The AMES Type 7, also known as the Final GCI, was a British ground-based radar system introduced during World War II by the Royal Air Force (RAF). The Type 7 was the first truly modern radar used by the Allies, providing a 360 degree view of the airspace around the station out to a distance of about . It allowed fighter interceptions to be plotted directly from the radar display, a concept known as ground controlled intercept, or GCI.
Earlier radars, like Chain Home (CH), provided the range and angle to a single target at a time. Arranging an intercept required a complex series of reports from multiple radars that were plotted at a central station, the filter room. In a seminal 1939 memo, Robert Hanbury Brown showed that this Dowding system resulted in an inherent inaccuracy of approximately , and the only way to reduce this would be to arrange the interception directly from the radar screen. He suggested spinning the radar antenna and the CRT display to produce a 360 degree picture of the airspace around the station, a concept he referred to as a plan position indicator, or PPI.
To test the concept, the AMES Type 8 was developed from the existing GL Mk. II radar, with a new antenna that was manually rotated to scan the area around the station. When the Type 8 was first used in December 1940 it proved extremely effective. Lessons learned on the Type 8 led to the production Type 7 design, which began deployment at the end of 1941. Starting in 1942, the installations began to be upgraded to permanent buildings known as "happidromes". From that point, the RAF's interest in the system waxed and waned along with the German bombing efforts and full deployment was repeatedly delayed. The stations began to take over most interception duties in 1943, but it was not until 1944 that the full network of 33 stations was completed. An American copy, the SCR-527, was not produced in quantity.
Late in the war, many UK radars were no longer staffed as the risk of German attack dwindled, and most were put into "care and maintenance" when the war ended. The detonation of the first Soviet atomic bomb in 1949 led to a re-evaluation of post-war alert status. As part of the ROTOR project, many Type 7's were re-activated, upgraded, and re-built in bomb-proof buildings. Most Type 7's were later displaced by the much more capable AMES Type 80, but a small number were kept to fill gaps in the Type 80's coverage, while others were kept as backup systems. Type 7's remained in backup service until the Linesman/Mediator program of the 1960s.
History Chain Home
In 1935, the newly formed Tizard Committee contacted well-known radio expert Robert Watson-Watt to pass comment on claims of a German death ray device based on radio waves. For help, Watt turned to his assistant Arnold Wilkins, who performed a number of back of the envelope calculations that demonstrated the idea was infeasible. When Watt asked him what use radio might be in an air war, Wilkins recalled reading a technical manual a few years earlier that mentioned the effect of aircraft on shortwave signals. The two prepared a memo stating that the death ray was highly improbable, but they should consider developing an aircraft detection system. Hugh Dowding, at that time the Air Member for Supply and Organisation in charge of research and development, was intensely interested and demanded a practical demonstration. Watt and Wilkins quickly arranged one in the Daventry Experiment. When he saw the signals, Watt claims to have exclaimed "Britain has become an island again!"
Watt strongly believed in deploying a radar system as rapidly as possible, suggesting they should "give them the third best to go on with; the second best comes too late, the best never comes". His solution was to deploy a slightly modified version of thunderstorm locators he had developed in the 1920s, which determined the direction to a storm by measuring the radio signal given off by lightning using an Adcock antenna and radio direction finder (RDF). To produce a radar, the signal given off by lightning was replaced by a powerful radio transmitter that lit up the sky in front of it. Reflections from aircraft were received on separate antennas using a similar RDF technique as the storm detectors. Although crude, it could be built using minor modifications of existing commercial shortwave broadcast electronics. These so-called Chain Home systems were being set up as early as 1936, and by 1939 a complete network was ready for battle.
In order to provide the maximum warning time, the CH radars were located as far forward as possible, along the coastline. This allowed stations in Kent to detect German aircraft while they were still forming up over France. However, this also meant the system could only locate targets over the water. They provided little or no coverage over the British Isles themselves, where tracking fell to the Royal Observer Corps (ROC) using binoculars and the Post Instrument. To collect all of this information and forward it to the pilots, Dowding set up what became known as the Dowding system. Reports from the ROC and CH systems were forwarded via telephone to RAF Fighter Command's headquarters and then sent out to the operational stations and forwarded to the fighters in flight via radio.
During early testing, RAF pilots noted that they could avoid detection by CH by flying at low altitudes. The Germans also discovered this early in the war when they noticed that aircraft on mine laying missions, which flew at low altitudes for much of their time, were not being intercepted. A solution was already at hand; the British Army had been working on the Coast Defense radar (CD Mk. I) to detect shipping in the Channel, and had noticed it also worked against low-altitude aircraft. Watt ordered 24 sets in 1939 under the name Chain Home Low (CHL). CHL used a manually rotated antenna to scan the sky, and the bearing to the target was indicated by the direction the antenna was pointed. This eliminated the multiple antennas and RDF used in CH, greatly simplifying installation and operation.
AI, PPI and GCI
Each CH station had slightly different calibration, the ROC's Post Instrument often produced different results, and all of the reports took time to flow back to the central plotting rooms and then out to the fighters. These effects meant that the information reaching the fighter controllers was always slightly inaccurate and a few minutes out of date. The system, as a whole, had an inherent accuracy of about . This was fine for daytime interceptions, but at night, when the pilot's eyesight was limited to perhaps , there was no way to guide the fighters to their targets close enough that they would be seen.
This possibility had been raised as early as 1936 in a memo by Henry Tizard, who felt that the Germans would suffer so stiffly at the hands of the RAF that they would turn to night bombing, as they had in World War I. While discussing the matter, Watt came to the conclusion that the most suitable solution was a radar with about 5 mile range that could be fit to the aircraft, allowing the CH system to get the fighter close enough that its own radar could take over. This eventually led to the AI Mk. IV radar, which entered service in prototype form in the summer of 1940.
In November 1939, Robert Hanbury Brown wrote a paper on the mathematics of the interception task. It demonstrated that the reporting delays in CH and the Dowding system would make the 5 mile range hard to achieve in practice. Even with the AI radars being developed, night interceptions would be very difficult to arrange, because relaying the messages through the system was simply too slow. He concluded that the only way to address this would be to have the radar operators control the fighters directly, eliminating the middle-men.
He went on to describe a radar system that rotated the antenna and the cathode ray tube (CRT) display at the same rate. When the time base generator triggered at the start of a broadcast, it would draw a line along the tube in the direction the radar was currently pointing. In CH, the amplified return signal deflected the CRT beam to cause a blip to appear; in this new system, the signal would instead cause the beam to briefly brighten, producing a small dot on the display. Using a long-lasting phosphor in the CRT, only these brighter spots would remain on the display when the radar rotated away from that angle. Hanbury Brown called this system the plan position indicator (PPI).
The result would be a number of bright dots on the display, directly indicating the angle and range of an aircraft. The display as a whole represented a map of the airspace around the radar station, showing every aircraft within that station's view. An operator looking at the display could calculate an interception by examining the locations of the dots on the display, instead of having to calculate the aircraft locations on a separate map and arrange it there. Better yet, any inaccuracy in the radar would apply to the fighter and its target equally, whereas the Dowding system tracked these using separate systems and were subject to large differences in measurements.
At the time there were more pressing issues getting CH and CHL into operation. These problems had improved by 1940, and in May a team at the newly renamed Telecommunications Research Establishment (TRE) had successfully motorized a CHL antenna to spin it, and had combined the separate transmit and receive antennas onto a single mount. The next month they successfully tested the first PPI display, which rotated the CRT display coils using a selsyn connected to the antenna motor.
Interim measures As Tizard had predicted, when the Battle of Britain failed to achieve victory over the RAF, the Luftwaffe turned their efforts to the night bombing effort that became The Blitz. The RAF found that the problems predicted in Brown's paper were proving true. In early night operations, most of the night fighters ended up flying around searching for targets they never found. The PPI would solve this problem, but it was clear it would not be ready for some time. The TRE began a rushed effort to develop some sort of stop-gap solution.
One solution was experimented with at RAF Foreness Point. Foreness was in the right position to see German bombers on their way to dropping mines in the Thames River. On these missions, the bombers would fly as low as possible to avoid detection, knowing there were no large objects in their way. Foreness was one of the first stations to receive a CHL in 1939, to detect these low-flying aircraft.
In early 1940, they began testing a solution to directly guide fighters. The interception started with the CHL station detecting a target and beginning to track it. Once its rough location was determined, an air traffic controller would direct an already airborne night fighter to fly along a line that would intersect the CHL beam at a shallow angle. When the fighter moved into the beam it would appear on the CHL display along with the bomber. The CHL operator would then take over guidance, giving the pilot instructions to turn left or right to stay centred in the beam as they tracked the target. As the fighter approached the target, it would eventually pick it up on its own radar and complete the interception. While this system worked, it proved extremely time consuming to use, and required careful planning to ensure the fighter arrived in the beam even as the radar operators were moving it to track the target.
Type 8