Chain Home, or CH for short, was the codename for the ring of coastal early warning radar stations built by the Royal Air Force (RAF) before and during the Second World War to detect and track aircraft.

Using commercial short wave radio hardware, Watt's team built a prototype pulsed transmitter and by June 1935 it detected an aircraft that happened to be flying past.

Over the next two years, additional stations were built while the problem of disseminating the information to the fighter aircraft led to the first integrated ground-controlled interception network, the Dowding system.

[10] Since 1915, Robert Watson-Watt had been working for the Met Office in a lab that was colocated at the National Physical Laboratory's (NPL) Radio Research Section (RRS) at Ditton Park in Slough.

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.

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.

In 1932, Winston Churchill and his friend, confidant and scientific advisor Frederick Lindemann travelled by car in Europe, where they saw the rapid rebuilding of the German aircraft industry.

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.

"[21] 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.

"[23] 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.

"[21] 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.

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.

"[25] 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.

[24] 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;[29] it went unclaimed.

"[36] Wilkins recalled the earlier report from the GPO, and noted that the wingspan of a contemporary bomber aircraft, about 25 m (82 ft), would be just right to form a half-wavelength dipole antenna for signals in the range of 50 m wavelength, or about 6 MHz.

[37] 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.

The group released several reports on these effects as a 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.

[53][55] 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".

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.

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.

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.

To address this problem, the system was later provided with a second pulse repetition frequency at 12.5 pps, which meant that a reflection would have to be from further than 6,000 miles (9,700 km) away before it would be seen during the next reception period.

But a much more accurate determination could be made by observing the "beat" rate of the composite echoes, the way they grew and diminished over time as they entered into different sections of the antenna reception pattern.

Additionally, that altitude was affected by the range, due to the curvature of the Earth, as well as any imperfections in the local environment, which caused the lobes to have different measurements depending on the target angle.

[84] From May to August 1939 the LZ130 Graf Zeppelin II made flights along Britain's North Sea coast to investigate the 100-metre-high radio towers that were being erected from Portsmouth to Scapa Flow.

In mid-June 1940, the Deutsche Versuchsanstalt für Luftfahrt (DVL, German Aeronautic Research Institute) set up a special group under the direction of Professor von Handel and found out that the signals originated from the installations on the coast of the English Channel.

The British had been aware that the Germans would determine the purpose of the system and attempt to interfere with it, and had designed in a variety of features and methods in order to address some of these issues even as the first stations were being built.

[90] By the opening of the Battle in July the German Luftwaffe operational units were well aware of CH, and had been informed by the DVL that they could not expect to remain undetected, even in clouds.

During the Air Battle of the Heligoland Bight in 1939, a German Freya radar detected the raid while it was still an hour away from its target, yet had no way to report this to any of the fighter units that could intercept it.

Although these new jammers were relatively sophisticated, CH operators quickly adapted to them by periodically changing the pulse repetition frequency (PRF) of their station's transmitter.