1800s
In 1887 the German physicist Heinrich Hertz began experimenting with radio waves in his laboratory. He found that radio waves could be transmitted through different types of materials, and were reflected by others. The existence of electromagnetic waves was predicted earlier by James Clerk Maxwell, but it was Hertz who first succeeded in generating and detecting radio waves experimentally.
1900s
By the 1900s a German engineer, Chistian Huelsmeyer, proposed the use of radio echoes to avoid collisions. He invented a device he called the telemobiloscope, which consisted of a simple spark gap aimed using a funnel-shaped metal antenna. When a reflection was seen by the two straight antennas attached to the receiver, a bell sounded. Although very simple, the system could detect shipping accurately up to about 3 km. Nevetheless the naval world seemed uninterested in his invention, and it was not put into production.
Nikola Tesla, in August 1917, proposed principles regarding frequency and power levels for primitive RADAR units. Tesla's study of high-voltage, high-frequency alternating currents led to this development. Tesla had formed the concept of using radio waves to detect objects at a distance. In the 1917 The Electrical Experimenter, Tesla stated the principles in detail.
Tesla stated, "For instance, by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed." Tesla also proposed the use of these standing electromagnetic waves along with pulsed reflected waves to determine the relative position, speed, and course of a moving object and other modern concepts of radar.
Tesla had first proposed that radio location might help find submarines (for which it is not well-suited) with a fluorescent screen indicator, though it was first applied successfully to locate aircraft (after their later proliferation) and surface ships during World War II. Emil Girardeau, working with the first French radar systems, stated he was building radar systems "conceived according to the principles stated by Tesla". Tesla first established principles regarding frequency and power level for the first primitive RADAR units in 1934.
In the autumn of 1922, A. H. Taylor and L. C. Young of the Naval Research Laboratory (NRL) in the U.S.A. detected a wooden ship using continuous wave (CW) wave-interference radar with a separated transmitting and receiving antenna. These men were the first inventors of radar. In June, 1930, L. A. Hyland of the NRL in the U.S. detected an airplane with this type of radar. Wave-interference radar can detect the presence of an object, but it cannot determine its location or velocity. That had to await the invention of pulse radar.
The first experiments with pulse radar were conducted at the NRL (U.S.) in 1934 and 1935. On April 28, 1936, the first pulse radar was demonstrated successfully at a range of 2.5 miles, but by June of that year, the range was extend to 25 miles. Pulse radar is the only way to detect the location, size, and velocity of objects, but its limitation for use on ships and planes was caused by its operation at low frequencies that required large antennas. However, in June, 1936, NRL researchers invented a 200 MHz pulse radar employing a duplex radar; that is, radar with the transmitting and receiving antennas located on the radar set.
On February 12, 1935, Robert Watson-Watt sent a memo of a proposed RADAR system to the British Air Ministry, entitled "Detection and location of aircraft by radio methods". In 1915 he joined the Royal Aircraft Factory at Ditton Park, in Hampshire, England, as a meteorologist, where he attempted to use radio signals generated by lightning strikes to map out thunderstorms. The difficulty in pinpointing the direction of these high-speed signals led to the use of rotating directional antennas, and in 1923 the use of oscilloscopes in order to display them in 2-D. At this point the only missing part of a functioning radar was the broadcaster.
In 1934, Watson-Watt was well established in the area of radio, and was approached by H.E. Wimperis from the Air Ministry, who asked about the use of radio to produce a 'death ray'. While he knew this to be unlikely, he pointed out that in the absence of progess, 'meanwhile attention is being turned to the still difficult, but less unpromising, problem of radio detection and numerical considerations on the method of detection by reflected radio waves will be submitted when required.' Watson-Watt and his assistant Arnold Wilkins published a report on the topic in February 1935, titled The Detection of Aircraft by Radio Methods.
By the time World War 2 began, viable radar technology existed in the oscilloscope type SCR-270 Radar.
Microwaves
Meanwhile in Germany, Hans Eric Hollmann had been working for some time in the field of microwaves, which were to later become the basis of almost all radar systems. In 1935 he published Physics and Technique of Ultrashort Waves, which was then picked up by researchers around the world. At the time he had been most interested in their use for communications, but he and his partner Hans-Karl von Willisen had also worked on radar-like systems.
In the autumn of 1934 their company, GEMA, built the first commercial radar system for detecting ships. Operating in the 50 cm range it could detect ships up to 10 km away, similar in purpose to Huelsmeyer's earlier device. In the summer of 1935 a pulse radar was developed with which they could spot the ship the Königsberg 8 km away, with an accuracy of up to 50 m, enough for gun-laying. The same system could also detect an aircraft at 500 m altitude at a distance of 28 km. The military implications were not lost this time around, and construction of land and sea-based versions took place as Freya and Seetakt.
World War II
At this point both the United Kingdom and Nazi Germany knew of each other's ongoing efforts in their arms race. Both nations were intensely interested in the other's developments in the field, and engaged in an active campaign of espionage and false leaks about their respective equipment. But it was only in Britain that the usefulness of the system became obvious, so while the German systems had the edge technologically (operating on much shorter wavelengths) only Britain started true mass deployment of both the radars and the control systems needed to support them.
Research had been initiated by Sir Henry Tizard's Aeronautical Research Committee in 1935 and, from 1940, was based at the Telecommunications Research Establishment (TRE).
UK
see also: H2S radar,
Chain Home
Shortly before the outbreak of World War II several radar stations known as Chain Home (or CH) were constructed in the south of England. As one might expect from the first radar to be deployed, CH was a simple system. The broadcast side was formed from two 300' (100 m) tall steel towers strung with a series of cables between them. The output of a powerful 50 MHz radio of about 200 kW (up to 800 kW in later models) was fed into these cables, pulsed at about 50 times a second. A second set of 240' (73 m) tall wooden towers were used for reception, with a series of crossed antennas at various heights up to 215' (65 m). Most stations had more than one set of each antenna, tuned to operate at different frequencies.
The CH radar was read with an oscilloscope. When a pulse was sent out into the broadcast towers, the scope was triggered to start its beam moving horizontally across the screen very rapidly. The output from the receiver was amplified and fed into the vertical axis of the scope, so a return from an aircraft would deflect the beam upward. This formed a spike on the display, and the distance from the left side - measured with a small scale on the bottom of the screen - would give the distance to the target. By rotating the receiver antennas to make the display disappear, the operator could determine the direction (this is the reason for the cross shaped antennas), the size of the vertical displacement indicated something of the number of aircraft involved, and by comparing the strengths returned from the various antennas up the tower, the altitude could be determined.
CH proved highly effective during the Battle of Britain, and is often credited with allowing the RAF to defeat the much larger Luftwaffe forces. Whereas the Luftwaffe had to hunt all over to find the RAF fighters, the RAF knew exactly where the Luftwaffe bombers were, and could converge all of their fighters on them. The RDF stations only worked over the sea, and the positions of enemy aircraft over land had to be relayed by observers and aircraft.
Very early in the battle the Luftwaffe made a series of small raids on a few of the stations, but they were returned to operation in a few days. In the meantime the operators took to broadcasting radar-like signals from other systems in order to fool the Germans into believing that the systems were still operating. Eventually the Germans gave up trying to bomb them. The Luftwaffe apparently never understood the importance of radar to the RAF's efforts, or they would have assigned them a much higher priority -- it is clear they could have knocked them out continually if they wished.
In order to avoid the CH system the Luftwaffe adopted other tactics. One was to approach Britain at very low levels, below the sight line of the radar stations.
This was countered to some degree with a series of shorter range stations built right on the coast, known as Chain Home Low (CHL). These radars had originally been intended to use for naval gun-laying and known as Coastal Defense (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) itself. 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 WAAFs until more reliable motorized movements were installed in 1941.
Later adaptations
Similar systems were later adapted with a new display to produce the Ground Controlled Intercept stations starting in late 1941. In these systems the antenna was rotated mechanically, followed by the display on the operators console. That is, instead of a single line across the bottom of the display from left to right, the line was rotated around the screen at the same speed as the antenna was turning.
The result was a 2-D display of the air around the station with the operator in the middle, with all the aircraft appearing as dots in the proper location in space. These so-called Plan Position Indicators (PPI) dramatically simplified the amount of work needed to track a target on the operator's part. Such a system with a rotating, or sweeping, line is what most people continue to associate with a radar display.
Rather than avoid the radars, the Luftwaffe took to avoiding the fighters by flying at night and in bad weather. Although the RAF was aware of the location of the bombers, there was little they could do about them unless the fighter pilots could see the opposing planes. However, just this eventuallity had already been foreseen, and Watson-Watt (likely at the urging of Tizzard) had already started work on a miniaturized radar system suitable for aircraft, the so-called AI (airborne interception) set. Initial sets were available in 1941 and fitted to Bristol Blenheim aircraft, replaced quickly with the better performing Bristol Beaufighter, which quickly put an end to German night- and bad-weather bombing over England. Mosquito night intruders were fitted with AI Mk VIII and later derivatives which, along with a device called "Serrate" to allow them to track down German night fighters from their Lichtenstein B/C and SN2 radar emissions, as well as a device named "Perfectos" that tracked German IFF, allowed the Mosquito to find and destroy German night fighters. As a counter measure the German night fighters employed Naxos ZR radar detectors.
Magnetron
The next major development in the history of radar was the invention of the cavity magnetron by Randall and Boot of Birmingham University in early 1940. This was a small device which generated much more powerful microwaves than previous devices, which in turn allowed for the detection of much smaller objects and the use of much smaller antennas. The secrecy of the device was so high that it was decided in 1940 to move production to the USA, which resulted in the creation of the MIT Radiation Lab to develop the device further.
German developments
German developments mirrored those in the United Kingdom, but it appears radar received a much lower priority until later in the war. The Freya radar was in fact much more sophisticated than its CH counterpart, and by operating in the 1.2 m wavelength (as opposed to ten times that for the CH) the Freya was able to be much smaller and yet offer better resolution. Yet by the start of the war only eight of these units were in operation, offering much less coverage.
However the Germans did not have an airborne system of any sort deployed until 1942, leaving them with the problem of having to get their fighters into that 300 m range solely with ground-based equipment. To fill this need another system known as Würzburg was deployed, starting in 1941.
Würzburg
Unlike other systems, the Würzburg was mounted on a highly directional parabolic antenna that was sensitive in only one direction. This made it useless for finding the targets, but once guided to one by an associated Freya it could track it with extreme accuracy: later models were accurate to 0.2 degrees or less. In order to do this the radar sent out two lobes and the return of each was shown on the display. By keeping the returns from both the same strength, the operator kept the Würzburg pointed directly at the target.
The downfall of the German radar network was that it could only track a single aircraft per Würzburg. In fact the system required two Würzburgs per interception, one for the target, and one for the fighter. This meant that as a raid developed, only a few night fighters could be directed at any one time, as only a small number of the eventual 5,000 Würzburgs would be within their 25 km range at any one time.
Comparison
Compared to the British PPI systems, the German system was far more labour intensive. This problem was compounded by the lackadaisical approach to command staffing. It was several years before the Luftwaffe had a command and control system nearly as sophisticated as the one set up by Watt before the war, after seeing the confusion too much information caused during one test.
German airborne radar units followed a similar pattern. Early Lichtenstein BC units were not deployed until 1942, and as they operated on the 2 m wavelength they required large antennas. By this point in the war the British had become experts on jamming German radars, and when a BC-equipped Ju 88 night fighter landed in England one foggy night, it was only a few weeks before the system was rendered completely useless. By late 1943 the Luftwaffe was starting to deploy the greatly improved SN-2, but this required huge antennas that slowed the planes as much as 50 km/h. Jamming the SN-2 took longer, but was accomplished. A 9 cm wavelength system known as Berlin was eventually developed, but only in the very last months of the war.
Cold War
After World War II, the United States and Canada built a chain of radars in the Arctic, the Distant Early Warning Line, to warn of Soviet bomber attack. In the late 1950s the Ballistic Missile Early Warning System was added to warn of ICBM launches.
Further reading
- Barrett, Dick, "All you ever wanted to know about British air defence radar (http://www.radarpages.co.uk/index.htm)". The Radar Pages. (History and details of various British radar systems)
- ES310 "Introduction to Naval Weapons Engineering.". (Radar fundementals section) (http://www.fas.org/man/dod-101/navy/docs/es310/syllabus.htm)
- Robert Buderi: The invention that changed the world: the story of radar from war to peace, Simon & Schuster, 1996. ISBN 0-349-11068-9
- R.V. Jones, Most Secret War. R.V. Jones's account of his part in British Scientific Intelligence between 1939 and 1945, working to anticipate the German's radar, radio navigation and V1/V2 developments.
External links
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