Linesman/Mediator


Linesman/Mediator

Linesman was the name given to the plan for the revamp of the air defence of the UK, this was originally conceived as part of the "Ahead" plan. As with any major plan it was subject to much review both prior to and post implementation; plans were scaled down and changes made which now are difficult to fix in chronological terms, and even more difficult to justify. Early on in the planning the need for an up-to-date air defence system and a civilian air traffic control system were combined.

Linesman/Mediator was the dual-purpose civil and military radar network in the United Kingdom. Against a background of the Russian H-bomb and supersonic bombers Linesman was the answer to the Cold War defence policies, replacing the earlier ROTOR and Type 80 Master Radar Stations that were the product of World War 2. Linesman with a small network of only five large radar sites and an associated control center near London was the air defence element of the project; Mediator was the civilian air traffic control element.

Linesman developed over the years until it was replaced by the Integrated United Kingdom Air Defence Ground Environment System (IUKADGES).

History and Plan

Linesman, whilst part of the "Ahead" plan, evolved out of the study into combining radar information carried out in the late 1950s by the Royal Radar Establishment (RRE) at Malvern with the participation of the Automatic Telephone and Electric Company (ATE) of Liverpool. The concept was to reduce the complexity of the existing distributed ROTOR system using multiple Master Radar Stations (MRS) with a single site designated "L1" (Linesman 1). L1 would be able to direct all of the air command using a complete air picture of the UK.

During the initial stages of the project RRE became Royal Signals and Radar Establishment (RSRE) and, in 1962, ATE was merged with Plessey.

Linesman was a comprehensive upgrade of the UK air defence system; along with work on the signaling and communications issues, upgrades to the existing radars were also planned. The existing Type 80 radars used in the MRS network were powerful but relatively easy to jam. Upgrades were aimed primarily at offering improved jamming resistance in the event of a noisy ECM enviornment. To augment the existing Type 80 a new Type 84 radar had already been proposed. Linesman proposed that three new radars were developed, two primary long-range search radars; Marconi's Type 84 and the AEI Type 85 "Blue Yeoman", with additional height fining capability being provided by the Decca HF 200 height-finder. To improve system resilience to jamming the two primary radars operated on different frequency bands, the Type 84 in "L" band, the Type 85 in "S" band. As the system developed more systems were added to improve the ECM capability of the system.

During various reviews of Linesman and the "Ahead" plan it was decided to incorporate civilian air traffic control control, the "Mediator" portion or the plan. Linesman and Mediator were now considered jointly, however this led to escalating costs and the subsequent need to scale down the original plan.

Whilst the L1 was originally intended to be underground at Bawburgh cost cutting between 1960-62 led to a prototype of the L1 system being set up above ground in Building 123 at West Drayton in 1963. The Mediator element set up LATCC (London Air Traffic Control Centre) in a building near by. LATCC used the Linesman data in their Marconi Myriad computers to handle all air traffic control in the London area. Here the main manufacturers (Plessey -data processing and Marconi - displays) carried out enhanced development. Each of the Linesman sites was able to operate on its own, but in general the network was intended to be controlled from the centralized "L1" site (now at West Drayton). Each of the field stations sent data to L1 over microwave links, eventually phone lines, where it was recombined to form a country-wide view of the airspace; a Recognized Air Picture (RAP).

Main Locations in UK

As well as the L1 the system comprised three main sites, these were, RAF Neatishead in Norfolk, RAF Staxton Wold in Yorkshire, RAF Boulmer in Northumberland. It was understood from the start that the L1 site, above ground and made largely of glass, was subject to air attack; a second "L2" site was originally planned to be built underground at Prestwick. However this work was never carried out.

Additional facilities extending the Linesman system were located at RAF Saxa Vord, RAF Benbecula, RAF Bishops Court, RAF Bentley Priory.

As with the ROTOR plan buildings were given "R" designations, this included underground bunkers as well as above ground technical and operations buildings. Details of buildings are included where necessary.

Main Equipment

The core system principle was to take the input from all sensors and enable '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.

The core to the system were the Type 84 & 85 primary radars. Height finding capability was provided by the Type 85 (one of the first 3D radars) and the HF200 Height Finders. As Linesman progressed additional systems were introduced to aid ECCM capability and communications. Each primary radar had an associated secondary radar (IFF), SSR 750, with the smaller secondary radar mounted on the main primary radar aerial.

Primary Radar T84

The Type 84 was a primary "L" band radar. The main transmitting equipment was contained in a building over which the aerial was mounted, this building was given the type designation "R15". In the R15 building contained the transmission equipment. The transmitter was a magnetron located in single story building and reaching the aerial through a rotating joint in the rotating cabin before being fed to the hornstacks on the aerial. The aerial 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/secondary radar 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 received signals from both the primary and secondary radars were passed for processing from the Type 84 Radar building to the main technical building for a Linesman site - the R12 building.

Primary Radar T85

The Type 85 radar operated in the "S" band part of the spectrum. By comparison with the Type 84 it was massive, a MOPA system with 12 klystron transmitters and 60 receivers. The equipment was housed in the main technical block for the Radar site, the R12 building, the aerial being mounted on top of the building. The Type 85 ended up using the same design of antenna, although only "one side" of it, the aerial for the associated secondary radar being mounted underneath, rather than above.

The Type 85, however, offered an extremely advanced ECCM system enabling the shifting frequencies on the fly, or as it is now known frequency agility. The radar had 12 transmitters which were grouped in four bands, A, B, D & E. Each transmitter had a peak power output of 5Mw, giving 60Mw total power output, this massive power was routed through waveguide switches what enabled the the aerial to produce a standard "cosec squared" beam or, in conditions of intense ECM the power concentrated into a beam 6 degrees high aimed at the target.

As well as the frequency agility and sheer power the Type 85 had multiple receivers enabling the return signals to be detected through the heaviest jamming, this included "dicky fix" receivers to combat carcinatron jamming. Post reception processing, including double integration loops enabling the comparison and discarding of spurious returns enhanced the ECCM capability still further. Whilst not having the MTI facility the Type 84 had, the frequency band and processing ensured the Type 85 produced a clear picture even under the most arduous conditions.

As the transmission and reception of the Type 85 was based on 12 beams it was also a 3D radar. Data from the radar was passed to the Automatic Height Finder which compared comparative strength of returns from a target between beams. Given the range and known angle it is a simple process to calculate height. However this was being done hundreds of times a second on multiple targets, in the late 60s and early 70s it was a considerable feat.

Secondary Radar

The secondary radar used in the system was the SSR750. Each primary radar had an associated secondary radar, at RAF Neatishead there was an additional "freestanding" SSR. Secondary Radar information was associated with the primary (search) radar and presented on the same display. This gave operators a range of information that far exceeded the traditional, 'range and direction". Secondary radar was a duel, military and civil, system that worked by interrogating a transponder on-board the aircraft, receiving and translating a coded reply.

Height Finding

There were three height finding facilities with in the Linesman system. One was built into the Type 85 radar, the second was a stand-alone system using the HF200 radar and the third was based on the SSR interrogating the aircrafts systems.

The Type 85 working with the Auto Height Computer enabled and operator to select a target and have, by triangulation in the radars 12 beams to obtain a height.

The HF200 provided another means of finding the height of a target. This radar, rather than continuosly rotating, nodded on a bearing selected by the operator. The bearing derived from the main primary radar would enable a reading of the height to be sent back to the operator.

The secondary radar system SSR750 as part of the joint military civil functionality could interrogate a transponder on board as aircraft for the height. This height, automatically derived from the on board altimeter was in the the late 60s and early 70s shown to the operator on a box mounted above his screen. Later, with the advent of plot extraction it could be displayed alongside the the aircraft on the traditional PPI display.

Passive Detection

A third system was later added to the network, the RX12874 "Winkle" passive jamming-detection system or PD system. PD consisted of a series of high-speed rotating antennas separated by many miles, combined with similar signals captured from a Type 85 radar.

The three Type 85 radars located at Neatishead, Staxton Wold, and Boulmer used the associated high speed PD aerials to make a PD baseline. For this the two aerials (T85 and PD) 300 miles apart had to be in sync to ensure they swept the same area of sky at the same time, sync and turning information was sent over microwave link. For example, the Type 85 at Neatishead used the signals captured from the high speed aerial at Staxton Wold to detect jamming aircraft. Each Type 85 with its associated high speed aerial was able to provide a PD baseline of several hundred miles to the north and south. There was a high speed aerial provided to the north of RAF Boulmer, but there was no similar extension to the south of RAF Neatishead.

The signals were combined at one of the radar stations (at the location of the Type 85 forming one end of the baseline) 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.

Links

In the original design processed radar signals were returned from the radar station to L1 and LATCC via microwave links. In the 1960s and 70s this consisted of processed, but by todays standard raw, video and turning information (ie the angle of azimuth of the radar aerial). Received signals from the PD equipment and aerial turning synchronisation information were transmitted over the same links.

In the late 70s plot extraction equipment was introduced, this took the primary and associated secondary radar outputs, combined and processed them before sending them over telephone lines to the L1. The RPEARDS (Radar Plot Extraction And Remote Display Equipment) was a hardwired computer that processed, combined and transmitted the signals. The memory was magnetic core store that had the capacity of some 1000 words, each of over 60 bits in length, transmission over the telephone line was at 2400 baud using GMSK. At the time it was leading edge technology.

Operation

Legacy

Linesman built on Rotor which had built on Chain Home and the lessons of the second world war. During the dangerous and tense period that was the cold war, where the detection and interception of Russian bombers was a weekly, if not daily, event, Linesman enabled the protection and policing of UK air space. IUKADGES and subsequent developments have all built on the legacy of these systems, systems which stem from the Dowding System. An air defence system that enables the quick and accurate deployment of assets to intercept a threat conserves resources and targets them to where they are most use.

Conceived, proven and tested in the heat of the Battle of Britain the system is still sound, even if the technology has improved manyfold, and the threat, post 9/11, significantly altered.

Whilst a lot of the Linesman systems and above ground evidence has gone it is still possible to get a feeling for what operating and working at the height of the cold war was like at the RADAR Museum at RAF Neatishead, http://www.radarmuseum.co.uk/ .


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