The history of radar (where radar stands for radio detection and ranging) 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 inventor Christian Hülsmeyer who first used them to build a simple ship detection device intended to help avoid collisions in fog (Reichspatent Nr. 165546 in 1904). True radar which provided directional and ranging information, such as the British Chain Home early warning system, was developed over the next two decades. The development of systems able to produce short pulses of radio energy was the key advance that allowed modern radar systems to come into existence. By timing the pulses on an oscilloscope, the range could be determined and the direction of the antenna revealed the angular location of the targets. The two, combined, produced a "fix", locating the target relative to the antenna. In the 1934–1939 period, eight nations developed independently, and in great secrecy, systems of this type: the United Kingdom, Germany, the United States, the USSR, Japan, the Netherlands, France, and Italy. In addition, Britain shared their information with the United States and four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries also developed their own radar systems. During the war, Hungary was added to this list. The term RADAR was coined in 1939 by the United States Signal Corps as it worked on these systems for the Navy. Progress during the war was rapid and of great importance, probably one of the decisive factors for the victory of the Allies. A key development was the magnetron in the UK, which allowed the creation of relatively small systems with sub-meter resolution. By the end of hostilities, Britain, Germany, the United States, the USSR, and Japan had a wide variety of land- and sea-based radars as well as small airborne systems. After the war, radar use was widened to numerous fields, including civil aviation, marine navigation, radar guns for police, meteorology, and medicine. Key developments in the post-war period include the travelling wave tube as a way to produce large quantities of coherent microwaves, the development of signal delay systems that led to phased array radars, and ever-increasing frequencies that allow higher resolutions. Increases in signal processing capability due to the introduction of solid-state computers has also had a large impact on radar use.
Significance The place of radar in the larger story of science and technology is argued differently by different authors. On one hand, radar contributed very little to theory, which was largely known since the days of Maxwell and Hertz. Therefore, radar did not advance science, but was instead a matter of technology and engineering. Maurice Ponte, one of the developers of radar in France, states:
The fundamental principle of the radar belongs to the common patrimony of the physicists; after all, what is left to the real credit of the technicians is measured by the effective realisation of operational materials. But others point out the immense practical consequences of the development of radar. Far more than the atomic bomb, radar contributed to the Allied victory in World War II. Robert Buderi states that it was also the precursor of much modern technology. From a review of his book:
... radar has been the root of a wide range of achievements since the war, producing a veritable family tree of modern technologies. Because of radar, astronomers can map the contours of far-off planets, physicians can see images of internal organs, meteorologists can measure rain falling in distant places, air travel is hundreds of times safer than travel by road, long-distance telephone calls are cheaper than postage, computers have become ubiquitous and ordinary people can cook their daily dinners in the time between sitcoms, with what used to be called a radar range. In later years radar was used in scientific instruments, such as weather radar and radar astronomy.
Early contributors
Heinrich Hertz In 1886–1888 the German physicist Heinrich Hertz conducted his series of experiments that proved the existence of electromagnetic waves (including radio waves), predicted in equations developed in 1862–4 by the Scottish physicist James Clerk Maxwell. In Hertz's 1887 experiment he found that these waves would transmit through different types of materials and also would reflect off metal surfaces in his lab as well as conductors and dielectrics. The nature of these waves being similar to visible light in their ability to be reflected, refracted, and polarized would be shown by Hertz and subsequent experiments by other physicists.
Guglielmo Marconi Radio pioneer Guglielmo Marconi noticed radio waves were being reflected back to the transmitter by objects in radio beacon experiments he conducted on March 3, 1899, on Salisbury Plain. In 1916 he and British engineer Charles Samuel Franklin used short-waves in their experiments, critical to the practical development of radar. He would relate his findings 6 years later in a 1922 paper delivered before the Institution of Electrical Engineers in London:
I also described tests carried out in transmitting a beam of reflected waves across country ... and pointed out the possibility of the utility of such a system if applied to lighthouses and lightships, so as to enable vessels in foggy weather to locate dangerous points around the coasts ... It [now] seems to me that it should be possible to design [an] apparatus by means of which a ship could radiate or project a divergent beam of these rays in any desired direction, which rays, if coming across a metallic object, such as another steamer or ship, would be reflected back to a receiver screened from the local transmitter on the sending ship, and thereby immediately reveal the presence and bearing of the other ship in fog or thick weather.
Christian Hülsmeyer In 1904, Christian Hülsmeyer 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 the presence of ships up to 3 kilometres (1.6 nmi), and Hülsmeyer planned to extend its capability to 10 kilometres (5.4 nmi). 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. Hülsmeyer also received a patent amendment for estimating the range to the ship. Using a vertical scan of the horizon with the telemobiloscope mounted on a tower, the operator would find the angle at which the return was the most intense and deduce, by simple triangulation, the approximate distance. This is in contrast to the later development of pulsed radar, which determines distance via two-way transit time of the pulse.
Germany A radio-based device for remotely indicating the presence of ships was built in Germany by Christian Hülsmeyer in 1904. This has been recognized by the Institute of Electrical and Electronics Engineers as the invention of the first working radar system by inauguration of an IEEE Historic Milestone in October 2019. Over the following three decades in Germany, a number of radio-based detection systems were developed but none were pulsed radars. This situation changed before World War II. Developments in three leading industries are described.
GEMA In the early 1930s, physicist Rudolf Kühnhold, Scientific Director at the Kriegsmarine (German navy) Nachrichtenmittel-Versuchsanstalt (NVA—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-Günther 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), for the effort. From the start, the firm was always called simply GEMA. Work 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 Barkhausen-Kurz tube regenerative receiver connected to Schultes dipole antenna array, while von Willisen used a 48 cm transmitter, first connected to a Yagi antenna and then a parabolic antenna at 13.5 cm and 48 cm. In June 1934, a large steamer was detected by Doppler-beat interference at a distance of about 2 km (1.2 mi) with the 48 cm equipment, and 4 km with the 13.5 cm equipment. In October, strong reflections were observed from an aircraft that happened to fly through the continuous-wave beam. Yet, keeping the transmitted signal out of the receiver continued to be a problem. Kühnhold then shifted the GEMA work to a pulse-modulated system. A new 52 cm (600 MHz) Philips magnetron with better frequency stability was used. It was modulated with 2- μs pulses at 2000 per second. The transmitting antenna was an array of 10 pairs of dipoles with a reflecting mesh. The broad band heterodyne receiver used Acorn tubes from RCA, and the receiving antenna had three pairs of dipoles and incorporated lobe switching. A Braun tube (a CRT) was used for displaying the range. The equipment was first tested at a NVA site at the Lübecker Bay near Pelzerhaken. During May 1935, it detected returns from woods across the bay at a range of 15 km (9.3 mi). It had limited success, however, in detecting a research ship, Welle, only a short distance away. The receiver was then rebuilt, becoming a super-regenerative set with two intermediate-frequency stages. With this improved receiver, the system readily tracked vessels at up to 8 km (5.0 mi) range. In September 1935, a demonstration was given to the Commander-in-Chief of the Kriegsmarine. The system performance was excellent; the range was read off the Braun tube with a tolerance of 50 meters (less than 1 percent variance), and the lobe switching allowed a directional accuracy of 0.1 degree. Although this apparatus was not put into production, GEMA was funded to develop similar systems operating around 50 cm (500 MHz) and 2.4 m. These became the Seetakt for the Kriegsmarine and the Freya for the Luftwaffe (German Air Force) respectively.The navy needed the shorter wavelengths for surface targets, while the air force needed the extended range with the longer wavelengths. The 50 cm magnetron was replaced with the GEMA TS1 triode, while the magnetron replaced the Barkhausen tube. Kühnhold remained with the NVA, but also consulted with GEMA. He is considered by many in Germany as the Father of Radar. During 1933–6, Hollmann wrote the first comprehensive treatise on microwaves, Physik und Technik der ultrakurzen Wellen (Physics and Technique of Ultrashort Waves), Springer 1938.
Telefunken In 1933, when Kühnhold at the NVA was first experimenting with microwaves, he had sought information from Telefunken on microwave tubes. (Telefunken was the largest supplier of radio products in Germany) There, Wilhelm Tolmé Runge had told him that no vacuum tubes were available for these frequencies. 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, now 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 5 km (3.1 mi) distance. This led the Luftwaffe to fund the development of a 50 cm (600 MHz) gun-laying system, the Würzburg.
Lorenz 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 (DFA – Device for Detection) was built. It used a type DS-310 tube (similar to the Acorn) operating at 70 cm (430 MHz) and about 1 kW power, it had identical transmitting and receiving antennas made with rows of half-wavelength dipoles backed by a reflecting screen. In early 1936, initial experiments gave reflections from large buildings at up to about 7 km (4.3 mi). 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 7 km (4.3 mi) and aircraft at 4 km (2.5 mi) were attained. The success of this experimental set was reported to the Kriegsmarine, but they showed no interest; 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 DFA was then demonstrated to the Heer (German Army), and they contracted with Lorenz for developing Kurfürst (Elector), a system for supporting Flugzeugabwehrkanone (Flak, anti-aircraft guns).
United Kingdom
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 developed the use of radio signals generated by lightning strikes to map out the position of thunderstorms. The difficulty in pinpointing the direction of these fleeting signals using rotatable directional antennas led, in 1923, to the use of oscilloscopes in order to display the signals. The operation eventually moved to the outskirts of Slough in Berkshire, and in 1927 formed the Radio Research Station (RRS), Slough, an entity under the Department of Scientific and Industrial Research (DSIR). Watson Watt was appointed the RRS Superintendent. In 1934, the Air Ministry established the Committee for the Scientific Survey of Air Defence (CSSAD) with the goal of finding "how far recent advances in scientific and technical knowledge can be used to strengthen the present methods of defence against hostile aircraft". This "Tizard Committee" was named after its chair Sir Henry Tizard. H. E. Wimperis, Director of Scientific Research at the Air Ministry and a member of the Tizard Committee, had read about a German newspaper article claiming that the Germans had built a death ray using radio signals, accompanied by an image of a very large radio antenna. Both concerned and potentially excited by this possibility, but highly skeptical at the same time, Wimperis looked for an expert in the field of radio propagation who might be able to pass judgement on the concept. Watt, Superintendent of the RRS, 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. After discussing this with his scientific assistant, Arnold F. 'Skip' Wilkins, Wilkins quickly produced a back-of-the-envelope calculation that showed the energy required would be enormous. 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". Over the following several weeks, Wilkins considered the radio detection problem. He outlined an approach and backed it with detailed calculations of necessary transmitter power, reflection characteristics of an aircraft, and needed receiver sensitivity. He proposed using a directional receiver based on Watt's lightning detection concept, listening for powerful signals from a separate transmitter. Timing, and thus distance measurements, would be accomplished by triggering the oscilloscope's trace with a muted signal from the transmitter, and then simply measuring the returns against a scale. Watson Watt sent this information to the Air Ministry on February 12, 1935, in a secret report titled "The Detection of Aircraft by Radio Methods". In 1935, Arnold Wilkins set up his receiving equipment six miles from the BBC 49.8 meter shortwave transmitter, and recorded the reflection from a passing Handley Page Heyford bomber passing between the two. Witnessed by Albert Rowe, the Air Ministry allocated research funds and classified the project Highly Secret. This experiment was later reproduced by Wilkins for the 1977 BBC television series The Secret War episode "To See a Hundred Miles". Based on pulsed transmission as used for probing the ionosphere, a preliminary system was designed and built at the RRS by the team. Their existing transmitter had a peak power of about 1 kW, and Wilkins had estimated that 100 kW would be needed. Edward George Bowen was added to the team to design and build such a transmitter. Bowens’ transmitter operated at 6 MHz (50 m), had a pulse-repetition rate of 25 Hz, a pulse width of 25 μs, and approached the desired power. In 1935, testing began at Orford Ness Six wooden towers were erected, two for stringing the transmitting antenna, and four for corners of crossed receiving antennas. On June 17, the first target was detected — a Supermarine Scapa flying boat at 17 mi (27 km) range. Watson Watt, Wilkins, and Bowen are generally credited with initiating what would later be called radar in this nation. 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 secretary of the Tizard Committee, Albert Percival Rowe, coined the acronym RDF as a cover for the work, meaning Range and Direction Finding but suggesting the already well-known Radio Direction Finding. Late in 1935, responding to Lindemann's recognition of the need for night detection and interception gear, and realizing existing transmitters were too heavy for aircraft, Bowen proposed fitting only receivers, what would later be called bistatic radar. Frederick Lindemann's proposals for infrared sensors and aerial mines would prove impractical. It would take Bowen's efforts — at the urging of Tizard, who became increasingly concerned about the need — to see air-to-surface-vessel (ASV) radar and, through it, aircraft interception (AI) radar, to fruition. In 1937, Bowen's team set their crude ASV radar, the world's first airborne set, to detect the Home Fleet in dismal weather. Only in spring 1939, "as a matter of great urgency" after the failure of the searchlight system Silhouette, did attention turn to using ASV for air-to-air interception (AI). Demonstrated in June 1939, AI got a warm reception from Air Chief Marshal Hugh Dowding, and even more so from Churchill. This proved problematic. Its accuracy, dependent on the height of the aircraft, meant that CH, capable of only 4 mi (6.4 km), was not accurate enough to place an aircraft within its detection range, and an additional system was required. Its wooden chassis had a disturbing tendency to catch fire (even with attention from expert technicians), so much so that Dowding, when told that Watson-Watt could provide hundreds of sets, demanded "ten that work". The Cossor and MetroVick sets were overweight for aircraft use and the RAF lacked night fighter pilots, observers, and suitable aircraft. In 1940, John Randall and Harry Boot developed the cavity magnetron, which made ten-centimetre (wavelength ) radar a reality. To aid Chain Home in making height calculations, at Dowding's request, the Electrical Calculator Type Q (commonly called the "Fruit Machine") was introduced in 1940. The solution to night intercepts would be provided by Dr. W. B. "Ben" Lewis, who proposed a new, more accurate ground control display, the Plan Position Indicator (PPI), a new Ground-Controlled Interception (GCI) radar, and reliable AI radar. The AI sets would ultimately be built by EMI. GCI was unquestionably delayed by Watson-Watt's opposition to it and his belief that CH was sufficient, as well as by Bowen's preference for using ASV for navigation, despite Bomber Command disclaiming a need for it, and by Tizard's reliance on the faulty Silhouette system.
Air Ministry
In March 1936, the work at Orfordness was moved to Bawdsey Manor, nearby 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 Chain Home (CH) equipment was assembled as a prototype. There were equipment problems when the Royal Air Force (RAF) first exercised the prototype station in September 1936. These were cleared by the next April, and the Air Ministry started plans for a larger network of stations. Initial hardware at CH stations was as follows: The transmitter operated on four pre-selected frequencies between 20 and 55 MHz, adjustable within 15 seconds, and delivered a peak power of 200 kW. The pulse duration was adjustable between 5 and 25 μs, with a repetition rate selectable as 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. Four 360-foot (110 m) steel towers supported transmitting antennas, and four 240-foot (73 m) wooden towers supported cross-dipole arrays at three different levels. A goniometer was used to improve the directional accuracy from the multiple receiving antennas. By the summer of 1937, 20 initial CH stations were in check-out operation. A major RAF exercise was performed before the end of the year, and was such a success that £10,000,000 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. In May 1938, Rowe replaced Watson Watt as Superintendent at Bawdsey. In addition to the work on CH and successor systems, there was now major work in airborne RDF equipment. This was led by E. G. Bowen and centered on 200-MHz (1.5 m) sets. The higher frequency allowed smaller antennas, appropriate for aircraft installation. 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.
British Army In 1931, at the Woolwich Research Station of the Army's Signals Experimental Establishment (SEE), W. A. S. Butement and P. E. Pollard had examined pulsed 600 MHz (50-cm) signals for detection of ships. Although they prepared a memorandum on this subject and performed preliminary experiments, for undefined reasons the War Office did not give it consideration. As the Air Ministry's work on RDF progressed, Colonel Peter Worlledge of the Royal Engineer and Signals Board met with Watson Watt and was briefed on 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. This was under E. Talbot Paris and the staff included Butement and Pollard. The Cell's work emphasize two general types of RDF equipment: gun-laying (GL) systems for assisting anti-aircraft guns and searchlights, and coastal-defense (CD) systems for directing coastal artillery and defense of Army bases overseas. Pollard led the first project, an early warning RDF code-named Mobile Radio Unit (MRU). This truck-mounted system was designed as a small version of a CH station. It operated at 23 MHz (13 m) with a power of 300 kW. A single 105-foot (32 m) 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 a range of 60 miles (97 km). The Air Ministry also adopted this system as a mobile auxiliary to the CH system. In early 1938, Butement started the development of the Radar, Coast Defense, Mark I system based on Bowen's evolving 200-MHz (1.5-m) airborne sets. The transmitter had a 400 Hz pulse rate, a 2-μs pulse width, and 50 kW power (later increased to 150 kW). Butement also introduced beam and the radar equation. By May 1939, the CD MK II RDF could detect aircraft flying as low as 500 feet (150 m) and at a range of 25 mi (40 km).
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. HMSS started RDF work in September 1935. Initial efforts, under R. F. Yeo, were in frequencies between 75 MHz (4 m) and 1.2 GHz (25 cm). All of the work was under the utmost secrecy; it could not even be discussed with other scientists and engineers at Portsmouth. A 75 MHz range-only set was eventually developed and designated Type 79X. Basic tests were done using a training ship, but the operation was unsatisfactory. In August 1937, the RDF development at HMSS changed, with many of their best researchers brought into the activity. John D. S. Rawlinson was made responsible for improving the Type 79X. To increase the efficiency, he decreased the frequency to 43 MHz ( 7 metre wavelength). Designated Type 79Y, it had separate, stationary transmitting and receiving antennas. In 1938, Type 79Y was tested at sea, detecting aircraft at 30 and 50 miles (48 and 80 km). The systems were then deployed on HMS Sheffield and HMS Rodney.
United States 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; thus, they will be described separately.
United States Navy In the autumn of 1922, Albert H. Taylor and Leo C. Young at the U.S. Naval Aircraft Radio Laboratory were conducting communication experiments when they noticed that a wooden ship in the Potomac River was interfering with their signals. They prepared a memorandum suggesting that this might be used for ship detection in a harbor defense, but their suggestion was not taken up. In 1930, Lawrence A. Hyland working with Taylor and Young, now at the U.S. Naval Research Laboratory (NRL) in Washington, D.C., used a similar arrangement of radio equipment to detect a passing aircraft. This led to a proposal and patent for using this technique for detecting ships and aircraft. A simple wave-interference apparatus can detect the presence of an object, but it cannot determine its location or velocity. That had to await the invention of pulsed radar, and later, additional encoding techniques to extract this information from a CW signal. When Taylor's group at the NRL were unsuccessful in getting interference radio accepted as a detection means, Young suggested trying pulsing techniques. This would also allow the direct determination of range to the target. In 1924, Hyland and Young had built such a transmitter for Gregory Breit and Merle A. Tuve at the Carnegie Institution of Washington for successfully measuring the height of the ionosphere. Robert Morris Page was assigned by Taylor to implement Young's suggestion. Page designed a transmitter operating at 60 MHz and pulsed 10 μs in duration and 90 μs between pulses. In December 1934, the apparatus was used to detect a plane at a distance of one mile (1.6 km) flying up and down the Potomac. Although the detection range was small and the indications on the oscilloscope monitor were almost indistinct, it demonstrated the basic concept of a pulsed radar system. Based on this, Page, Taylor, and Young are usually credited with building and demonstrating the world's first pulsed radar. An important subsequent development by Page was the duplexer, a device that allowed the transmitter and receiver to use the same antenna without overwhelming 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 of long-range targets. The experiments with pulsed radar were continued, primarily in improving the receiver for handling the short pulses. In June 1936, the NRL's first prototype radar system, now operating at 28.6 MHz, was demonstrated to government officials, successfully tracking an aircraft at distances up to 25 miles (40 km). Their radar was based on low frequency signals, at least by today's standards, and thus required large antennas, making it impractical for ship or aircraft mounting.
Antenna size is inversely proportional to the operating frequency; therefore, the operating frequency of the system was increased to 200 MHz, allowing much smaller antennas. The frequency of 200 MHz was the highest possible with existing transmitter tubes and other components. In 1937, it was tested installed on the USS Leary, with a Yagi antenna mounted on a gun barrel. Based on success of the sea trials, the NRL further improved the system. Page developed the ring oscillator, allowing multiple output tubes and increasing the pulse-power to 15 kW in 5-μs pulses. A 20-by-23 ft (6 x 7 m), stacked-dipole "bedspring" antenna was used. In laboratory test during 1938, the system, now designated XAF, detected planes at ranges up to 100 miles (160 km). 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. Designated CXAM, deliveries started in May 1940. The acronym RADAR was coined from "Radio Detection And Ranging". One of the first CXAM systems was placed aboard the USS California, a battleship that was sunk in the Japanese attack on Pearl Harbor on December 7, 1941.
United States Army 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) and Lt. Colonel (Dr.) William R. Blair was appointed the SCL Director. Among other activities, the SCL was made responsible for research in the detection of aircraft by acoustical and infrared radiation means. Blair had performed his doctoral research in the interaction of electromagnet waves with solid materials, and naturally gave attention to this type of detection. Initially, attempts were made to detect infrared radiation, either from the heat of aircraft engines or as reflected from large searchlights with infrared filters, as well as from radio signals generated by the engine ignition. Some success was made in the infrared detection, but little was accomplished using radio. In 1932, progress at the Naval Research Laboratory (NRL) on radio interference for aircraft detection was passed on to the Army. While it does not appear that any of this information was used by Blair, the SCL did undertake a systematic survey of what was then known throughout the world about the methods of generating, modulating, and detecting radio signals in the microwave region. In 1934, the SCL was directed by the Chief of the Army Signal Corps to investigate microwave radio position-finding (RPF). In 1934 and 1935 tests showed Doppler-shifted signals, using a bi-static arrangement. Blair was evidently not aware of the success of a pulsed system at the NRL in December 1934. In an internal 1935 note, Blair had commented:
Consideration is now being given to the scheme of projecting an interrupted sequence of trains of oscillations against the target and attempting to detect the echoes during the interstices between the projections. In 1936, W. Delmar Hershberger and Robert H. Noyes built a 75 watt, 110 MHz (2.73 m) transmitter with pulse modulation and a receiver patterned on the one at the NRL. In October 1936, Paul E. Watson became the SCL Chief Engineer and led the project. A field setup near the coast was made with the transmitter and receiver separated by a mile. On December 14, 1936, the experimental set detected at up to 7 mi (11 km) range aircraft flying in and out of New York City. In 1937, Ralph I. Cole and William S. Marks lead used separate antennas for the receiver and azimuth-elevation detection, illuminating a bomber. Observing, Secretary of War Henry A. Woodring placed orders the next day. In 1938, the 1.5 meter SCR-268 used Major James C. Moore designed lobe switching antennas. Assisted by Western Electric and Westinghouse production started by Western Electric in 1939, entering service in 1941. Even before the SCR-268 entered service, it had been greatly improved. In a project led by Major (Dr.) Harold A. Zahl, two new configurations evolved – the 2.83 meter mobile SCR-270 and the fixed base SCR-271 (fixed-site). In 1940, Westinghouse started deliveries. The Army deployed five of the first SCR-270 sets around the island of Oahu in Hawaii, where one of these radars detected a flight of aircraft involved in the Attack on Pearl Harbor.
USSR On 7 May 1895, Alexander Stepanovich Popov, a physics instructor at the Imperial Russian Navy's Torpedo School of the Naval Warfare Institute in Kronstadt, presented a paper to the Russian Physical and Chemical Society on his coherer lightning detector. In March 1896, he added a spark-gap transmitter and transmitted a Morse code message in a demonstration to the same society. In 1897, he demonstrated communication between two ships in the Baltic Sea. He also noted interference beat caused by the passage of a third vessel. In 1899, Popov collaborated with Eugène Adrien Ducretet in building wireless sets in Paris, based on Popov's apparatus, and under license in the Siemens & Halske St. Petersburg factory. In 1900, after Popov's receiver was patented in Russia, England and France, the Russian Imperial Navy adopted wireless communication. By 1924, 50 radio stations were operating in Russia. The Red Army's Glavnoe Artilleriyskoe Upravlenie (GAU, Main Artillery Administration), seeking help with radio location, were assisted by Yu. K. Korovin of the Tsentral’naya Radiolaboratoriya (TsRL, Central Radio Laboratory) in Leningrad. On 3 January 1934, he was able to show reflections from aircraft using 50 centimeter transmitter. The TsRL were then engaged to develop radio location to assist searchlights and anti-aircraft guns. In 1934, following a Russian Academy of Sciences conference for the Voiska Protivo-vozdushnoi aborony (PVO), Pavel Oshchepkov was able to initiate radio location work with Abram Fedorovich Ioffe's Leningrad Physics and Technology Institute (LEPI), the Ukrainian Institute of Physics and Technology (UIPT), his Special Construction Bureau (SCB), and Svetlana. In October 1934, the PVO ordered five of LEPI Bistro sets for experimentation. Operating at 4.7 meters, the Bistatic radar system built by B.K. Shembel's team could only determine the presence of a target, and a rough direction, but not its range. In 1936, Shembel's team in Mikhail A. Bonch-Bruevich's Nauchno-issledovatelsky institut-9 (NII-9, Scientific Research Institute #9), used a UIPT 18 centimeter magnetron to build Burya. Though capable of range, elevation, and azimuth detection, it was still insufficient for Aa guns. Burya-2 and Burya-3, incorporated three receiving antennas paired vertically and horizontally, and included lobe switching, but it was still a continuous-wave radar (CW). Simultaneously in 1936, Oshehepkov at the SCB, and V.V. Tsimbalin at the LPTI, developed a 4 meter pulsed radar system. However, this bistatic system could not directly detect range with the pulses. In June 1937, all of the 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 svyazi RKKA (NIIIS-KA, Scientific Research Institute of Signals of the Red Army), had initially opposed research in radio-location, favoring 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. Writing later about the Purge and subsequent effects, General Lobanov commented that it led to the development being placed under a single organization, and the rapid reorganization of the work. In 1938, NII-9's D. S. Stogov developed Reven (Rhubarb), a slightly better version of Bistro. This became the basis for the mobile system Radio Ulavlivatel Samoletov (RUS) RUS-1. This CW bi-static system used a truck-mounted 4.7 meter transmitter, and two truck-mounted receivers separated by 40 km. In 1940, the system entered service, of which 45 were built. Yet the system lacked range capabilities. In 1940, an oscilloscope was used to display range, and designated RUS-2. Modified with range-finding and a duplexer, an additional 15 systems were built. One set was tested aboard the Soviet cruiser Molotov as Redut-k by Aksel Berg. In 1938, the Scientific Research Institute of Signals of the Red Ary (NIIS-KA)' A. I. Shestako continued work on Oshchepkov's 4 meter pulsed radar system. Ioffe's turned it into a mobile system, Redut, using Yagi antennas. In 1939 it was field tested, indicating longer range and better direction-angle measurements t