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Linesman/Mediator was a dual-purpose civil and military radar network in the United Kingdom. It replaced the earlier ROTOR and Type 80 Master Radar Stations with small network of only five large radar sites and an associated control center near London. Linesman was the air defence element of the project, and Mediator the air traffic control element.

Linesman evolved out of work on combining radar information carried out by the Royal Radar Establishment (RRE) at Malvern in the late 50s with the participation of the Automatic Telephone and Electric Company (ATE) of Liverpool. The idea was to reduce the complexity of the distributed systems like ROTOR or Master Radar Stations (MRS) with a single site, "L1", able to direct all of the air command. RRE became Royal Signals and Radar Establishment (RSRE) and ATE was merged with Plessey in 1962.

Along with work on the signaling and communications issues, upgrades to the existing radars were also planned. The existing Type 80 radar used in the MRS network was powerful, but relatively easy to jam. Upgrades were aimed primarily at offering improved jamming resistance in the event of a noisy ECM enviornment. Three new radars were developed for the project, Marconi's Type 84 and the AEI Type 85 "Blue Yeoman" long-range search radars, and the Decca HF 200 height-finder that could be used with either.

The Type 84 was built with two 60 foot by 21 foot elliptical parabolic antennas placed back-to-back, one acting as the radar, and the other as an IFF system. In practice the original IFF system was never installed, although more modern systems with a much smaller antennas were normally installed on the "front" dish, either below the feed horn, or on top of the main antenna.

The Type 85 ended up using the same antenna, although only "one side" of it, and used the same variety of IFF antennas. The Type 85, however, offered an extremely advanced system for shifting frequencies on the fly. The feed horns were powered by twelve klystrons tuned to slightly different frequencies, which were switched from horn to horn such that any "slice" vertically through the sky saw a continually changing frequency. The klystrons themselves could be shifted to one of four slightly different frequencies separated by 60 MHz from their base frequency, and selected one of the four after every pulse was sent. Additionally the outputs of the klystrons were mixed so that each feed horn was sent the output of two klystrons, creating a single lobe with two 300 MHz separated frequencies. Jamming such a system would be extremely difficult; an aircraft would always be covered by two frequencies at any one time, and these would change continually. The only practical solution would be to jam all of the 48 possible frequencies, dividing the jammer's power among them. It was expected that the Type 85 would be useful at 200 miles even in a strong ECM enviornment.

A third system was later added to the network, the RX12874 "Winkle" passive jamming-detection system. Winkle consisted of a series of high-speed rotating antennas separated by many miles, combined with additional signals captured from a Type 85 radar. These were combined at one of the radar stations in a phase correlator that produced a series of possible locations and plotted them as a series of blips on a unique "phi-theta" display. The operators manually adjusted gains in order to reduce the number of blips, and then sent that information to a remote display where it could be combined with normal data from the Type 84/85. The idea was to locate any specialty-equipped jammer aircraft within a larger attack, allowing them to be prioritized for attack, thereby lowering the ECM load on other radars.

In 1963 a prototype of the L1 system was set up in Building 123 at West Drayton where the main manufacturers (Plessey -data processing and Marconi - displays) carried out enhanced development. Each of the Linesman sites was able to operate on it's own, but in general the network was intended to be controlled from the centralized "L1" site at Drayton. Each of the field stations sent data to L1 over phone lines, where it was recombined to form a country-wide view of the airspace. It was understood from the start that the L1 site was subject to air attack, and a second "L2" site was originally planned to be built underground in northern England. However this work was never carried out.

Civilian control, the "Mediator" portion, was also added while the project continued, forming up as the London Air Traffic Control Centre (LATCC) in a separate building nearby. LATCC used the Linesman data in their Marconi Myriad computers to handle all air traffic control in the London area.

A large scale review was carried out to rationalize what could be done with the developed hardware resulting in 'The Hub Concept' where the system resources were reorganized to provide an air data processing system. This produced an RAP, (Recognised Air Picture) whereby all aircraft and air movements could identified and tracked within the UK air space. Intercepts etc were undertaken at other sites using the information feeds from L1. It continued in operation until the late 70s, early 80s.