullard EF50 is of interest because it marked a significant departure from the conventional types used in Britain at the time. The usual Bakelite base and internal glass pinch were replaced by an all-glass base. Elimination of the stem and pinch resulted in a considerable reduction in length of the internal wires. The valve had nine chromium-iron pins, which were sealed into the glass base and arranged uniformly around a central metallic spigot, which was keyed in order to facilitate insertion into the valveholder. The spigot was joined to an external metal screen that covered the whole base, with small holes to allow the pins to come through. Because of the screening provided, it was possible to bring all the connections out to the base, avoiding the need for a top cap connection.

In early 1938 John F. Coales initiated the development of 600 MHz (50cm wavelength) equipment. The higher frequency allowed narrower beams (needed for air search) and antennas more suitable for shipboard use. The first 50cm set was Type 282, with 25 KW output and a pair of Yagi arrays incorporating lobe-switching, given sea trials in June 1939. This set detected low-flying aircraft at 4 Km and ships at 8 Km range.

World War 2

Immediately after the war began in September 1939, the Air Ministry RDF development at Bawdsey was temporarily relocated to University College, Dundee. A year later the operation moved to near Worth Matravers in Dorset on the southern coast of England, and was named the Telecommunications Research Establishment (TRE).


In a final wartime move, the TRE relocated to Malvern College in Great Malvern. All the Air Ministry radar systems were given the official designation Air Ministry Experimental Station (AMES) plus a Type number.

Chain Home

Shortly before the outbreak of World War II, several RDF (radar) stations known as Chain Home (CH) were constructed along the South and East coasts of Britain, based on the successful model at Bawdsey. The broadcast aerials were two 90m steel towers strung with a series of antennas between them. A second set of 73m wooden towers were used for reception, with a series of crossed antennas at various heights up to 65m. Typical CH operating conditions were: Frequency 20 to 30 MHz (wavelength 15 to 10m), Peak power 350kW, Pulse repetition frequency 25 and 12.5 pps, and Pulse length 20?s. The CH output was read with an oscilloscope. When a pulse was sent out into the broadcast towers, the oscilloscope was triggered to start its beam moving horizontally across the screen. The output from the receiver was amplified and fed into the vertical axis of the oscilloscope, so a return from an aircraft deflected 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, gave 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, while the size of the vertical displacement gave an estimate of the number of aircraft involved. By comparing the strengths returned from the various antennas on the tower, the altitude of the targets could also be determined fairly accurately.


Chain Home coverage

CH proved highly effective during the Battle of Britain, and is often credited with enabling the RAF to defeat the much larger Luftwaffe forces. Whereas the Luftwaffe had to hunt to find the RAF fighters, the RAF knew exactly where the German bombers were, and could converge all their fighters onto them. The RAF fighters could therefore operate as if they were a much larger force. In addition, the CH system allowed pilots to rest on the ground, instead of flying continuous standing patrols, and they only needed to scramble when the air threat was imminent. This reduced the pilots' workloads, reduced wear on engines, and reduced fuel 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 it was possible to return the stations to operation within a few days. Meanwhile, radar-like signals were broadcast 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 German High Command apparently never understood the importance of radar to the RAF's efforts, or they would have assigned these stations a much higher priority, but even a concerted effort would not have had much effect on the transmitters, as their structure allowed a blast to pass through the spaces in the metal lattice.

Systems similar to CH were later adapted with a new display to produce the Ground-Controlled Intercept (GCI) stations in January 1941. In these systems the antenna was rotated mechanically, in synchronism with the display on the operator's console, i.e. instead of a single line across the bottom of the display from left to right, the line was rotated radially around the screen at the same speed as the antenna was turning. The result was a 2D display of the air space around the station, with the operator in the centre of the display and all the aircraft appearing as dots in their proper location in space. Called Plan Position Indicators (PPI), these dramatically simplified the amount of work needed to track a target on the operator's part. Philo Taylor Farnsworth, the American inventor of all-electronic television in 1927, contributed to this by creating the Iatron cathode ray tube (CRT) that could store an image from a few ms up to a few hours. One version of this CRT kept an image illuminated with a persistence of about 1s before fading, and this slow-to-fade display tube proved to be a very useful addition to the evolution of radar.


Battle of Britain operations room at RAF Uxbridge

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 CH 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 systems 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 the Women's Auxiliary Air Force (WAAF) until more reliable motorized movements were installed in 1941.

The Luftwaffe then took to avoiding the fighters by flying at night and in bad weather. Although the RAF control stations were 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 starting in 1936 developed a miniaturized RDF system suitable for aircraft, the Air Interception (AI) set. At the same time Air to Surface Vessel (ASV) sets were also developed, making a significant contribution to the defeat of the German U-boats.

Initial AI sets were available in 1939 and they were fitted to Bristol Blenheim aircraft, replaced quickly with the better performing Bristol Beaufighter. These quickly put an end to German night-bombing and bad-weather bombing over Britain. Mosquito night intruders were fitted with AI Mk VIII, "Serrate" (a device to track German night fighters from their Lichtenstein signal emissions, and "Perfectos" (a device that tracked German Identification Friend or Foe (IFF) signals, allowing the Mosquito to find and destroy German night fighters.

The next major development in the history of radar was the invention of the cavity magnetron by John Randall and Harry Boot of Birmingham University in early 1940. This was a small device that generated microwave frequencies much more efficiently than previous devices, allowing the development of practical centimetric radar, operating 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 sets. The cavity magnetron was perhaps the single most important invention in the history of radar and played a major part in the Allies' victory. In the Tizard Mission during September 1940, it was given free to the U.S., 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, Edward George Bowen was sent as the RDF expert in the Tizard Mission to the U.S. This led to the creation of the Radiation Laboratory (Rad Lab) at MIT to further develop the device and its applications.

When the cavity magnetron was first developed, its use in microwave RDF sets was held up because the duplexers for VHF were destroyed by the new higher-powered transmitter. This critical problem was solved in early 1941 by the T-R switch developed at the Clarendon Laboratory of Oxford University, allowing a pulse transmitter and receiver to share the same antenna without destabilizing the sensitive receiver. The combination of the magnetron, the T-R 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 V1 flying bomb flight paths to London, are credited with destroying many of the flying bombs before they reached their target.

At the time that the Air Ministry had its RDF development centre in Bawdsey, an Army Cell was attached to initiate its own programmes. These programs were for a Gun Laying (GL) system to in assist aiming antiaircraft guns and searchlights and a Coastal Defence (CD) system for directing coastal artillery. While at Bawdsey, the Army Cell developed a GL system code-named Transportable Radio Unit (TRU). Operating at 60MHz (6m wavelength) with 50kW power, the TRU had two vans for the electronic equipment plus a power van; it used a 35m erectable tower to support a transmitting antenna and two receiving antennas. When the war started and the Air Ministry activities were relocated to Dundee in Scotland, the Army Cell became a part of a new developmental center at Christchurch in Dorset on the south coast. John D. Cockcroft a physicist from Cambridge University and later a Nobel Prize Laureate, became the Director. With enlarged activities, the facility became the Air Defence Research and Development Establishment (ADRDE) in mid-1941. A year later, the ADRDE relocated to Great Malvern, Worcestershire. In 1944, this was reorganized as the Radar Research and Development Establishment (RRDE).

As the war started, GL Mk I sets were used overseas by the British Army in Malta and Egypt in 1939-40. Seventeen sets were sent to France with the British Expeditionary Force; most of these were destroyed at the Dunkirk evacuation in late May 1940, but a few were captured and gave the Germans their first full information on British RDF hardware. An improved version, GL Mk II, was used throughout the war; some 1,700 sets were put into service, including over 200 supplied to the Soviet Union. In early 1938, Alan Butement began the development of a Coastal Defense (CD) system that involved some of the most significant advancements in the early history of radar. Early tests showed that the CD set had much better capabilities for detecting aircraft at low altitudes than the existing CH. Consequently, the CD was also adopted by the Air Defence to augment the CH stations; in this role it was designated the Chain Home Low (CHL).

Representatives of the Experimental Department of His Majesty’s Signal School (HMSS) in the Royal Navy had been invited to demonstrations of the equipment being developed by the Air Ministry at Orfordness and Bawdsey Manor. Located at Portsmouth in Hampshire, the Experimental Department had an excellent capability for developing wireless valves (vacuum tubes), and, in fact, had provided the tubes used by Bowden in the initial transmitter at Orford Ness. With excellent research facilities of its own, the Admiralty decided to have its RDF development at the HMSS. This remained in Portsmouth after the start of the war, but in 1942, it was moved inland to safer locations at Witley and Haslemere, both in a county in the South East of England named Surrey. These two operations became the Admiralty Signal Establishment (ASE). The Royal Navy’s first successful RDF was the Type 79Y Surface Warning, tested at sea in early 1938. This 43MHz (7m wavelength) 70kW set used fixed transmitting and receiving antennas and had a range of 48 to 80Km, depending on the antenna heights. By 1940, this became the Type 281, increased in frequency to 85MHz (3.5m wavelength) and in power to between 350 and 1,000kW, depending on the pulse width. With steerable antennas, it was also used for Gun Control. This was first used in combat on 17 March 1941, allowing the British Royal Navy to essentially annihilate the Regia Marina (Royal Italian Navy) off the coast of Greece. Type 281B was a modification to use a common transmitting and receiving antenna. The Type 281, including the B-version, was likely the most used metric system of the Royal Navy throughout the war.

During World War 2 radar:


Chain Home station at Great Baddow

The Chain Home (CH) sys­tem provided the vital advance informa­tion that helped the Royal Air Force (RAF) to win the Battle of Britain. Once the radar system was in place it required a great number of trained tech­nicians to operate it. The designs and capabilities of this radar development were also given to the United States in 1940 in order to utilize their resources.


WW2 radar operation, equipment, Plan Position Indicator (PPI) display, and radar plotting table

In Britain the RAF radar training school was established at RAF Yatesbury, Wiltshire. RAF Yatesbury was the first military air base in the whole of Europe. In 1916 the Royal Flying Corps developed Yatesbury Field to train pilots. There were two camps either side of the minor road from the A4 to the village itself. The West camp comprised the Officers' and Men’s quarters with the usual facilities and had three large hangars. The East camp was adjacent to the A4 and again had hangars and workshops. The airfield opened in November 1916 with No. 55 Reserve Squadron arriving from Filton, Bristol equipped with Avro 504A and Scout D aircraft. Although the First World War ended in November 1918, training continued into 1919, when the Squadrons were sent to Yatesbury to be disbanded. The Station finally closed in early 1920. The land was returned to the original owners and reverted to farmland, and this remained the case until 1936, when the Government became alarmed by the rise of Hitler and German militarism. It was decided to train more pilots in the existing Elementary and Reserve Flying Schools, where anyone could learn basic flying skills. The Bristol Aeroplane Company (BAC) had been operating a School at Filton in Bristol since 1923 and was asked to set up another. So in 1935 they purchased part of the former Yatesbury Western Airfield and built the Flying School, which opened in early 1936. Training was carried out with Tiger Moth aircraft. This continued until the outbreak of war in September 1939, when pilot training was transferred away to other Stations to allow the field to be used for training airborne wireless operators.



From 1936 onwards, RAF Yatesbury and RAF Compton Bassett were major Radio and Radar Training Schools, and RAF Townsend was a satellite landing ground. In 1938 the RAF realised it would need a large number of radio operators so built No. 2 Electrical and Wireless School (later renamed No. 2 Radio School), a camp of wooden huts. The theory of wireless and Morse code were taught on the ground, and Dominie and Proctor aircraft were used for the aerial training. Over 50,000 men successfully passed out from 1939 to 1945, when the war ended. In 1942 a heavily guarded compound was built at the Eastern end of the camp to teach the new top-secret radar. This was originally known as No. 9 RDF School but was quickly changed to No. 9 Radio School. Over 19,000 men and women were trained there. After closure, the Radar and Wireless training school transferred to RAF Locking.

The Flying School was briefly used to train pilots after the war, but in 1947 it was abandoned. From 1954 to 1958 it was converted to RAF Cherhill, 27 Group Headquarters.

  
Cold War Air Radar Fitter and Special Gears courses at Yatesbury in 1956 and 1957

The camp was used for square bashing for a while, but with the start of the Cold War training of radar operators, mechanics and fitters began. Large numbers of personnel passed through because of the high proportion of National Servicemen in the RAF. With the end of National Service in 1961 demand reduced, so in 1965 the camp finally closed. Over 70,000 personnel were successfully trained during this period.

In 1969 the wooden huts were demolished and the land was returned to agriculture. With the exception of the gymnasium, the only brick building on the camp, the other buildings were abandoned and left to rot. The aircraft hangars and air strip, although now farmland, can still be seen from the A4.

In 2002 proposals were put forward to modify the Flying School for housing and, after a great deal of bureaucracy, work started in 2007. This included the repair of the WW1 hangars.

The RRDE

The Radar Research and Development Establishment (RRDE) was founded in Malvern, Worcestershire, in the shadow of the Malvern Hills. Another laboratory, the Telecommunications Research Establishment (TRE) was founded on a different site in Malvern. The RRDE and TRE merged in 1953 to form the Radar Research Establishment (RRE) for research, development and production in the areas of radar, electronics, and computer hardware and software. The Royal Radar Establishment was a renaming of the Radar Research Establishment. Initially the establishment was under the control of the Ministry of Supply, but in 1959 control passed to the Ministry of Aviation. When the Ministry of Aviation was abolished in 1967 responsibility was passed to the Ministry of Technology, then in 1970 to the Ministry of Aviation Supply, and finally in 1971 to the Ministry of Defence (MoD).

In 1972, in conjunction with George Gray and Ken Harrison of the University of Hull, new stable liquid crystals were developed, which were an immediate success in the electronics industry and consumer products. In 1979 RRE was awarded the Queen's Award for Technological Achievement for the joint-development of the long-lasting materials that made liquid crystal displays possible.


RSRE Malvern, viewed east from the Malvern Hills

In 1979 the RRE merged with the Services Electronic Research Laboratory (SERL), formerly at Baldock, and the Signals Research and Development Establishment (SRDE), formerly at Christchurch, to form the Royal Signals and Radar Establishment (RSRE). The RSRE motto was "Ubique Sentio" (Latin for I sense everywhere). There were out-stations at the ex-RAF airfields at Defford and Pershore. In April 1991 RSRE amalgamated with other defence research establishments to form the Defence Research Agency (DRA), which in April 1995 amalgamated with other organisations to form the Defence Evaluation and Research Agency (DERA). On 02.06.2001 this became independent of the MoD, with approximately two thirds of the organisation incorporated into Qinetic, a commercial company originally owned by the MoD, and the remainder into the fully government-owned laboratory, the Defence Science and Technology Laboratory (DSTL). In 2003 the Carlyle Group bought a private equity stake (around 30%) in QinetiQ.

Some of the most important technologies developed from work at RSRE are radar, thermal imaging, liquid crystal displays and speech synthesis. Contributions to computer science made by the RSRE include Algol68R (one of the first viable implementations of Algol68), the VIPER high integrity microprocessor, and the ELLA hardware description language.

Appendix 1: Vanastra

During World War 2, because of fear in Britain that RAF Yatesbury could become a target, the military sought another loca­tion where the threat of enemy attack did not exist. No. 31 Radio School RAF Clinton (later No. 5 Radio School Royal Canadian Air Force (RCAF)) was therefore set up in Vanastra, Ontario in July 1942. This was a top secret radar training school, the first of its kind in the North American continent. The Vanastra site was per­fectly situated in an isolated area on the Lake Huron coastline. Having exhausted Canada's supply of experienced radio men, the Air Force commissioned 13 Canadian universities to train new technicians to man the radar controls.

Appendix 2: The Secrets of Radar Museum

After the war the Secrets of Radar Museum was set up in London, Ontario. Many of the materials in this muse­um came from the original training base at RAF Clinton, Ontario. Unfortunately it closed, and the materials were put in storage. Fearing that the artefacts, as well as their histo­ry, might be lost forever, enthusiasts re-established the collection in a cottage behind Parkwood Hospital in London, Ontario that once housed ailing war veterans, and the Secrets of Radar Museum became a reality at the beginning of March 2003. With its collection of photographs, maps and artefacts, the museum is ded­icated to the preservation of the experiences, anecdotes and histories of the men and women who helped build, develop, oper­ate, maintain and defend the radar establishments in Canada, and abroad. A number of items housed in the museum were put on display at the first Radar Reunion in Coventry, England in 1991. It was in that year that the British Government excused the radar workers from their Official Secrets Act oath of secre­cy, allowing them to disclose their expe­riences for the first time. There are no actual radar devices at the Secrets of Radar Museum, as the Canadian government ordered them destroyed at the end of the war. However, there are a number of pieces of communications and electronic equipment, as well as memorabilia. The panels from the Coventry reunion detail the history, development and uses of radar, using maps, photos, newspaper articles, personal anecdotes, uniforms, medals, and notes and textbooks used by the radar students.

Appendix 3: The Paris gaol attack

In 1944 two Gestapo agents dis­guised in Canadian uniforms were able to penetrate the Dutch resistance, which had been retrieving allied airmen from across Europe. As a result, 158 Dutch resistance members were impris­oned in a large stone gaol just north of Paris. Coin­cidentally, the jail had been designed by the same architect who planned the Barton Street Jail in Hamilton, Ontario. Canadian staff were therefore able to plan a rescue effort based on the Barton Street blueprints. Using five Mosquito aircraft, the entire raid required five 500-pound bombs and lasted less than 10 minutes. Precision was imperative in order to destroy the non-supporting walls at the back of the prison. The mission was successful, and 156 of the prisoners managed to escape out of the back of the cells into the arms of the waiting French resistance.

Appendix 4: RAF Chicksands


RAF Chicksands was a Royal Air Force station in Bedfordshire. It was one of the original Y stations which supplied Enigma cipher transmissions to Bletchley Park. The Crown Commissioners bought the Chicksands estate on 15 April 1936, and it was then requisitioned by the Royal Navy. After nine months the RAF took over operations and established a signal intelligence collection unit there, which operated throughout World War 2 and afterwards. In 1950 the site was subleased to the United States Air Force serving as the base of the 6940th radio Squadron, responsible for continued communications and SIGINT operation throughout the Cold War. The RAF continued to act as a host unit for the resident USAF units, including over time the 6950th United States Air Force Security Squadron, later becoming the 6950th Electronic Security Group and the 7274th Air Base Group. In 1962 a huge 440m diameter AN/FLR-9 Wullenweber antenna array was constructed at Chicksands to form part of the Iron Horse HF direction finding network. This atenna array, dubbed the Elephant Cage, was dismantled in 1996 when the USAF withdrew from the site, handing it back to the British Armed Forces. The site was closed in 1997 when responsibility for the camp was taken over by the British Army Intelligence Corps, who moved the Corps Headquarters to the site from Ashford, Kent along with Intelligence Training.

Appendix 5: RAF Fylingdales


The original RAF Fylingdales, North Yorkshire, radomes were three golfball-like structures housing scanners


The modern early warning system for detection of missiles at RAF Fylingdales. This houses a Solid State Phased Array Radar, and is a 40m high triangular pyramid structure with three faces. During the Cold War, radar stations such as RAF Fylingdales were used to detect possible missile launches and aerial reconnaissance planes from the USSR.

Appendix 6: RAF Menwith Hill


RAF Menwith Hill is a Royal Air Force station near Harrogate, North Yorkshire which provides communications and intelligence support services to the United Kingdom and the United States of America. The site contains an extensive satellite ground station, and is a communications intercept and missile warning site described as the largest electronic monitoring station in the world.

Appendix 7: British airborne radars


Hawker Hunter with airborne radar within nose radome

  
Gloster Meteor Night Fighter NF 12 and Gloster Javelin with airborne radar nose radomes

Airborne radars used by Britain during WW2 and afterwards include the British Air Interception (AI) series AI Mark III used on the Bristol Blenheim, the AI-17 Post-war British Air Interception radar built by Decca codename "Yellow Lemon" and used on the Gloster Javelin, the AI-18 built by GEC and used on the De Havilland Sea Vixen, the improved US-built APS-21 system used on the new Meteor night-fighters NF12 and NF14 (first flew on 21 April 1953), the US-built Westinghouse AN/APQ-43 airborne radar exported to the UK with designation AI-22 for the Gloster Javelin FAW.2 to FAW.6 (replacement for AI-17), and the AI-23 built by Ferranti and used on the English Electric Lightning.

Appendix 8: The Malvern Ministry Of Supply Automatic Integrator and Computer (MOSAIC)


The MOSAIC, a second implementation of the ACE design of NPL

The Automatic Computing Engine (ACE) was an early electronic stored-program computer design produced by Alan Turing at the invitation of John R. Womersley, superintendent of the Mathematics Division of the National Physical Laboratory (NPL). The use of the word Engine was in homage to Charles Babbage and his Difference Engine and Analytical Engine. Turing's technical design Proposed Electronic Calculator was the product of his theoretical work in 1936 "On Computable Numbers" and his wartime experience at Bletchley Park where the Colossus computers had been successful in breaking German military codes. In his 1936 paper Turing described his idea as a "universal computing machine", but it is now known as the Universal Turing Machine. On 19 February 1946 Turing presented a detailed paper to the NPL Executive Committee, giving the first reasonably complete design of a stored-program computer. However, because of the strict and long-lasting secrecy around the Bletchley Park work, he was prohibited (because of the Official Secrets Act) from explaining that he knew that his ideas could be implemented in an electronic device. The better-known EDVAC design presented in the First Draft of a Report on the EDVAC (dated June 30, 1945), by John von Neumann, who knew of Turing's theoretical work, received much publicity, despite its incomplete nature and lack of attribution of the sources of some of the ideas. Turing's report on the ACE was written in late 1945 and included detailed logical circuit diagrams. He felt that speed and size of memory were crucial and he proposed a high-speed memory of what would today be called 25 KB, accessed at a speed of 1 MHz. The ACE implemented subroutine calls, whereas the EDVAC did not, and what also set the ACE apart from the EDVAC was the use of Abbreviated Computer Instructions, an early form of programming language. Initially, it was planned that Tommy Flowers, the engineer at the Post Office Research Station at Dollis Hill in north London, who had been responsible for building the Colossus computers, should also build the ACE, but because of the secrecy around his wartime achievements and the pressure of post-war work, this was not possible.

Turing's colleagues at the NPL, not knowing about Colossus, thought that the engineering work to build a complete ACE was too ambitious, so the first version of the ACE that was built was the ACE Pilot, a smaller version of Turing's original design, with 1450 thermionic valves, and mercury delay lines for its main memory. Each of the 12 delay lines could store 32 instructions or data words of 32 bits. The ACE Pilot ran its first program on May 10, 1950, at which time it was the fastest computer in the world with a clock speed of 1MHz.

The second implementation of the ACE design was the Ministry of Supply Automatic Integrator and Computer (MOSAIC). This was built by Allen Coombs and William Chandler from Dollis Hill, who had worked with Tommy Flowers on building the ten Colossus computers. MOSAIC was installed at the Telecommunications Research Establishment (TRE) which soon became the Royal Radar Establishment (RRE) at Malvern and ran its first program in late 1952 or early 1953. It was used to calculate aircraft trajectories from radar data, but details of it are still secret.

The principles of the ACE design were used in the Bendix Corporation's G-15 computer. The engineering design was done by Harry Huskey, who had been in the ACE section at NPL in 1947, and he later contributed to the hardware designs for the EDVAC. The first G-15 was commissioned in 1954 and, because it was a relatively small single-user machine, it was considered to be the first personal computer by some people.

The first production versions of the ACE Pilot, the English Electric DEUCE, of which 31 were sold, were delivered in the spring of 1955.

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Version: 02 19 October 2010 updated by Dr John Wilcock