History of radar

Encyclopedia : H : HI : HIS : History of radar

The history of radar began in the 1900s when engineers invented reflection devices. Around the 1930s, radar stations were being deployed.

Contents

Before the twentieth century

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

The extraordinary technical mutation which provoked the radar in electronics was naturally accompanied by an abundant flowering of memoires, among which legend and technical nationalism were regrettably not always absent. The fundamental principle of the radar belongs to the common patrimony of the physicists : after all, what is left to the real credit of the technicians is measured by the effective realisation of operational materials". — Maurice Ponte [in France 1934]

Christian Huelsmeyer

In 1904 Christian Huelsmeyer gave public demonstrations in Germany and the Netherlands of the use of radio echoes to detect ships so that collisions could be avoided, which consisted of a simple spark gap aimed using a multipole antenna. When a reflection was picked up by the two straight antennas attached to the separate receiver, a bell sounded. The system detected presence of ships up to 3 km, and he planned to extend its capability to 10km. It did not provide range information, only warning of a nearby metal object, and would be periodically "spun" to check for ships in bad weather. He patented the device, called the telemobiloscope, but due to lack of interest by the naval authorities the invention was not put into production.

Nikola Tesla

Nikola Tesla, in August 1917, proposed principles regarding frequency and power levels for primitive radar units. In the 1917 The Electrical Experimenter, Tesla stated the principles in detail:

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

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

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

In 1927, Camille Guitton and Pierret experimented with wavelengths going down to 16 cm. Other engineers, Mesny and David, noticed repeatedly since 1931 that an aircraft flying between a transmitter and a receiver would perturbate the communication. This was the basis of a device put into operational use in 1935 by the French military radio branch to detect airplanes flying over a given zone.
In 1934, Henri Guitton (the son of the former, and engineer at the Compagnie de Télégraphie Sans Fil) resumes his father's experiments after initial reports made by the U.S. Naval Research Laboratory in 1930 (see below) and brings improvements to the magnetron. Emile Girardeau,[link], another engineer at the CSF, recalled in a testimony that they were at the time intending to build radar systems "conceived according to the principles stated by Tesla". The CSF received the [French patent] (no. 788.795, "New system of location of obstacles and its applications") on July 20, for a device detecting obstacles (icebergs, ships, planes) using pulses of ultra-short wavelengths produced by a magnetron. The radar was experimented from November to Decemeber 1934 aboard cargo Oregon, with two transmitters working at 80 cm and 16 cm wavelengths. Coasts were detected from a range of 10-12 nautical miles. The shortest wavelength was chosen for the final design, which equipped the liner Normandie as soon as mid-1935 for operational use.

Naval Research Laboratory

In the autumn of 1922, Albert H. Taylor and Leo C. Young of the U.S. Naval Research Laboratory (NRL) were conducting communication experiments when they noticed that a wooden ship in the Potomac River was interfering with their signals; in effect, they had demonstrated the first continuous wave (CW) interference radar with separated transmitting and receiving antennas. In June, 1930, Lawrence A. Hyland of the NRL in the U.S. detected an airplane with this type of radar. Simple 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, and later, additional encoding techniques to extract this information from a CW signal.

Under Robert M. Page, experiments with pulse radar were conducted at the NRL in 1934 and 1935. On April 28, 1936, their first pulse radar was demonstrated successfully at a range of 2.5 miles, but by June of that year, the range was extended to 25 miles. Their radar was based on low frequency signals, at least by today's standards, and thus required large antennas, making it impractical for ship or aircraft mounting.

Robert Watson-Watt

In 1915 Robert Watson-Watt joined the Meteorological Office as a meteorologist. Working at an outstation at Aldershot, in Hampshire, England, he developed the use of radio signals generated by lightning strikes to map out the position of thunderstorms. The difficulty in pinpointing the direction of these high-speed signals led to the use of rotating directional antennas, and in 1923 the use of oscilloscopes in order to display them. An operator would periodically rotate the antenna and look for "spikes" on the oscilloscope to find the direction of a storm. At this point the only missing part of a functioning radar was the transmitter.

By 1934 Watson-Watt was well established in the area of radio as head of the Radio Research Station at Ditton Park near Slough. He was approached by H.E. Wimperis from the Air Ministry, who asked about the use of radio to produce a 'death ray', after hearing Germans claims to have built such a device. Watt quickly wrote back that this was unlikely, and 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 on February 12, 1935, titled The Detection of Aircraft by Radio Methods.

The Daventry Experiment 26 February 1935, set up by A.F.Wilkins and his driver, Dyer, to demonstrate the feasibility of RADAR.

On February 26 1935 Watson-Watt and Arnold Wilkins demonstrated to an observer from the Air Ministry Committee the detection of an aircraft. The previous day Wilkins had set up receiving equipment in a field near Upper Stowe, Northamptonshire, and this was used to detect the presence of a Handley Page Heyford bomber at ranges up to 8 miles by means of the radio waves which it reflected from the nearby Daventry shortwave radio transmitter of the BBC, which operated at a wavelength of 49m. This convincing demonstration, known as The Daventry Experiment, led immediately to development of radar in the UK.

Hans Hollmann

Meanwhile in Germany, Hans 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 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. This device was similar in purpose to Huelsmeyer's earlier system, and like it, did not provide range information.

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 the start of World War II both the United Kingdom and Nazi Germany knew of each other's ongoing efforts in their "battle of the beams". Both nations were intensely interested in the other's developments in the field, and engaged in an active campaign of espionage and false leaks about their respective equipment. 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). But much of the credit belongs to Watson-Watt, who turned from the technical side of radar to building up a usable network of machines and the people to run them. After watching a demonstration in which his radar operators were attempting to locate an "attacking" bomber, he noticed that the primary problem was not technological, but worker overload. By 1940 Watt had built up a layered organization that efficiently passed information along, and was able to track large numbers of aircraft.

UK

Chain Home

Shortly before the outbreak of World War II several radar stations known as Chain Home (or CH) were constructed along the South and East coasts of Britain. 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 50,000 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 to the target (this is the reason for the cross shaped antennas), while the size of the vertical displacement indicated something of the number of aircraft involved. By comparing the strengths returned from the various antennas up the tower, the altitude could be determined to some degree of accuracy.

CH proved highly effective during the Battle of Britain, and is often credited with allowing the RAF to defeat the much larger Luftwaffe forces. Whereas the Luftwaffe had to hunt all over to find the RAF fighters, the RAF knew exactly where the Luftwaffe bombers were, and could converge all of their fighters on them. CH was a force multiplier in today's terms, allowing the RAF fighters to operate more effectively, as if they were a much larger force operating as the same effectiveness as the Germans.

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. But they didn't.

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

Ground Controlled Intercept

An annotated picture of one of the first Plan Position Indicator images - Pembroke and Milford Haven as seen on the PPI of an early H2S screen

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.

Airborne Intercept

Rather than avoid the radars, the Luftwaffe took to avoiding the fighters instead 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.

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

Initial AI sets were available in 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.

Centimetric radar

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 microwave frequencies much more efficiently than previous devices, allowing the development of practical centimetric radar. Centimetric radar allowed for the detection of much smaller objects and the use of much smaller antennas than the earlier lower frequency radars.

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

The British need to produce the magnetron in large quantities was so great that Edward George Bowen was sent as the radar expert in the Tizard Mission to the USA in 1940, which resulted in the creation of the MIT Radiation Lab to develop the device further. Half of the radar deployed during World War II were designed at the RadLab, including over 100 different radar systems costing $1.5 billion.

Germany

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.

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

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

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

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

Airborne radars

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

Early Lichtenstein BC units were not deployed until 1942, and as they operated on the 2 m wavelength 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.

US

Early U.S. work on radar started with the Signal Corps, which had been working on an early type of VHF radar equipment at Fort Monmouth and Camp Evans in New Jersey for use with coast artillery.

By 1940 the U.S. Navy had developed the CXAM radar that was very similar to the British Chain Home system. The Navy had also produced by 1940 an experimental 10-centimeter radar but it had insufficient transmitter power.

The major impetus given to US radar technology was through the Tizard Mission in 1940, which transferred British technology, in particular the powerful cavity magnetron and led to the creation of the MIT Radiation Lab, a major center for research employing almost 4,000 people at its peak during the Second World War. By the time the US entered World War II, operational radar technology existed in the oscilloscope type SCR-270 radar.

It was in 1942 that the neologism and acronym RADAR was coined by the U.S. Navy.

The acronym RADAR is still in use by the US Navy, and as a mnemonic device to describe its components, they have come up with a new acroynm, ARMPIT (Antenna, Receiver, Modulator, PowerSupply, Indicator, Transmitter).

Cold War

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

External links

Hollmann, Martin, "[Radar Family Tree]". [Radar World].
Penley, Bill, and Jonathan Penley, "[Early Radar History] - an Introduction". 2002.
Buderi, "[Telephone History: Radar History]". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.)
Sinnott, D.H., "[The Development of Over-the-Horizon Radar in Australia]"
[The Secrets of Radar Museum] (Canada's involvement in WWII Radar)
[Historic Radar Archive]
[The Radar Pages]

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.