ROTOR As the threat of German air attack on the United Kingdom faded in 1944, the wartime network of
Chain Home (CH) and
AMES Type 7 sites was progressively turned off, a process that accelerated with the end of the war in 1945. At the time, it was believed it would be at least another ten years before there would be another major war, so the
Royal Air Force turned its attention to
research and development of
radar as they felt there was little reason to deploy any systems when better ones would be available by the time they would be needed. Various events in 1949, notably the
Berlin Airlift and the testing of the
first Soviet nuclear bomb, led to studies into rapidly re-implementing the air defence system. Most influential was the Cherry Report, which outlined a set of equipment upgrades at existing WWII-era radar sites to improve their performance, along with an entirely new communications network to coordinate the response. Adopted as
ROTOR, the new system split the UK's airspace into six "sectors" controlled from underground bunkers, using thousands of miles of
telex lines to pass information around. Deployment of ROTOR would have several phases. The first re-used existing radars and control methods and was to be operational by 1952. The second would replace the Chain Home radars with a new
Microwave Early Warning (MEW) set beginning around 1957. More stations would be added and the communications systems updated over time, with the final network being fully implemented by the late 1950s.
Master Radar Stations The entire ROTOR plan was upset by an experimental radar known as "Green Garlic" that was developed at the
Telecommunications Research Establishment. This combined an experimental low-noise receiver with a new high-power
cavity magnetron and a lash-up antenna. The system more than doubled the effective range for early warning, and when married to a more suitable antenna, provided good detection of bomber-sized aircraft out to . This filled most of the requirements for MEW, but would be available years earlier. ROTOR plans were initially adjusted to introduce these radars in 1953; they were known in production form as the
AMES Type 80. But even as these began to be installed, further upgrades to the design pushed the detection range out even further and greatly increased accuracy. This allowed a single radar to detect the enemy out to the
radar horizon, with enough accuracy to direct fighters to the targets and get them within range of the fighters'
aircraft interception radars. With the introduction of these Type 80 Mark III's, the ROTOR plan was obsolete. There was no longer any need to send information to a central plotting room, as the radar operators could do everything directly from their displays. Many of the underground bunkers, recently completed at great expense, were sold off as these Master Radar Stations came online in the late 1950s.
Carcinotron In 1953 the French electronics firm
CSF introduced a new type of
microwave-generating
vacuum tube, later known as the
carcinotron. The carcinotron could generate about a kilowatt of output signal, compared to megawatts for the latest examples of the magnetron and newer examples of the
klystron. However, it had the ability to be rapidly tuned across a wide
bandwidth simply by changing a single input voltage, something the magnetron could not do and the klystron only do within a limited bandwidth. Previously,
jamming a radar was a time-consuming process that required an operator to listen for enemy radars on a receiver, isolate signals from potentially threatening radars, and then set up a transmitter on that frequency. This was effective against radars using magnetrons, which cannot change their operational frequency. Against other types of transmitters, the operator on the ground would notice the jamming and change their frequency, starting the jamming process over again. If several radars could see the aircraft, keeping the jammers properly tuned could be an impossible task. In contrast, the carcinotron could tune so rapidly that one could simply sweep its output through the bandwidth of any potential enemy broadcasts. As long as this was done quickly enough that every radar would see the jamming signal during the flight time of its pulses, in the order of a few milliseconds, then the signal from the carcinotron would mix with the radar's own and cause false signals to appear on the display. Although the carcinotron's signal was small, it was stronger than that of the high-power radar sources after travelling to the target. The resulting signal was strong enough that it would overwhelm the radar's own signal, filling the radar display with noise and rendering the aircraft invisible. To test whether such a system would be effective, the RAF purchased a carcinotron from CSF and fitted it to an aircraft christened "Catherine". In tests beginning in late 1954, the jammer proved to be able to render the area around the aircraft unreadable, even when the aircraft was below the radar horizon. In one test, any aircraft to either side of the jammer was invisible, meaning a single powerful jammer could hide an entire formation of aircraft. As the jammer aircraft approached the radar station, the signal would be picked up in the radar antenna's
side lobes, until the entire display was filled with noise and nothing could be tracked anywhere. It appeared that the decade-long effort to provide radar coverage for the UK was rendered useless.
Plan Ahead As the nature of the threat of the carcinotron became clear, the Air Ministry began looking for potential solutions. Two concepts emerged. One was to simply overpower the carcinotron; although it could produce about a kilowatt in total, it would have to spread that signal across all the frequencies being used by the various radars. This meant the amount of energy at any one frequency was limited, and estimated to be about 10 Watt per megahertz of bandwidth. If the receiver accurately filtered the return signal tightly around the broadcast frequency, it might only receive a few watts of jamming. With enough transmitted power, in the order of 10 MW, the signal reflected off a target at 200 miles would be about 11 W, allowing the radar's signal to remain visible. This solution only worked if the carcinotron was forced to spread its signal over a wide frequency range, and would not work if it could concentrate its signal over a smaller number of frequencies. To ensure this, the radar's signals had to be randomly spread across a wide range. This could not be accomplished with the magnetron, whose output frequency is a function of its physical construction, but was possible using new high-power klystrons. This possibility was developed under the name "Blue Riband", using twelve klystrons that were randomly mixed together to produce an output signal on two widely separated frequencies. Another possibility was to use the carcinotron's own signal as the tracking source. If a single jammer is sending out a signal, its location can be determined through simple
triangulation from two stations by drawing the measured angles on a shared map. But if there is more than one jammer, each station will measure several angles and it is not easy to tell which of the measured crossing points contains the jammer aircraft. The introduction of the
correlator introduced a way to eliminate this ambiguity. This concept was explored in a system known as "
Winkle" that used two widely separated antennas, several correlators, and a computer to calculate the location of the jammer. It was later noted that one of the two antennas could be the Blue Riband, thereby reducing the number of sites that had to be built. Winkle proved able to track the jammer even while it was below the
radar horizon, allowing the aircraft to be driven off before it could approach close enough to be effective against Blue Riband. By 1957, a new network combining these systems was outlined under the name "Ahead". The system would cover the entire British Isles, like ROTOR and the Type 80s before it. Because the new systems had even longer range than the Type 80, the number of stations would be smaller, with the baseline deployment having only five. A number of Type 80s would be retained in the new network purely for early warning, in locations where interceptions would not be taking place and the anti-jamming performance was not needed.
Changing threats Prior to this period, the air defence mission in the UK was based on a damage-limitation model which aimed to reduce the amount of damage on the UK while inflicting losses on the enemy that would make follow-up attacks impossible. This sort of concept is best illustrated by the
Battle of Britain, where the RAF was able to cause enough damage on the German bomber force that daylight raids had to be abandoned. Even in the post-war era with early nuclear bombs, this basic concept was still the prevailing strategic concept. The introduction of the
hydrogen bomb rendered this concept useless. In this case, a single aircraft escaping interception could cause enormous damage. Estimating the number of bombers that would reach their targets, it appeared any such attack would result in the UK being devastated. In this environment, the only way to protect the UK from destruction was to ensure the attack never occurred, and the only way to do that was to offer a significant deterrent. By 1954 there were serious concerns being expressed about the usefulness of air defences. If the goal was to deter an attack, all that was required was enough warning to ensure the
V bomber force would successfully launch. Directing fighters to their targets and shooting them down with
surface to air missiles seemed like a useless gesture if an attack was underway, given that there was no situation where these weapons would seriously change the outcome of the war. By 1956, all pretext of general defence was dropped and the RAF adopted the policy that the only worthwhile mission was early warning and short-term defence of the V bomber bases while they launched, the "protection of the deterrent" mission. This led to a smaller network of only three stations covering the Midlands area, under the new name "Plan Ahead". Even this mission was ultimately eliminated with the release of the
1957 Defence White Paper. This paper explored the changes to the strategic outlook with the introduction of the
ballistic missile. It noted that missiles able to reach the UK from eastern Europe were already available, and by the mid-1960s these would be armed with strategic weapons with enough accuracy to attack the bomber bases. Although a Soviet bomber attack was still possible, especially low-altitude sneak attacks, such attacks would simply portend the arrival of the missiles – there was no credible scenario where the Soviets used an all-bomber attack, if they released their strategic weapons they would use all of them. If this was the case, there was really no purpose to the air defence beyond the most minimal systems needed for identification. By 1957 even the concept of defending the V bombers was abandoned; if there was credible warning of any sort of attack the bombers had to launch immediately because in all scenarios missiles would be landing shortly. The key requirement was not air defence, but rapid warning of a missile attack. In October 1957 the UK approached the US about deploying a station of the
Ballistic Missile Early Warning System (BMEWS) in the UK in a location that would cover attacks on the V bomber bases.
New mission With the changes brought about by the White Paper, there appeared to be no need for Plan Ahead. However, air planners soon pointed out a critical problem. If the Soviets equipped an aircraft with a powerful carcinotron, they could fly it well off the shore and jam the relatively limited frequency band of the BMEWS. This would obscure a missile attack, and the V bombers would be forced to launch until the nature of the threat was determined. If the Soviets repeated this trick, the bomber force and its crews would be quickly worn out. The solution to this problem was to build a much smaller defensive network with enough performance to attack a jammer aircraft while it was still far enough away that the BMEWS would still be operational. This led to a new Ahead layout in 1958 with three stations arranged in a triangle covering the V bomber bases and extending to cover the BMEWS. Even this limited deployment was heavily criticised within the government, not the least of which by Prime Minister
Harold Macmillan himself. He desired to move the UK's own force to missiles as well, at which point the network would be superfluous. Eventually, the plan was approved only if all other air defence radars were cancelled, and accordingly, work on the
Blue Joker system ended in 1960.
Air traffic control Through the 1950s, the RAF had become accustomed to treating the airspace above as their own to use as they saw fit. At that time, the RAF's fighters and bombers were jet powered and flew at altitudes and speeds that the civilian aircraft, mostly propeller-driven, could not touch. The introduction of the first
jetliners, and their rapid expansion in the late 1950s, had led to a number of close calls between civilian and military traffic. This would only get worse over time. Some sort of system would be needed to cover the high-altitude traffic across the entire UK, and this led to the
National Air Traffic Control Service (NATS) under
Laurence Sinclair, who were planning an extensive network of their own based on the new
Decca DASR-1 and
Marconi S264 radars. Macmillan remained sceptical of Plan Ahead and asked the Chief Scientific Advisor to the Ministry of Aviation,
Solly Zuckerman, to consider it. Zuckerman stated there was no way to significantly reduce the estimated cost of the system and still have a military use. However, he suggested one solution would be to use it as the basis for a shared military/civilian
air traffic control network and thus share the costs that would otherwise require two complete networks. A series of follow-up studies by NATS all agreed with Zuckerman, pointing out that a military network would need complete information on civilian flights anyway in order to filter out contacts that were scheduled jetliner traffic. There appeared to be no reason not to merge the networks, and the result would be a network once again covering the entire UK. Only minor changes were made to the military network. One was to move the location of the inland radar, originally
RAF Bramcote, to a new location further north on the eastern shoreline,
RAF Boulmer, which was intended to cover the BMEWS location now that it had also moved to its final position at
RAF Fylingdales. This meant the original triangular layout was now an extended line, but this had little effect on the overall performance. The other change was to move the southern-sector control centre from
RAF Bawburgh to be beside the new London Area Control Center in
West Drayton, which would significantly reduce the cost of the telecommunications equipment. This led to a firestorm of protest within the RAF, because while it was true this would help communications with the civilian network, it made communications with the military radars more difficult, potentially able to be jammed. The argument over this issue raged, but no changes were made in the immediate term.
Linesman/Mediator By 1961 any remaining argument was overruled, and on 21 February the Treasury released funding for the newly-christened Linesman/Mediator. ==Implementation==