Editing Chain Home (section)

==Development==

===Prior experiments===
From the earliest days of [[radio]] technology, signals had been used for navigation using the [[radio direction finding]] (RDF) technique. RDF can determine the bearing to a radio transmitter, and several such measurements can be combined to produce a [[radio fix]], allowing the receiver's position to be calculated.<ref>{{cite web |title=Federal Standard 1037C, Glossary of Telecommunication Terms |year=1996 |website=General Services Administration |url=http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm |access-date=25 May 2015 |archive-date=2 March 2009 |archive-url=https://web.archive.org/web/20090302235918/http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm |url-status=dead }}</ref> Given some basic changes to the broadcast signal, it was possible for the receiver to determine its location using a single station. The UK pioneered one such service in the form of the [[Orfordness Beacon]].<ref>{{cite book |first1=B. |last1=Sitterly |first2=D. |last2=Davidson |url=http://www.loran-history.info/downloads/MIT-Radiation-Lab-Series-V4-Long-Range-Navigation-LORAN.pdf |title=The LORAN System |publisher=McGraw Hill |date=1948 |page=4 |access-date=8 August 2016 |archive-date=25 September 2012 |archive-url=https://web.archive.org/web/20120925163857/http://www.loran-history.info/downloads/MIT-Radiation-Lab-Series-V4-Long-Range-Navigation-LORAN.pdf |url-status=dead }}</ref>

Through the early period of radio development it was widely known that certain materials, especially metal, reflected radio signals. This led to the possibility of determining the location of objects by broadcasting a signal and then using RDF to measure the [[Bearing (navigation)|bearing]] of any reflections. Such a system saw patents issued to Germany's [[Christian Hülsmeyer]] in 1904,<ref name=bauer>{{cite book |last=Bauer |first=Arthur |title=Christian Hülsmeyer and about the early days of radar inventions |publisher=Foundation Centre for German Communications and Related Technologies |url=http://aobauer.home.xs4all.nl/Huelspart1def.pdf |date=15 January 2005}}</ref> and widespread experimentation with the basic concept was carried out from then on. These systems revealed only the bearing to the target, not the range, and due to the low power of radio equipment of that era, they were useful only for short-range detection. This led to their use for iceberg and collision warning in fog or bad weather, where all that was required was the rough bearing of nearby objects.<ref name=bauer/>

The use of radio detection specifically against aircraft was first considered in the early 1930s. Teams in the UK, US,{{sfn|Bowen|1998|p=6}} Japan,<ref>{{cite book |first= Shigeru |last=Nakajima |chapter=The history of Japanese radar development to 1945 |pages=245–258 |title=Radar Development to 1945 |editor-first=Russell |editor-last=Burns |publisher=Peter Peregrinus |year=1988 |isbn= 978-0863411397}}</ref> Germany<ref name=holl>{{cite web|author=Hollmann |url=http://www.radarworld.org/germany.html |title=Radar Development in Germany |publisher=Radarworld.org |access-date=10 February 2013}}</ref> and others had all considered this concept and put at least some amount of effort into developing it. Lacking ranging information, such systems remained of limited use in practical terms; two angle measurements could be used, but these took time to complete using existing RDF equipment and the rapid movement of the aircraft during the measurement would make coordination difficult.<ref name=holl/>

===Radio research in the UK===
[[File:Robert Watson-Watt.jpg|thumb|upright=0.8|[[Robert Watson-Watt]]'s position with the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] placed him at the centre of a network of researchers whose knowledge of radio physics was instrumental to the rapid development of radar.]]

Since 1915, Robert Watson-Watt had been working for the [[Met Office]] in a lab that was colocated at the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]]'s (NPL) [[Radio Research Station (UK)|Radio Research Section]] (RRS) at [[Ditton Park]] in [[Slough]]. Watt became interested in using the fleeting radio signals given off by [[lightning]] as a way to track [[thunderstorm]]s, but existing RDF techniques were too slow to allow the direction to be determined before the signal disappeared. In 1922,{{sfn|Bowen|1998|p=7}} he solved this by connecting a [[cathode-ray tube]] (CRT) to a directional [[Adcock antenna]] array, originally built by the RRS but now unused. The combined system, later known as [[huff-duff]] (from HF/DF, high frequency direction finding), allowed the almost instantaneous determination of the bearing of a signal. The Met Office began using it to produce storm warnings for aviators.{{sfn|Watson|2009|p=39}}

During this period, [[Edward Victor Appleton|Edward Appleton]] of [[King's College, Cambridge]] was carrying out experiments that would lead to him winning the [[Nobel Prize in Physics]]. Using a BBC transmitter set up in 1923 in [[Bournemouth]] and listening for its signal with a receiver at [[Oxford University]], he was able to use changes in wavelength to measure the distance to a reflective layer in the atmosphere then known as the [[Kennelly–Heaviside layer|Heaviside layer]]. After the initial experiments at Oxford, an NPL transmitter at [[Teddington]] was used as a source, received by Appleton in an out-station of King's College in the East End of London. Watt learned of these experiments and began conducting the same measurements using his team's receivers in Slough. From then on, the two teams interacted regularly and Watt coined the term [[ionosphere]] to describe the multiple atmospheric layers they discovered.<ref>{{cite book |first=Robert |last=Clark |title= Sir Edward Appleton G.B.E., K.C.B., F.R.S. |url=https://books.google.com/books?id=fZg4BQAAQBAJ |publisher= Elsevier |year=2013 |pages=39–45, 51, 53|isbn=9781483153766 }}</ref>

In 1927 the two radio labs, at the Met Office and NPL, were combined to form the Radio Research Station (with the same acronym, RRS), run by the NPL with Watt as the Superintendent.{{sfn|Bowen|1998|p=7}} This provided Watt with direct contact to the research community, as well as the chief signals officers of the [[British Army]], [[Royal Navy]] and [[Royal Air Force]]. Watt became a well-known expert in the field of radio technology.{{sfn|Bowen|1998|p=7}} This began a long period where Watt agitated for the NPL to take a more active role in technology development, as opposed to its pure research role. Watt was particularly interested in the use of radio for long-range aircraft navigation, but the NPL management at Teddington was not receptive and these proposals went nowhere.{{sfn|Bowen|1998|p=9}}

===Detection of aircraft===
In 1931, [[Arnold Frederic Wilkins]] joined Watt's staff in Slough. As the "new boy", he was given a variety of menial tasks to complete. One of these was to select a new [[shortwave]] receiver for ionospheric studies, a task he undertook with great seriousness. After reading everything available on several units, he selected a model from the [[General Post Office]] (GPO) that worked at (for that time) very high frequencies. As part of their tests of this system, in June 1932 the GPO published a report, No. 232 ''Interference by Aeroplanes''. The report recounted the GPO testing team's observation that aircraft flying near the receiver caused the signal to change in intensity, an annoying effect known as [[fading]].{{sfn|Watson|2009|p=44}}

The stage was now set for the development of radar in the UK. Using Wilkins' knowledge that shortwave signals bounced off aircraft, a BBC transmitter to light up the sky as in Appleton's experiment, and Watt's RDF technique to measure angles, a complete radar could be built. While such a system could determine the angle to a target, it could not determine its range and provide a location in space. To do so, two such measurements would have to be made from different locations. Watt's huff-duff technique solved the problem of making rapid measurements, but the issue of coordinating the measurement at two stations remained, as did any inaccuracies in measurement or differences in calibration between the two stations.{{sfn|Clark|1997|p=30}}

The missing technique that made radar practical was the use of pulses to determine range by measuring the time between the transmission of the signal and reception of the reflected signal. This would allow a single station to measure angle and range simultaneously. In 1924, two researchers at the [[Naval Research Laboratory]] in the United States, [[Merle Tuve]] and Gregory Briet, decided to recreate Appleton's experiment using timed pulsed signals instead of the changing wavelengths.{{sfn|Seitz|Einspruch|1998|p=91}} The application of this technique to a detection system was not lost on those working in the field, and such a system was prototyped by [[W. A. S. Butement]] and [[P. E. Pollard]] of the British [[Signals Research and Development Establishment|Signals Experimental Establishment]] (SEE) in 1931. The [[War Office]] proved uninterested in the concept and the development remained little known outside SEE.<ref>{{cite journal |first=R.W. |last=Home |title= Butement, William Alan (1904–1990) |url=http://adb.anu.edu.au/biography/butement-william-alan-12274 |journal=Australian Dictionary of Biography |year=2007}}</ref>

==="The bomber will always get through"===<!--This is a quote in the original-->
[[File:Stanley Baldwin ggbain.35233.jpg|thumb|upright=0.8|right|[[Stanley Baldwin]]'s 1932 comments on future aerial warfare led to a "feeling of defencelessness and dismay." It was the UK's concern about this issue that led to so much support being given to radar development while other countries had a much more lackadaisical approach until the war started.]]

At the same time, the need for such a system was becoming increasingly pressing. In 1932, [[Winston Churchill]] and his friend, confidant and scientific advisor [[Frederick Lindemann, 1st Viscount Cherwell|Frederick Lindemann]] travelled by car in Europe, where they saw the rapid rebuilding of the German aircraft industry.<ref>{{cite web |url=http://www.historynet.com/lord-cherwell-churchills-confidence-man.htm |title=Lord Cherwell: Churchill's Confidence Man |first=Madhusree |last=Mukerjee |website=Historynet |date=29 September 2011}}</ref> It was in November of that year that [[Stanley Baldwin]] gave his famous speech, stating that "[[The bomber will always get through]]".<ref>{{cite book |first1=Keith |last1=Middlemas |first2=John |last2=Barnes |date=1969 |title=Baldwin: A Biography |url=https://archive.org/details/baldwinbiography0000midd |url-access=registration |publisher=Weidenfeld and Nicolson |page=[https://archive.org/details/baldwinbiography0000midd/page/722 722] |isbn=9780297178590 }}</ref>

In the early summer of 1934, the RAF carried out large-scale exercises with up to 350 aircraft. The forces were split, with bombers attempting to attack London, while fighters, guided by the [[Royal Observer Corps|Observer Corps]], attempted to stop them. The results were dismal. In most cases, the vast majority of the bombers reached their target without ever seeing a fighter. To address the one-sided results, the RAF gave increasingly accurate information to the defenders, eventually telling the observers where and when the attacks would be taking place. Even then, 70 per cent of the bombers reached their targets unhindered. The numbers suggested any targets in the city would be destroyed.<ref name=london/> Squadron Leader P. R. Burchall summed up the results by noting that "a feeling of defencelessness and dismay, or at all events of uneasiness, has seized the public."<ref name=london>{{cite news |title=The Air Attacks on London |newspaper=The Spectator |date=2 August 1934 |page=9 |url=http://archive.spectator.co.uk/article/3rd-august-1934/9/the-air-attacks-on-london}}</ref> In November, Churchill gave a speech on "The threat of Nazi Germany" in which he pointed out that the Royal Navy could not protect Britain from an enemy who attacked by air.<ref>{{cite AV media |title=The Threat of Nazi Germany |url=https://www.winstonchurchill.org/resources/speeches/1930-1938-the-wilderness/the-threat-of-nazi-germany | people =Winston Churchill |date=16 November 1934 |medium=Audio recording |access-date=19 May 2017}}</ref>

Through the early 1930s, a debate raged within British military and political circles about strategic airpower. Baldwin's famous speech led many to believe the only way to prevent the bombing of British cities was to make a strategic bomber force so large it could, as Baldwin put it, "kill more women and children more quickly than the enemy."<ref name = Times >{{Cite journal | title = Mr Baldwin on Aerial Warfare – A Fear for the Future |journal=The Times | place = London, ENG, UK | date = 11 November 1932 | page = 7 column B}}.</ref> Even the highest levels of the RAF came to agree with this policy, publicly stating that their tests suggested that "'The best form of defence is attack' may be all-too-familiar platitudes, but they illustrate the only sound method of defending this country from air invasion. It is attack that counts."<ref name=london/> As it became clear the Germans were rapidly rearming the ''Luftwaffe'', the fear grew RAF could not meet the objective of winning such a tit-for-tat exchange and many suggested they invest in a massive bomber building exercise.{{sfn|Clark|1997|p=28}}

Others felt advances in fighters meant the bomber was increasingly vulnerable and suggested at least exploring a defensive approach. Among the latter group was Lindemann, [[test pilot]] and scientist, who noted in ''[[The Times]]'' in August 1934 that "To adopt a defeatist attitude in the face of such a threat is inexcusable until it has definitely been shown that all the resources of science and invention have been exhausted."{{sfn|Clark|1997|pp=28-29}}

===Tales of destructive "rays"===
[[File:Grindell-Matthews death ray.jpg|thumb|A 1925 radio magazine photograph of Grindell-Matthews'<!--possessive, remember?--> death ray]]

In 1923–24 inventor [[Harry Grindell Matthews]] repeatedly claimed to have built a device that projected energy over long ranges and attempted to sell it to the War Office, but it was deemed to be fraudulent.{{sfn|Clarke|2014|pp=48-51}} His attempts spurred on many other inventors to contact the British military with claims of having perfected some form of the fabled electric or radio "[[death ray]]".{{sfn|Clarke|2014|pp=48-51}} Some turned out to be frauds and none turned out to be feasible.<ref name="Stephen Budiansky 2005, pages 192-193">{{cite book |first=Stephen |last=Budiansky |title=Air Power: The Men, Machines, and Ideas That Revolutionized War, from Kitty Hawk to Iraq |publisher=Penguin |date=2005 |pages=192–193}}</ref>

Around the same time, a series of stories suggested another radio weapon was being developed in Germany. The stories varied, with one common thread being a death ray, and another that used the signals to interfere with an engine's [[ignition system]] to cause the engine to stall. One commonly repeated story involved an English couple who were driving in the [[Black Forest]] on holiday and had their car fail in the countryside. They claimed they were approached by soldiers who told them to wait while they conducted a test, and were then able to start their engine without trouble when the test was complete. This was followed shortly thereafter by a story in a German newspaper with an image of a large radio antenna that had been installed on [[Feldberg (Black Forest)|Feldberg]] in the same area.{{sfn|Jones|1978|p=50}}

Although highly skeptical about claims of engine-stopping rays and death rays, the Air Ministry could not ignore them as they were theoretically possible.<ref name="Stephen Budiansky 2005, pages 192-193"/> If such systems could be built, it might render bombers useless.{{sfn|Watson|2009|p=44}} If this were to happen, the night bomber deterrent might evaporate overnight, leaving the UK open to attack by Germany's ever-growing air fleet. Conversely, if the UK had such a device, the population could be protected.{{sfn|Clark|1997|p=28}}

In 1934, along with a movement to establish a scientific committee to examine these new types of weapons, the RAF offered a £1,000 prize to anyone who could demonstrate a working model of a death ray that could kill a sheep at 100 yards;{{sfn|Heazell|2011|p={{pn|date=April 2024}}}} it went unclaimed.{{sfn|Watson|2009|p=44}}

===Tizard committee===
The need to research better forms of air defense prompted [[Harry Wimperis]]{{efn|Bowen suggests Tizard was the original impetus for the formation of the Committee and had approached Wimperis to back him up.{{sfn|Bowen|1998|p=4}}}} to press for the formation of a study group to consider new concepts. [[Charles Vane-Tempest-Stewart, 7th Marquess of Londonderry|Lord Londonderry]], then<!--not permanently--> [[Secretary of State for Air]], approved the formation of the [[Committee for the Scientific Survey of Air Defence]] in November 1934, asking [[Henry Tizard]] to chair the group, which thus became better known to history as the [[Tizard Committee]].<ref>{{cite book |first=David |last=Zimmerman |url=https://books.google.com/books?id=k76Q1P4HEmwC&pg=PA23 |title=Top Secret Exchange: The Tizard Mission and the Scientific War |publisher=McGill-Queen's Press |year=1996 |page=23|isbn=9780750912426 }}</ref>

When Wimperis sought an expert in radio to help judge the death-ray concept, he was naturally directed to Watt. He wrote to Watt "on the practicability of proposals of the type colloquially called 'death ray'".{{sfn|Jones|1978|p=19}} The two met on 18 January 1935,{{sfn|Watson|2009|pp=44-45}} and Watt promised to look into the matter. Watt turned to Wilkins for help but wanted to keep the underlying question a secret. He asked Wilkins to calculate what sort of radio energy would be needed to raise the temperature of {{convert|8|imppt}} of water at a distance of {{convert|5|km}} from {{convert|98|to|105|°F}}. To Watt's bemusement, Wilkins immediately surmised this was a question about a death ray. He made a number of [[back-of-the-envelope calculation]]s<ref>{{cite journal |first=B.A. |last=Austin |title=Precursors To Radar – The Watson-Watt Memorandum and the Daventry Experiment |url=http://www.bawdseyradar.org.uk/wp-content/uploads/2012/12/Wilkins-Calculations.pdf |journal=International Journal of Electrical Engineering & Education |volume=36 |year=1999 |issue=4 |pages=365–372 |doi=10.7227/IJEEE.36.4.10 |s2cid=111153288 |url-status=dead |archive-url=https://web.archive.org/web/20150525040134/http://www.bawdseyradar.org.uk/wp-content/uploads/2012/12/Wilkins-Calculations.pdf |archive-date=25 May 2015}}</ref> demonstrating the amount of energy needed would be impossible given the [[state of the art]] in electronics.{{sfn|Watson|2009|p=45}}

According to [[Reginald Victor Jones|R. V. Jones]], when Wilkins reported the negative results, Watt asked, "Well then, if the death ray is not possible, how can we help them?"<ref>{{cite book |first=Reginald Victor |last=Jones |url=https://books.google.com/books?id=hkftkibM5fkC&pg=PT19 |title=Most Secret War |publisher=Penguin |date=2009 |page=19|isbn=9780141957678 }}</ref> Wilkins recalled the earlier report from the GPO, and noted that the [[wingspan]] of a contemporary [[bomber aircraft]], about {{convert|25|m|ft|abbr=on}}, would be just right to form a [[Dipole antenna|half-wavelength dipole antenna]] for signals in the range of 50&nbsp;m wavelength, or about 6&nbsp;MHz. In theory, this would efficiently reflect the signal and could be picked up by a receiver to give an early indication of approaching aircraft.{{sfn|Watson|2009|p=45}}

=== "Less unpromising" ===
[[File:Arnold Frederic Wilkins.jpg|thumb|upright=0.8|Arnold Wilkins carried out most of the theoretical and practical work that proved radar could work.]]
Watt wrote back to the committee saying the death ray was extremely unlikely, but added:

{{quote|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.{{sfn|Watson|2009|p=45}}}}

The letter was discussed at the first official meeting of the Tizard Committee on 28 January 1935. The utility of the concept was evident to all attending, but the question remained whether it was actually possible. [[Albert Rowe (physicist)|Albert Rowe]] and Wimperis both checked the maths and it appeared to be correct. They immediately wrote back asking for a more detailed consideration. Watt and Wilkins followed up with a 14 February secret memo entitled ''Detection and Location of Aircraft by Radio Means''.<ref>{{cite tech report |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a110586.pdf |title=New Eye for the Navy: The Origin of Radar at the Naval Research Laboratory |first=David |last=Allison |date=29 September 1981 |publisher= Naval Research Laboratory |page=143}}</ref> In the new memo, Watson-Watt and Wilkins first considered various natural emanations from the aircraft – light, heat and radio waves from the engine ignition system – and demonstrated that these were too easy for the enemy to mask to a level that would be undetectable at reasonable ranges. They concluded that radio waves from their own transmitter would be needed.{{sfn|Watson|2009|p=45}}

Wilkins gave specific calculations for the expected reflectivity of an aircraft. The received signal would be only 10<sup>−19</sup> times as strong as the transmitted one, but such sensitivity was considered to be within the state of the art.{{sfn|Bowen|1998|p=9}} To reach this goal, a further improvement in receiver sensitivity of two times was assumed. Their ionospheric systems broadcast only about 1&nbsp;kW,{{sfn|Bowen|1998|p=9}} but commercial shortwave systems were available with 15 amp transmitters (about 10&nbsp;kW) that they calculated would produce a signal detectable at about {{convert|10|miles|km}}. They went on to suggest that the output power could be increased as much as ten times if the system operated in pulses instead of continuously, and that such a system would have the advantage of allowing range to the targets to be determined by measuring the time delay between transmission and reception on an [[oscilloscope]].{{sfn|Watson|2009|p=45}} The rest of the required performance would be made up by increasing the [[Gain (electronics)|gain]] of the antennas by making them very tall, focusing the signal vertically.{{sfn|Bowen|1998|p=10}} The memo concluded with an outline for a complete station using these techniques. The design was almost identical to the CH stations that went into service.{{sfn|Watson|2009|p=45}}

===Daventry experiment===
[[File:Jock Herd in Ditton Park radio van.jpg|thumb|This Morris Commercial T-type van, originally used as a portable radio reception testbed, was later refitted for the Daventry Experiment. It is shown in 1933, being operated by "Jock" Herd.]]

The letter was seized on by the Committee, who immediately released £4,000 to begin development.{{efn|Some sources say £2,000.}} They petitioned [[Hugh Dowding]], the [[Air Member for Supply and Research]], to ask the Treasury for another £10,000. Dowding was extremely impressed with the concept, but demanded a practical demonstration before further funding was released.{{sfn|Watson|2009|p=46}}{{sfn|Gough|1993|p=2}}

Wilkins suggested using the new 10&nbsp;kW, 49.8&nbsp;m [[BBC]] [[Borough Hill]] shortwave station in [[Daventry]], Northamptonshire as a suitable ''ad hoc'' transmitter. The receiver and an oscilloscope were placed in a delivery van the RRS used for measuring radio reception around the countryside. On 26 February 1935,{{efn|This, coincidentally, was the same day Hitler officially created the ''Luftwaffe''.<ref>{{cite web |website=History Channel |title=Hitler organizes Luftwaffe |url=http://www.history.com/this-day-in-history/hitler-organizes-luftwaffe}}</ref>}} they parked the van in a field near [[Upper Stowe]] and connected it to wire antennas stretched across the field on top of wooden poles. A [[Handley Page Heyford]] made four passes over the area, producing clearly notable effects on the CRT display on three of the passes.{{sfn|Gough|1993|p=3}} A memorial stone was placed at the site of the test.<ref>{{Cite web|url=https://www.geograph.org.uk/photo/386034|title=Geograph:: Birth of Radar Memorial (C) Jeff Tomlinson|website=www.geograph.org.uk}}</ref>

Observing the test were Watt, Wilkins, and several other members of the RRS team, along with Rowe representing the Tizard Committee. Watt was so impressed he later claimed to have exclaimed: "Britain has become an island again!"{{sfn|Watson|2009|p=46}}

Rowe and Dowding were equally impressed. It was at this point that Watt's previous agitation over development became important; NPL management remained uninterested in practical development of the concept, and was happy to allow the Air Ministry to take over the team.{{sfn|Bowen|1998|p=8}} Days later, the Treasury released £12,300 for further development,{{sfn|Watson|2009|p=46}} and a small team of the RRS researchers were sworn to secrecy and began developing the concept.{{sfn|Bowen|1998|p=8}} A system was to be built at the RRS station, and then moved to [[Orfordness]] for over-water testing. Wilkins would develop the receiver based on the GPO units, along with suitable antenna systems. This left the problem of developing a suitable pulsed transmitter. An engineer familiar with these concepts was needed.{{sfn|Watson|2009|p=47}}

===Experimental system===
[[Edward George Bowen]] joined the team after responding to a newspaper advertisement looking for a radio expert. Bowen had previously worked on ionosphere studies under Appleton, and was well acquainted with the basic concepts. He had also used the RRS' RDF systems at Appleton's request and was known to the RRS staff.{{sfn|Bowen|1998|p=8}} After a breezy interview, Watson-Watt and Jock Herd stated the job was his if he could sing the [[Hen Wlad Fy Nhadau|Welsh national anthem]]. He agreed, but only if they would sing [[National anthem of Scotland|the Scottish one]] in return. They declined, and gave him the job.{{sfn|Bowen|1998|p=9}}

Starting with the BBC transmitter electronics, but using a new transmitter [[Valve (electronics)|valve]] from the Navy, Bowen produced a system that transmitted a 25&nbsp;kW signal at 6&nbsp;MHz (50&nbsp;metre wavelength), sending out 25&nbsp;μs long pulses 25 times a second.{{sfn|Watson|2009|p=47}} Meanwhile, Wilkins and L.H. Bainbridge-Bell built a receiver based on electronics from [[Ferranti]] and one of the RRS CRTs. They decided not to assemble the system at the RRS for secrecy reasons. The team, now consisting of three scientific officers and six assistants, began moving the equipment to Orfordness on 13 May 1935. The receiver and transmitter were set up in old huts left over from [[World War I]] artillery experiments, the transmitter antenna was a single [[dipole]] strung horizontally between two {{convert|75|foot}} poles, and the receiver a similar arrangement of two crossed wires.{{sfn|Bowen|1998|pp=11–13}}

The system showed little success against aircraft, although echoes from the ionosphere as far as 1,000&nbsp;miles away were noted. The group released several reports on these effects as a [[disinformation|cover story]], claiming that their ionospheric studies had been interfering with the other experiments at the RRS at Slough, and expressing their gratitude that the Air Ministry had granted them access to unused land at Orfordness to continue their efforts.{{sfn|Watson|2009|p=48}} Bowen continued increasing the voltage in the transmitter, starting with the 5000&nbsp;[[volt]] maximum suggested by the Navy, but increasing in steps over several months to 12,000&nbsp;V, which produced pulses of 200&nbsp;kW.{{sfn|Bowen|1998|p=14}} Arcing between the valves required the transmitter to be rebuilt with more room between them,{{sfn|Watson|2009|p=48}} while arcing on the antenna was solved by hanging copper balls from the dipole to reduce [[corona discharge]].{{sfn|Bowen|1998|p=13}}

By June the system was working well, although Bainbridge-Bell proved to be so skeptical of success that Watt eventually returned him to the RRS and replaced him with Nick Carter.{{sfn|Bowen|1998|p=14}} The Tizard Committee visited the site on 15 June to examine the team's progress. Watt secretly arranged for a [[Vickers Valentia]] to fly nearby, and years later insisted that he saw the echoes on the display, but no one else recalls seeing these.{{sfn|Bowen|1998|p=15}}

Watt decided not to return to the RRS with the rest of the Tizard group and stayed with the team for another day.{{sfn|Bowen|1998|p=16}} With no changes made to the equipment, on 17 June the system was turned on and immediately provided returns from an object at {{convert|17|mi|km|abbr=on}}. After tracking it for some time, they watched it fly off to the south and disappear. Watt phoned the nearby [[Seaplane Experimental Station]] at [[Felixstowe]] and the superintendent stated that a [[Supermarine Scapa]] [[flying boat]] had just landed. Watt requested the aircraft return to make more passes.{{sfn|Bowen|1998|p=16}} This event is considered the official birth date of radar in the UK.{{sfn|Watson|2009|p=50}}

Aircraft from [[RAF Martlesham Heath]] took over the job of providing targets for the system, and the range was continually pushed out. During a 24 July test, the receiver detected a target at {{convert|40|mi|abbr=on}} and the signal was strong enough that they could determine the target was actually three aircraft in close formation. By September the range was consistently 40&nbsp;miles, increasing to {{convert|80|miles}} by the end of the year, and with the power improvements Bowen worked into the transmitter, was over {{convert|100|mi|abbr=on}} by early 1936.{{sfn|Bowen|1998|p=16}}

===Planning the chain===
[[File:Bawdsey For Wiki Russ.jpg|thumb|upright=1.5|right|Watson-Watt suggested using [[Bawdsey Manor]] in Suffolk as a development site, after Wilkins noticed it on a Sunday drive while working at [[Orfordness]]]]

In August 1935, [[Albert Percival Rowe|Albert Rowe]], secretary of the Tizard Committee, coined the term "Radio Direction and Finding" (RDF), deliberately choosing a name that could be confused with "Radio Direction Finding", a term already in widespread use.{{sfn|Watson|2009|p=50}}

In a 9 September 1935 memo, Watson-Watt outlined the progress to date. At that time the range was about {{convert|40|mi|abbr=on}}, so Watson-Watt suggested building a complete network of stations {{convert|20|mi|abbr=on}} apart along the entire east coast. Since the transmitters and receivers were separate, to save development costs he suggested placing a transmitter at every other station. The transmitter signal could be used by a receiver at that site as well as the ones on each side of it.{{sfn|Watson|2009|p=51}} This was quickly rendered moot by the rapid increases in range. When the Committee next visited the site in October, the range was up to {{cvt|80|mi}}, and Wilkins was working on a method for height finding using multiple antennas.{{sfn|Watson|2009|p=51}}

In spite of its ''ad hoc'' nature and short development time of less than six months, the Orfordness system had already become a useful and practical system. In comparison, the [[acoustic mirror]] systems that had been in development for a decade were still limited to only {{cvt|5|mi}} range under most conditions, and were very difficult to use in practice. Work on mirror systems ended, and on 19 December 1935, a £60,000 contract{{efn|Bowen puts the sum at £1,000,000.{{sfn|Bowen|1998|p=21}}}} for five{{efn|Gough says seven}} RDF stations along the south-east coast was sent out, to be operational by August 1936.{{sfn|Gough|1993|p=3}}{{sfn|Watson|2009|p=51}}

The only person not convinced of the utility of RDF was Lindemann. He had been placed on the Committee at the insistence of his friend, Churchill, and proved unimpressed with the team's work. When he visited the site, he was upset by the crude conditions, and apparently, by the box lunch he had to eat.{{sfn|Bowen|1998|p=20}} Lindemann strongly advocated the use of [[infrared]] systems for detection and tracking and numerous observers have noted Lindemann's continual interference with radar. As Bowen put it,

{{quote|Within a few months of his joining the Committee, what had previously been an innovative and forward-looking group became riven with strife. It was strictly Lindemann versus the rest, with his hostility to radar and his insistence on totally impractical ideas about intercepting enemy aircraft by means of wires dangled from balloons, or by infrared, which at that time simply did not have the sensitivity to detect aircraft at long range.{{sfn|Bowen|1998|p=20}}}}

Churchill's backing meant the other members' complaints about his behaviour were ignored. The matter was eventually referred back to [[Philip Cunliffe-Lister, 1st Earl of Swinton|Lord Swinton]], the new Secretary of State for Air. Swinton solved the problem by dissolving the original Committee and reforming it with Appleton in Lindemann's place.{{sfn|Watson|2009|p=51}}{{sfn|Bowen|1998|p=20}}

As the development effort grew, Watt requested a central research station be established "of large size and with ground space for a considerable number of mast and aerial systems".{{sfn|Watson|2009|p=51}} Several members of the team went on scouting trips with Watt to the north of Orfordness but found nothing suitable. Then Wilkins recalled having come across an interesting site about {{convert|10|mi|abbr=on}} south of Orfordness, some time earlier while on a Sunday drive. He recalled it because it was some {{cvt|70|–|80|ft}} above sea level, which was unusual in that area. The large [[manor house]] on the property would have ample room for experimental labs and offices. In February and March 1936, the team moved to [[Bawdsey Manor]] and established the [[Air Ministry Experimental Station]] (AMES). When the scientific team left in 1939, the site became the operational CH site [[RAF Bawdsey]].{{sfn|Watson|2009|p=52}}

While the "ness team" began moving to Bawdsey, the Orfordness site remained in use. This proved useful during one demonstration when the new system recently completed at Bawdsey failed. The next day, Robert Hanbury-Brown and the new recruit Gerald Touch started up the Orfordness system and were able to run the demonstrations from there. The Orfordness site was not closed until 1937.{{sfn|Heazell|2011|p=[https://books.google.com/books?id=FrUTDQAAQBAJ&pg=PT275 280]}}

===Into production===
[[File:Watson Radar.jpg|thumb|The first working<!--workable? really?--> radar unit constructed by Watson-Watt and his team. The four widely separated NT46 valves can be seen. Production units were largely identical.|alt=]]

The system was deliberately developed using existing commercially available technology to speed introduction.<ref>{{cite web|last=Waligorski |first=Martin |url=http://spitfiresite.com/2010/04/from-peace-to-war-royal-air-force-rearmament-programme-1934-1940.html |title=From Peace to War – Royal Air Force Rearmament Programme, 1934–1940 |publisher=Spitfiresite.com |date=10 April 2010 |access-date=10 February 2013}}</ref> The development team could not afford the time to develop and debug new technology. Watt, a pragmatic engineer, believed "third-best" would do if "second-best" would not be available in time and "best" never available at all.<ref name="Vectorsite.net">{{cite web |title=Longwave Radar at War / Early American Radar Efforts |publisher=Vectorsite.net |url=http://www.vectorsite.net/ttwiz_02.html |access-date=10 February 2013 |archive-date=17 February 2013 |archive-url=https://web.archive.org/web/20130217041924/http://www.vectorsite.net/ttwiz_02.html |url-status=usurped}}</ref> This led to the use of the 50&nbsp;m wavelength (around 6&nbsp;MHz), which Wilkins suggested would resonate in a bomber's wings and improve the signal. Unfortunately, this also meant that the system was increasingly blanketed by noise as new commercial broadcasts began taking up this formerly [[Shortwave radio|high-frequency spectrum]]. The team responded by reducing their own wavelength to 26&nbsp;m (around 11&nbsp;MHz) to get clear spectrum. To everyone's delight, and contrary to Wilkins' 1935 calculations, the shorter wavelength produced no loss of performance.{{sfn|Bowen|1998|p=20}} This led to a further reduction to 13&nbsp;m, and finally the ability to tune between 10 and 13&nbsp;m, (roughly 30-20&nbsp;MHz) to provide some [[frequency agility]] to help avoid jamming.{{sfn|Bowen|1998|p=21}}

Wilkins' method of height-finding was added in 1937. He had originally developed this system as a way to measure the vertical angle of transatlantic broadcasts while working at the RRS. The system consisted of several parallel dipoles separated vertically on the receiver masts. Normally the RDF [[Radiogoniometer|goniometer]] was connected to two crossed dipoles at the same height and used to determine the bearing to a target return. For height finding, the operator instead connected two antennas at different heights and carried out the same basic operation to determine the vertical angle. Because the transmitter antenna was deliberately focused vertically to improve gain, a single pair of such antennas would only cover a thin vertical angle. A series of such antennas was used, each pair with a different centre angle, providing continuous coverage from about 2.5 degrees over the horizon to as much as 40 degrees above it. With this addition, the final remaining piece of Watt's original memo was accomplished and the system was ready to go into production.{{sfn|Gough|1993|p=5}}{{sfn|Bowen|1998|p=21}}

Industry partners were canvassed in early 1937, and a production network was organized covering many companies. [[Metropolitan-Vickers]] took over design and production of the transmitters, [[AC Cossor]] did the same for the receivers, the Radio Transmission Equipment Company worked on the goniometers, and the antennas were designed by a joint AMES-GPO group. The Treasury gave approval for full-scale deployment in August, and the first production contracts were sent out for 20 sets in November, at a total cost of £380,000.{{sfn|Gough|1993|p=5}} Installation of 15 of these sets was carried out in 1937 and 1938. In June 1938 a London headquarters was established to organize the rapidly growing force. This became the Directorate of Communications Development (DCD), with Watt named as the director. Wilkins followed him to the DCD, and [[A. P. Rowe]] took over AMES at Bawdsey. In August 1938, the first five stations were declared operational and entered service during the [[Munich Agreement|Munich crisis]], starting full-time operation in September.{{sfn|Gough|1993|p=6}}

===Deployment===
[[File:Chain home coverage.jpg|thumb|Radar coverage 1939–1940]]

During the summer of 1936, experiments were carried out at [[RAF Biggin Hill]] to examine what effect the presence of radar would have on an air battle.<ref>{{cite web |url=http://histru.bournemouth.ac.uk/Oral_History/Talking_About_Technology/radar_research/biggin_hill_exp.html |title=Sir Henry and the 'Biggin Hill Experiment' |publisher=Histru.bournemouth.ac.uk |access-date=10 February 2013}}</ref> Assuming RDF would provide them 15 minutes' warning, they developed interception techniques putting fighters in front of the bombers with increasing efficiency. They found the main problems were finding their own aircraft's location, and ensuring the fighters were at the right altitude.<!--Positive control of each squadron, & constant micromanagement of heading, were drawbacks, according to H R Allen in ''Who Won the Battle of Britain''-->

In a similar test against the operational radar at Bawdsey in 1937, the results were comical. As Dowding watched the ground controllers scramble to direct their fighters, he could hear the bombers passing overhead. He identified the problem not as a technological one, but in the reporting. The pilots were being sent too many reports, often contradictory. This realization led to the development of the [[Dowding system]], an extensive network of telephone lines reporting to a central "filter room" in London where the reports from the radar stations were collected and collated, and fed back to the pilots in a clear format. The system as a whole was enormously manpower intensive.<!--The filter rooms actually slowed the information transfer process, according to Allen-->

By the outbreak of war in September 1939, there were 21 operational Chain Home stations. After the [[Battle of France]] in 1940 the network was expanded to cover the west coast and Northern Ireland. The Chain continued to be expanded throughout the war, and by 1940 it stretched from [[Orkney]] in the north to [[Weymouth, Dorset|Weymouth]] in the south. This provided radar coverage for the entire Europe-facing side of the British Isles, able to detect high-flying targets well over France. Calibration of the system was carried out initially using a flight of mostly civilian-flown, impressed [[Cierva C.30|Avro Rota]] [[autogyro]]s flying over a known landmark, the radar then being calibrated so that the position of a target relative to the ground could be read off the CRT. The Rota was used because of its ability to maintain a relatively stationary position over the ground, the pilots learning to fly in small circles while remaining at a constant ground position, despite a headwind.

The rapid expansion of the CH network necessitated more technical and operational personnel than the UK could provide, and in 1940, a formal request was made by the [[British High Commission, Ottawa]] to the Canadian Government, appealing for men skilled in radio technology for the service of the defence of Great Britain. By the end of 1941, 1,292 trained personnel had enlisted and most were rushed to England to serve as radar mechanics.<ref>{{cite book |last=Grande |first=George Kinnear |title=Canadians on Radar: Royal Canadian Air Force, 1940-45 |publisher=The Canadian Radar History Project |date=2000 |page=III-3}}</ref>

===Battle of Britain===
During the battle, Chain Home stations – most notably the one at [[Ventnor]], [[Isle of Wight]] – were attacked several times between 12 and 18 August 1940. On one occasion a section of the radar chain in Kent, including the Dover CH, was put out of action by a lucky hit on the power grid. Though the wooden huts housing the radar equipment were damaged, the towers survived owing to their open steel girder construction. Because the towers survived intact and the signals were soon restored, the ''Luftwaffe'' concluded the stations were too difficult to damage by bombing and left them alone for the remainder of the war.{{fact|date=September 2021}}<!--Neither did the Germans understand how vulnerable the generators, & phone lines, were-->

===Upgrades===
Chain Home was the primary radar system of the UK for only a short time. By 1942, many of its duties had been taken over by the far more advanced [[AMES Type 7]] ground-controlled interception ([[Ground control intercept|GCI]]) radar systems. Whereas CH scanned an area perhaps 100 degrees wide and required considerable effort to take measurements, the Type 7 scanned the entire 360-degree area around the station, and presented it on a [[plan position indicator]], essentially a real-time two-dimensional map of the airspace around the station. Both fighters and bombers appeared on the display, and could be distinguished using [[Identification friend or foe]] (IFF) signals. The data from this display could be read directly to the intercepting pilots, without the need for additional operators or control centres.

With the deployment of GCI, CH became the early warning portion of the radar network. To further simplify operations and reduce manpower requirements, the job of plotting the targets became semi-automated. An [[analogue computer]] of some complexity, known as "The Fruit Machine", was fed information directly from the operator console, reading the goniometer setting for bearing, and the range from the setting of a dial that moved a mechanical pointer along the screen until it lay over a selected target. When a button was pushed, the Fruit Machine read the inputs and calculated the X and Y location of the target, which a single operator could then plot on a map, or relay directly over the telephone.<ref name="Vectorsite.net"/>

The original transmitters were constantly upgraded, first from 100&nbsp;kW of the Orfordness system to 350&nbsp;kW for the deployed system, and then again to 750&nbsp;kW during the war in order to offer greatly increased range. To aid in detection at long range, a slower 12.5 pulse per second rate was added. The four-tower transmitter was later reduced to three towers.

===Big Ben===
[[File:Bundesarchiv Bild 141-1879, Rakete V2 nach Start cropped.jpg|thumb|right|Attempts to attack the heavily camouflaged and highly mobile V-2 were unsuccessful, but CH did help provide some early warning]]

The British had no answer to the [[V-2 rocket]] bombardment that began in September 1944. The missiles flew too high and too fast to be detected during their approach, leaving no time even for an [[air raid warning]] to be sounded. Their supersonic speed meant that the explosions occurred without warning before the sound of their approach reached the target. The government initially tried to pass them off as explosions in the underground gas mains. It was clear this was not the case, and eventually, examples of the V-2 falling in its final plunge were captured on film.

In response, several CH stations were re-organized into the "Big Ben" system to report the V-2s during launch. No attempt was made to try to find the location of the launch; the radio-goniometer was simply too slow to use. Instead, each of the stations in the network, Bawdsey, Gt. Bromley, High St, Dunkirk and Swingate (Dover) were left set to their maximum range settings and in the altitude measuring mode. In this mode, the radar had several stacked ''lobes'' where they were sensitive to signals. As the missile ascended it would pass through these lobes in turn, causing a series of blips to fade in and out over time. The stations attempted to measure the ranges to the target as they flew through each of these lobes and forwarded that by telephone to a central plotting station.{{sfn|Neale|1985|p=83}}

At the station, these range measurements were plotted as arcs on a chart, known as ''range cuts''. The intersections of the arcs defined the approximate area of the launcher. Since the missile approached the target as it climbed, each of these intersections would be closer to the target. Taking several of these, in turn, the trajectory of the missile could be determined to some degree of accuracy, and air raid warnings sent to likely areas.{{sfn|Neale|1985|p=83}}

Success in this task was aided by the missile fuselage profile, which acted as an excellent quarter-wave reflector for 12&nbsp;m band HF radar.<ref>{{cite web|author=Dick Barrett |url=http://www.radarpages.co.uk/mob/ch/chainhome10.htm |title=Chain Home |publisher=The Radar Pages |date=19 March 2002 |access-date=10 February 2013}}</ref> [[RAF Fighter Command]] was also informed of the launch in an effort to attack the sites. However, the German launch convoys were motorized, well camouflaged and highly mobile, making them extremely difficult to find and attack. The only known claim was made when [[Supermarine Spitfire]] pilots of [[No. 602 Squadron RAF]] squadron came across a V-2 rising from a wooded area, allowing a quick shot of unknown result.<ref>{{cite news |url=http://www.scotsman.com/lifestyle/the-spitfire-and-the-hunt-for-the-v2-1-1399596 |title=The Spitfire and the hunt for the V2 |date=14 November 2004 |newspaper=The Scotsman}}</ref>

===ROTOR===
The British radar defences were rapidly run down during the last years of the war, with many sites closed and others placed on "care and maintenance". However, immediate postwar tensions with the [[Soviet Union]] resulted in recommissioning of some wartime radars as a stopgap measure. Specific radars were remanufactured to peacetime standards of quality and reliability, which gave significant increases in range and accuracy. These rebuilt systems were the first phase of Chain Home's replacement system, [[ROTOR]], which progressed through three phases from 1949 to 1958.<ref>{{cite web|url=http://www.thetimechamber.co.uk/beta/sites/military/rotor-radar-stations |title=The ROTOR Project |publisher=TheTimeChamber |date=24 January 2013 |access-date=10 February 2013}}</ref>

It had been pointed out from the start that due to the inherent timing of the interception task, about 23 minutes was required to carry out a single interception from initial detection. If the target was a high-speed jet bomber, this required about {{convert|240|miles}} initial detection range.{{sfn|McCamley|2013|p=86}} CH, even in its upgraded form, was barely capable of this under the best conditions. The GCI radars were not even close to this, and the entire ROTOR system relied on a new radar system becoming available by 1957 at the latest. In one of the few instances of this occurring, this requirement was actually beaten, with the first [[AMES Type 80]] systems entering service in 1954.

The very last Chain Home Type 1 systems were retired in 1955 along with the wholesale demolition of most of the steel and timber towers.

===CH today===
[[File:CH Radar Mast - Stenigot - geograph.org.uk - 150615.jpg|alt=Stenigot Chain Home radar tower.|thumb|Stenigot Chain Home radar tower]]
Some of the steel transmitter towers remain, although the wooden receiver towers have all been demolished. The remaining towers have various new uses and in some cases are now protected as [[listed building]]s by order of [[English Heritage]].<ref>{{Cite web|url=https://www.pastscape.org.uk/SearchResults.aspx?rational=q&criteria=chain+home&search=ALL&sort=4&recordsperpage=10|title=PastScape search results Page|website=www.pastscape.org.uk}}</ref> One such {{convert|360|ft|m|adj=mid|-high}} transmitter tower can now be found at the [[BAE Systems]] facility at [[Great Baddow]] in Essex, on the former [[Marconi Research Centre]] site. It originally stood at RAF [[Canewdon]] in Essex and was moved to Great Baddow in 1956. This is the only surviving Chain Home tower still in its original, unmodified form with cantilever platforms at 50&nbsp;ft, 200&nbsp;ft and 360&nbsp;ft, and in 2019 was given Grade II listed status.<ref>{{National Heritage List for England|num=1456445|desc=Chain Home tower at Great Baddow|access-date=27 October 2019}}</ref> [[Swingate transmitting station]] in Kent (originally AMES 04 Dover) has two original towers (three until 2010) which are used for microwave relay; the towers lost their platforms in the 1970s. [[RAF Stenigot]] in Lincolnshire has another, almost complete tower, without its top platforms; it is used for training aerial erectors.

The only original Chain Home site which is still used as a military radar station is [[RRH Staxton Wold]] in North Yorkshire, although there are no remnants of the 1937 equipment as it was completely cleared and remodelled for the ROTOR replacement, the [[Linesman/Mediator]] system, in 1964.

The 240-foot timber receiver towers were some of the tallest wooden structures ever built in Britain. Two of these wooden towers were still standing in 1955, at Hayscastle Cross.<ref>{{cite web |url=http://www.subbrit.org.uk/sb-sites/sites/h/hayscastle_cross/index.shtml |title=Site Name: RAF Hayscastle Cross - West Coast Chain Home and West Coast Readiness ROTOR Radar Station |website=Subterranea Britannica |access-date=2009-06-01 |url-status=dead |archive-date=2009-05-01 |archive-url=https://web.archive.org/web/20090501184631/http://www.subbrit.org.uk/sb-sites/sites/h/hayscastle_cross/index.shtml}} shows these towers.</ref> Unlike the transmitter tower pictured here, those at Hayscastle Cross were guyed. The wooden reception towers at Stoke Holy Cross were demolished in 1960.<ref>{{cite web |url=http://www.poringlandarchive.co.uk/stoke-holy-cross/raf-stoke-holy-cross |title=RAF Stoke Holy Cross |website=Poringland Archive}}</ref> It is possible that a wooden tower standing at [[RAF Blakehill Farm]], Wiltshire, in the 1970s or 1980s was a Chain Home survivor.<ref>{{Cite web |last=Povey |first=Vincent |title=The AN/FLR-9 Type Antenna |url=https://www.rafblakehillfarm.co.uk/blakehill-anflr9-cold-war |access-date=2022-11-14 |website=RAF Station Blakehall Farm}}</ref>

Wilkins would later repeat the Daventry Experiment for the 1977 BBC Television series ''[[The Secret War (TV series)#Episode 2: "To See for a Hundred Miles" (12 January 1977)|The Secret War]]'' episode "To See For a Hundred Miles".