Linesman/Mediator was a dual-purpose civil and military radar network in the United Kingdom between the 1960s and 1984. The military side (Linesman) was replaced by the Improved United Kingdom Air Defence Ground Environment (IUKADGE), while the civilian side (Mediator) became the modern public-private National Air Traffic Services (NATS).
In the 1950s, the Royal Air Force was installing a radar network known as ROTOR using war-era radars like Chain Home along with new command centres. A new radar, the AMES Type 80, replaced all of the ROTOR radars and command centres with a series of nine Master Control Radars and a number of associated secondary radars. While these installations were in progress in the early 1950s, CSF introduced the carcinotron, which could output a radio signal at any desired frequency. This made it a very effective jamming system, and it appeared to render ground-based radars like Type 80 useless.
Considering several possible solutions, the RAF selected an experimental radar known as Blue Yeoman, later known as the AMES Type 85. This radar changed its frequency with every pulse, making it impossible for carcinotron operators to know what frequency to jam. The RAF initially proposed an extensive network similar to the Type 80s, known as the "1958 Plan". This was abandoned in the aftermath of the 1957 Defence White Paper, which saw the threat to be moving from bombers to ballistic missiles, and argued the system should be cancelled. In late 1958, a much smaller system with only three main radars and a single control centre became "Plan Ahead", with its primary purpose being to provide air cover and anti-jamming support for a new anti-missile BMEWS radar.
During this period, civilian air traffic was increasing dramatically and led to the 1962 formation of the National Air Traffic Control Services organization to handle national-scale air traffic control (ATC). Given enemy aircraft might hide among civilian ones, it was seen that combining data from Plan Ahead and the ATC system would have many advantages. Plan Ahead became Linesman and the ATC system Mediator. The systems would share locations in West Drayton (just north of Heathrow) and at Glasgow Prestwick Airport, with a third Mediator site in Preston, Lancashire. Ultimately all three Mediator sites were built, while only "L1" at West Drayton was completed as part of Linesman.
Whilst Mediator proceeded relatively smoothly, construction of Linesman was greatly delayed and it was not fully operational until March 1974. By that time the strategic threat had changed dramatically, and air strikes on the UK once again became a possibility. Linesman's single centralized L1 command centre was vulnerable, and the seaside radars even more so. Money set aside to improve Linesman was redirected to building its replacement, UKADGE. That system was further expanded with Marconi Martello and the Type 85s were stood down in the 1990s.
History 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.