'Chain Home' was the British ring of coastal early warning radar stations used in World War II to detect and track aircraft (1939/45).

Initially known as Radio Direction Finding, and given the official name Air Ministry Experimental Station Type 1 in 1940, the radar units themselves were also known as 'Chain Home' for most of their life. 'Chain Home' was the first early warning radar network in the world, and the first military radar system to reach operational status, and its effect on the outcome of the war made it one of the most powerful weapons of what is today known as the 'wizards' war'.

Late in 1934, the Tizard Committee asked radio expert Robert Watson-Watt to comment on the repeated claims of radio death rays and reports suggesting Germany had built some sort of radio weapon. His assistant, Arnold Wilkins, demonstrated that a death ray was impossible but suggested radio could be used for long-range detection. In February 1935, a demonstration was arranged by placing a receiver near a BBC shortwave transmitter and flying an aeroplane around the area. An oscilloscope connected to the receiver showed a pattern from the aircraft’s electro-magnetic reflection. Funding for further research and development followed swiftly. Using commercial shortwave radio hardware, Watt’s team built a prototype pulsed transmitter. On 17 June 1935, it successfully measured the angle and range of an aircraft that happened to be flying in the area. Basic development had been completed by the end of the year, with detection ranges on the order of 100 miles (160 km). Throughout 1936 attention was focused on a production version, and early 1937 saw the addition of height finding.

The first five stations, covering the approaches to London, were installed by 1937 and began full-time operation in 1938. Operational tests in that year, using early units, demonstrated the difficulties in relaying useful information to pilots airborne in fighters. This led to the formation of the first integrated ground-controlled interception network, the 'Dowding system', which collected and filtered this information into a single view of the airspace. Dozens of 'Chain Home' stations covering the majority of the eastern and southern coasts of the UK, along with a complete ground network with thousands of miles of dedicated telephone lines, were ready by the time World War II began in September 1939, and the 'Chain Home' system proved decisive during the Battle of Britain in 1940. The 'Chain Home' system could detect German aircraft while they were forming over France, giving RAF commanders ample time to marshal their entire force directly in the path of the raid. This had the effect of multiplying the effectiveness of the RAF to the point that it was as if they had three times as many fighters, allowing them to defeat frequently larger German forces. The 'Chain Home' network was continually expanded, with over 40 stations operational by the war’s end.

'Chain Home' was unable to detect aircraft at low altitude, and from 1939 was normally partnered with the 'Chain Home Low' system, or AMES Type 2, which could detect aircraft flying at any altitude over 500 ft (150 m). Ports were covered by the 'Chain Home Extra Low' system, which gave cover down to 50 ft (15 m) but at shorter ranges of approximately 30 miles (48 km). In 1942 the AMES Type 7 radar began to assume the task of tracking targets once detected, and 'Chain Home' moved entirely to the early warning role. Late in the war, when the threat of Luftwaffe bombing had ended, the 'Chain Home' system was used to detect V2 missile launches.

British radar systems were wound down after the war, but the start of the Cold War led to efforts to establish a new network as rapidly as possible. In spite of being outdated, the 'Chain Home' radars were pressed into service in the new ROTOR system until replaced by newer systems in the 1950s.

From the earliest days of radio technology, signals had been used for navigation using the radio direction finding technique, which can determine the bearing to a radio transmitter, and several such measurements can be triangulated to produce a radio fix, allowing the receiver’s position to be calculated. 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.

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 radio direction finding to measure the bearing of any reflections. Such a system saw patents issued to Germany’s Christian Hülsmeyer in 1904, and widespread experimentation with the basic concept was carried out after that. These systems revealed the bearing but not the range of the target, and as a result of 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.

The use of radio detection specifically against aircraft was first considered in the early 1930s. Teams in the UK, USA, Japan, Germany and others had all considered this concept and put at least some effort into its development. 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 radio direction finding equipment and the rapid movement of the target aeroplane during the measurement made co-ordination difficult.

In the UK, Robert Watson-Watt had been working since 1915 for the Meteorological Office in a laboratory co-located at the National Physical Laboratory’s Radio Research Section at Ditton Park in Slough. Watson-Watt became interested in the use of the fleeting radio signals given off by lightning as a way to track thunderstorms. Existing radio direction finding techniques were too slow to allow the direction to be determined before the signal disappeared. however, but 1922 Watson-Watt solved this by connecting a cathode ray tube to a directional antenna array, originally built by the Radio Research Section 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 Meteorological Office began using it to produce storm warnings for fliers.

During this same period, Edward Appleton of King’s College, Cambridge was carrying out experiments using a BBC transmitter set up in 1923 in Bournemouth and listening for its signal with a receiver at Oxford University. Appleton was able to use changes in wavelength to measure the distance to a reflective layer in the atmosphere then known as the Heaviside layer. After the initial experiments at Oxford, a National Physical Laboratory 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. Watson-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 Watson-Watt coined the term ionosphere to describe the multiple atmospheric layers they discovered.

In 1927 the two radio laboratories, at the Meteorological Office and National Physical Laboratory, were combined to form the Radio Research Station run by the latter and with Watson-Watt as the superintendent. This provided Watson-Watt with direct contact to the research community, as well as the chief signals officers of the three British armed forces. This began a long period in which Watson-Watt agitated for the National Physical Laboratory to take a more active role in technology development, as opposed to its pure research role. Watson-Watt was particularly interested in the use of radio for long-range aircraft navigation, but the National Physical Laboratory’s management at Teddington was not very receptive and the proposals foundered.

In 1931, Arnold Frederic Wilkins joined Watson-Watt’s staff in Slough. As the latest addition to the team, Wilkins was given a variety of menial tasks to complete. One of these was to select a new short-wave receiver for ionospheric studies, a task he undertook with great diligence. After reading everything available on several units, he selected a model from the General Post Office that worked at what was for that time very high frequencies. As part of their tests of this system, in June 1932 the General Post Office published a report, No. 232 Interference by Aeroplanes, which recounted the General Post Office testing team’s observation that aircraft flying near the receiver caused the signal to change in intensity, an annoying effect known as fading.

All was now ready for the development of radar in the UK. Using Wilkins' knowledge that short-wave signals bounced off aircraft, a BBC transmitter to light up the sky as in Appleton’s experiment, and Watson-Watt’s radio direction finding 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 its location in space. Watson-Watt’s HF/DF technique solved the problem of making rapid measurements, but the issue of co-ordinating the measurement at two stations remained, as did any inaccuracies in measurement or differences in calibration between the two stations.

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 USA, Merle Tuve and Gregory Briet, decided to recreate Appleton’s experiment using timed pulsed signals instead of the changing wavelengths. 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 Experimental Establishment in 1931. The War Office proved uninterested in the concept and the development remained little known outside the Signals Experimental Establishment.

At the same time, when the feeling that 'the bomber will always get through' had led to a general feeling of dismay and defencelessness, the need for such a system was becoming increasingly pressing. 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, and it was in November of that year that Stanley Baldwin, who had already served two of his three terms as British prime minister, gave his famous speech stating that 'the bomber will always get through'.

Early in the summer of 1934, the Royal Air Force 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 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 this singularly one-sided result, 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% of the bombers reached their targets unhindered. The numbers suggested any targets in the city would be completely destroyed. 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.' In November, Churchill gave a speech on 'The threat of Nazi Germany' in which he pointed out that the Royal Navy could not protect the UK from an enemy who attacked by air.

Through the early 1930s, a debate raged within British military and political circles about strategic air power. 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'. 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.' As it became clear the Germans were rapidly enlarging and rearming their new Luftwaffe, the fear grew that 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.

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 during 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.'

In 1923/24, the English 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. His attempts spurred many other inventors to contact the British military with claims of having perfected some form of the fabled electric or radio 'death ray'. Some turned out to be frauds and none emerged as feasible.

At about 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 claiming the at use of the signals to interfere with an engine’s ignition system and thereby to stall. One oft-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 after the test had been completed. This was followed shortly thereafter by a story in a German newspaper with an image of a large radio antenna which had been installed in the same area.

Although highly sceptical about claims of engine-stopping rays and death rays, the Air Ministry could not ignore them as they were theoretically possible. If such systems could be built, it might render the bomber useless, and were this to happen, the night bomber might evaporate overnight as a deterrent, 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.

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 (90 m): the prize went unclaimed.

The need to research better forms of air defence prompted Harry Wimperis, the director of scientific research at the Air Ministry, to press for the formation of a study group to consider new concepts. Lord Londonderry, then 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 known to history as the Tizard Committee.

When Wimperis sought an expert in radio to help judge the death ray concept, he was directed to Watson-Watt. He wrote to Watt 'on the practicability of proposals of the type colloquially called ''death ray'". The two men met on 18 January 1935, and Watson-Watt agreed to look into the matter. Watson-Watt turned to Wilkins for help, but wished 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 8 Imp pints (4.5 litres) of water at a distance of 3 mile (4.8 km) from 98° to 105° F (37° to 41° C). To Watt’s bemusement, Wilkins immediately surmised this was a question about a death ray. He made a number of back-of-the-envelope calculations demonstrating the amount of energy needed would be impossible given the state of the art in electronics.

According to Dr R. V. Jones, when Wilkins reported the negative results, Watson-Watt asked 'Well then, if the death ray is not possible, how can we help them?' Wilkins recalled the earlier report from the General Post Office and noted that the wingspan of a contemporary bomber aircraft, about 82 ft (25 m), would be just right to form a half-wavelength dipole antenna for signals in the range of 50-m wavelength, or about 6 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.

Watt wrote back to the committee saying the death ray was extremely unlikely, but added that '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.'

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 there nonetheless remained the question of whether or not it was technically possible. Albert Rowe, an Air Ministry scientific researcher, and Wimperis each checked the mathematics, which appeared to be correct. They immediately wrote back asking for a more detailed consideration. Watson-Watt and Wilkins followed on a 14 February with a secret memorandum titled Detection and Location of Aircraft by Radio Means. In the new memorandum, 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 an 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.

Wilkins gave specific calculations for the expected reflectivity of an aeroplane. The received signal would be only 10 to 19 times as strong as the transmitted signal, but such sensitivity was considered to be within the state of the art. To reach this goal, a further improvement in receiver sensitivity of two times was assumed. Their ionospheric systems broadcast only about 1 kW, but commercial short-wave systems were available with 15-amp transmitters (about 10 kW) that they calculated would produce a signal detectable at about 10 miles (16 km). They went on to suggest that the output power could be increased as much as 10 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 on an oscilloscope the time delay between transmission and reception. The rest of the required performance would be provided by increasing the gain of the antennae by making them very tall, focusing the signal vertically. The memorandum concluded with an outline for a complete station using these techniques. The design was almost identical to the 'Chain Home' stations which eventually entered service.

The letter was seized on by the Tizard Committee, which immediately released £4,000 to begin development and requested Air Vice Marshal H. T. C. 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 additional funding was released. Wilkins suggested using the new 10-kW, 49.8-m BBC Borough Hill short-wave station in Daventry as a suitable ad hoc transmitter. The receiver and an oscilloscope were placed in a delivery van the Radio Research Section used for measuring radio reception around the countryside. On 26 February 1935, the van was parked a field near Upper Stowe and connected to wire antennae stretched across the field on top of wooden poles. A Handley Page Heyford twin-engined biplane bomber made four passes over the area, producing clearly notable effects on the cathode ray tube display on three of the passes.

Observing the test were Watson-Watt, Wilkins and several other members of the Radio Research Section team, together with along with Rowe representing the Tizard Committee. Rowe and Dowding were highly impressed. It was at this point that Watson-Watt’s previous agitation over development became important. The National Physical Laboratory’s management remained uninterested in practical development of the concept, and was happy to allow the Air Ministry to take over the team. Only a few days later, the Treasury released £12,300 for further development, and the members of a small team of Radio Research Section researchers were sworn to secrecy and began work on the development of the concept. A system was to be built at the Radio Research Section’s station, and then moved to Orfordness for over-water testing. Wilkins was to develop the receiver based on the General Post Office units, together with suitable antenna systems.

This left the problem of developing a suitable pulsed transmitter, and an engineer familiar with these concepts was therefore required. 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 Radio Research Section’s radio direction finding systems at Appleton’s request and was known to the Radio Research Section’s staff. Watson-Watt and Jock Herd stated that the job was his if he could sing the Welsh national anthem. He agreed, but only if they would sing the Scottish one in return. They declined, and gave him the job.

Starting with the BBC transmitter electronics, but using a new transmitter valve from the Royal Navy, Bowen produced a system that transmitted a 25-kW signal at 6 MHz (50-m wavelength), transmitting pulses of 25-μs length 25 times per second. Meanwhile, Wilkins and L. H. Bainbridge-Bell built a receiver based on electronics from Ferranti and one of the Radio Research Section’s cathode ray tubes. To preserve secrecy, it was decided not to assemble the system at the Radio Research Section and 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 75-ft (23-m) poles, and the receiver a similar arrangement of two crossed wires.

The system showed little success against aircraft, although echoes from the ionosphere as far as 1,000 miles (1610 km) distant were noted. 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 Radio Research Station at Slough, and expressing their gratitude that the Air Ministry had granted them access to unused land at Orfordness to continue their efforts. Bowen continued increasing the voltage in the transmitter, starting with the 5,000-volt maximum suggested by the Royal Navy, but increasing in steps over several months to 12,000 volts, which produced pulses of 200 kW. Arcing between the valves required the transmitter to be rebuilt with more room between them, while arcing on the antenna was solved by hanging copper balls from the dipole to reduce corona discharge.

By June the system was working well, although Bainbridge-Bell proved to be so sceptical of success that Watt eventually returned him to the Radio Research Section and replaced him with Nick Carter. The Tizard Committee visited the site on 15 June to examine the team’s progress. Watt secretly arranged for a Vickers Valentia twin-engined biplane bomber/transport to fly close to the site, and years later insisted that he saw the echoes on the display, although no one person recalled seeing these.

Watt decided not to return to the Radio Research Section with the rest of the Tizard group and stayed with the team for another day. No changes made to the equipment, but on 17 June the system was activated and immediately provided returns from an object at a distance of 17 miles (27 km). After tracking it for some time, they watched it fly off to the south and disappear. Watt telephoned the nearby Seaplane Experimental Station at Felixstowe and the superintendent stated that a Supermarine Scapa twin-engined flying boat had just landed. Watt requested that the 'boat return to make more passes, and this event is considered the official birth date of radar in the UK.

Aircraft from RAF Martlesham Heath took over the job of providing targets for the system, and the range was continually extended. During a test on 24 July, the receiver detected a target at 40 miles (64 km) and the signal was strong enough that it could be determined that the target was actually three aircraft in close formation. By September the range was consistently 40 miles (64 km), increasing to 80 miles (130 km) by the end of the year, and with the power improvements Bowen worked into the transmitter, was more than 100 miles (160 km) by a time early in 1936.

In August 1935, A. P. Rowe, the secretary of the Tizard Committee, coined the term 'Radio Direction and Finding', deliberately choosing a name that could be confused with 'Radio Direction Finding', which was a term already in widespread use.

In a memorandum of 9 September 1935, Watson-Watt outlined progress to date. At that time the range was about 40 miles (64 km), so Watson-Watt suggested the construction of a complete network of stations spaced at 20-mile (32-km) intervals along the entire eastern coast. As 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. This was quickly rendered moot by the rapid increases in range. When the Tizard Committee next visited the site in October, the range was up to 80 miles (130 km), and Wilkins was working on a method for height finding using multiple antennae.

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 a range of only 5 miles (8 km) 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 for five Radio Direction and Finding stations along the south-eastern coast was sent out, to be operational by August 1936.

The only person who remained unconvinced of the utility of Radio Direction and Finding was Lindemann, who had been placed on the committee at the personal insistence of his long-time friend, Churchill, and proved completely unimpressed with the team’s work. When he visited the site, he was upset by the crude conditions and, apparently, the box lunch he had to eat. Lindemann strongly advocated the use of infra-red systems for detection and tracking, and many observers have noted Lindemann’s continual interference with radar.

Churchill’s backing meant the other members' complaints about his behaviour were ignored. The matter was eventually referred to Lord Swinton, the new Secretary of State for Air. Swinton solved the problem by dissolving the original committee and reforming it with Appleton in place of Lindemann.

As the development effort grew, Watson-Watt requested the creation of a central research station 'of large size and with ground space for a considerable number of mast and aerial systems'. 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 10 miles (16 km) to the south of Orfordness sometime earlier while on a Sunday drive. He recalled it because it was some 70 to 80 ft (21 to 24 m) above sea level, which was very odd in that area. What was really useful was the large manor house on the property, which 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. When the scientific team left in 1939, the site became the operational 'Chain Home' site RAF Bawdsey.

While the Orfordness 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 newly arrived Gerald Touch started up the Orfordness system and were able to run the demonstrations from there. The Orfordness site was not completely closed until 1937.

The system was deliberately developed on the basis of existing commercially available technology to speed introduction, for the development team could not afford the time to develop and debug new technology. Watson-Watt, a pragmatic engineer, believed that 'third-best' would do if 'second-best' would not be available in time and 'best' never available at all. This thinking led to the use of the 50-m wavelength (around 6 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 high-frequency spectrum. The team responded by reducing its own wavelength to 26 m (around 11 MHz) to get clear spectrum. To everyone’s delight, and contrary to Wilkins' 1935 calculations, the shorter wavelength produced no loss of performance. This led to a further reduction to 13 m, and finally the ability to tune between 10 and 13 m (approximately 30 to 20 MHz) to provide some frequency agility to help avoid jamming.

Wilkins’s 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 Radio Research Section, and comprised several parallel dipoles separated vertically on the receiver masts. Normally the RDF 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 antennae 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 antennae would cover only a thin vertical angle. A series of such antennae was used, each pair with a different centre angle, providing continuous coverage from about 2.5° over the horizon to as much as 40° above it. With this addition, the final remaining piece of Watt’s original memorandum was accomplished and the system was ready for production.

Industrial partners were sought and arranged early in 1937, and a production team was organised on the basis of many companies. Metropolitan-Vickers assumed responsibility for the the design and production of the transmitters, AC Cossor for the receivers, the Radio Transmission Equipment Company for the goniometers, and a joint AMES-General Post Office group for the antennae. The Treasury gave approval for full-scale deployment in August, and the first production contracts were issued in November for 20 sets at a total cost of £380,000. Installation of 15 of these was carried out in 1937 and 1938, and in June 1938 a London headquarters was created to organise the rapidly growing force. This became the Directorate of Communications Development, with Watson-Watt named as the director. Wilkins followed him to the directorate, and Rowe took over the Air Ministry Experimental Station at Bawdsey. The first five stations were declared operational in August 1938 and entered service during the Munich crisis, starting full-time operation in September.

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. Assuming that radar would provide 15 minutes' warning, interception techniques were developed to put fighters in front of the bombers with increasing efficiency. The main problems encountered were finding the location of the British fighters , and ensuring that the fighters were at the right altitude to effect an interception.

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 of technology but of communication, inasmuch as the pilots were being sent too many, and often contradictory, reports. This realisation led to the development of the 'Dowding system', a vast network of telephone lines reporting to a central filter room in London where the reports from the radar stations were collected and collated, ultimately but swiftly fed to the pilots in a clear format. The system as a whole was enormously intensive in its need for human resources.

By the outbreak of World War II in September 1939, there were 21 operational 'Chain Home' stations. After the 'Battle of France' in 1940 the network was expanded to cover the UK’s western coast and Northern Ireland. The expansion of Chain Home continued throughout the war, and by 1940 it stretched from the Orkney islands group in the north to Weymouth in the south. This provided radar coverage for the entire Europe-facing side of the British Isles, able to detect high-flying targets while they were still deep over France. Calibration of the system was carried out initially using a flight of mostly civilian-flown, impressed Avro Rota single-engined autogyros 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 cathode ray tube. 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 'Chain Home' network required more technical and operational personnel than the UK could provide, and in 1940 the British made a formal request to the Canadian government for men skilled in radio technology for deployment in the defence of the UK. By the end of 1941, 1,292 trained personnel had enlisted and most were rushed to England to serve as radar mechanics.

During the 'Battle of Britain', 'Chain Home' stations (most notably that at Ventnor on the 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 that at Dover, 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 as a result of 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 ignored them for the rest of the war. Had the Luftwaffe realised just how essential the radar stations were to the British air-defence system, it is likely that they would have expended an altogether greater effort to destroy them.

'Chain Home' was the primary radar system for the defence 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 radar systems. Whereas 'Chain Home' scanned an area perhaps 100° wide and required considerable effort to take measurements, the Type 7 scanned the entire 360° 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 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 ground-controlled interception, 'Chain Home' became the early warning portion of the radar network. To simplify operations further and to reduce manpower requirements, the job of plotting the targets became semi-automated. An analogue computer of some complexity, known simply 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.

The original transmitters were constantly upgraded, first from 100 kW in the Orfordness system to 350 kW in the deployed system, and then again to 750 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.

The German deployment of the V-2 ballistic rocket in September 1944 was initially met with no potential response. The weapons 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 the weapons' approach reached the target. The government initially tried to pass them off as explosions in the underground gas mains. However, 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 'Chain Home' stations were revised into the 'Big Ben' system to report the V-2 weapons as they were launched. 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, Great Bromley, High St, Dunkirk and Swingate near 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 a V-2 climbed after launch, it passed 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.

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 target areas.

Success in this task was aided by the V-2’s fuselage profile, which acted as an excellent quarter-wave reflector for 12-m band high-frequency radar. RAF Fighter Command was also informed of the launch in an effort to attack the sites. However, the German launch convoys were camouflaged and being motorised were highly mobile, making them extremely difficult to find and attack. The only known claim was made when Supermarine Spitfire pilots of No. 602 Squadron came across a V-2 rising from a wooded area, allowing a quick shot of unknown result.

The British radar defences were rapidly scaled down during the last years of the war, with many sites closed and others placed on a care-and-maintenance basis. However, immediate post-war tensions with the USSR resulted in the recommissioning of some wartime radars as an interim 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 the 'Chain Home' system’s replacement, ROTOR, which progressed through three phases from 1949 to 1958.

As noted above, the inherent timing of the interception task, about 23 minutes, was the result of the time of a single interception from the target’s initial detection. If the target was a high-speed jet bomber, this required an initial detection range of about 240 miles (390 km). Even in its upgraded form, 'Chain Home' was barely capable of this under the best conditions. The ground-controlled interception 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.