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Authors: Bickers Richard Townshend
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was 50 miles (80km) and it could not read height. The aerial, adapted from 200MHZ gun-laying radar that had also been designed at Bawdsey, was a gantry 20ft (6m) high, mounted on a 185ft (56m) tower.
The first CHL station began operating in November 1939. Mobile units were also being built. The vehicles on and in which they were installed became known as a âconvoyâ. The wavelength was 5.4 to 10m, frequency 30-56MHz, power output 40kW, range 90 miles (144km) and it indicated target height. The combined CH and CHL system gave the RAF an excellent probability of detecting virtually any intruders.
At the time of the 1936 reorganisation, the main question for Fighter Command was how to make the best use of radar. A control and reporting system (in RAF parlance âC & Râ) had to be devised, to report the presence of enemy aircraft and control the movements of the fighters sent up to intercept them. Overall tactical control would be vested in Headquarters Fighter Command. Tactical control within each Group area would be delegated to Group Headquarters. Once fighters were airborne they would be controlled directly by their Sector. This meant that each Group and Sector, as well as HQ Fighter Command, had to have an Operations Room, in which the air activity within its area of responsibility could be shown. The general situation map (commonly referred to as the plotting table) in a Sector Ops Room needed to show the whole sector area and a large part of the adjacent sector or sectors. In a Group Ops Room, the whole Group area and part of the adjacent Group or Groups had to be displayed. In the Fighter Command Ops Room, a picture of the whole of the British Isles was necessary.
In addition to the CH stations, another source of information had to be integrated: the Observer Corps. This organisation originated in 1914 on the outbreak of World War I. The Royal Naval Air Service being responsible for home defence, the Police were instructed to report to the Admiralty by telephone when enemy aircraft were seen or heard. In 1916, the Army, of which the Royal Flying Corps was an arm, took over from the Admiralty. Cordons of civilian observers were now positioned at a radius of 30 miles (48km) around vulnerable areas, to inform the WarOffice when they saw or heard enemy aircraft and, if possible, to give an estimate of course and height. In 1921 the Observer Corps was restructured into observation posts that reported to observation centres reporting in turn to Fighting Area HQ, which was responsible for the defence of Great Britain.
By the time the RAF was restructured in 1936, the Observer Corps had grown in numbers. It took over from radar the tracking by sound and sight, and reporting, of aircraft when they crossed inland over the coast. The Observer Corps was a body of mostly part-time civilian members. At the end of the war the accuracy of their estimations was assessed most commendably. Height: visual, average error 10 per cent up to 20,000ft (6,060m); sound, 20 per cent. Strength: visual, exact; sound, good.
With three systems (Chain Home, Chain Home Low and the Observer Corps) reporting aircraft movements, a means of resolving disparities and duplication had to be found. Filtering was the name given to this process. At Bawdsey, an experimental filter room was set up in July 1937, to sort out the plots passed by the three CH stations then operating â Bawdsey itself, Dover and Canewdon (Essex) â and telling the filtered plots through to the Ops Room at Fighter Command during the annual exercise in August. On November 8, 1938, this was closed and the Filter Room opened at HQ Fighter Command.
A report from Bawdsey on August 30, after the annual exercise, noted:
âInformation is to be told to Groups and thence broadcast simultaneously to Sectors:
âExperience with Biggin Hill has shown that Sectors require information accurately and speedily at a rate of one plot per minute per
The first CHL station began operating in November 1939. Mobile units were also being built. The vehicles on and in which they were installed became known as a âconvoyâ. The wavelength was 5.4 to 10m, frequency 30-56MHz, power output 40kW, range 90 miles (144km) and it indicated target height. The combined CH and CHL system gave the RAF an excellent probability of detecting virtually any intruders.
At the time of the 1936 reorganisation, the main question for Fighter Command was how to make the best use of radar. A control and reporting system (in RAF parlance âC & Râ) had to be devised, to report the presence of enemy aircraft and control the movements of the fighters sent up to intercept them. Overall tactical control would be vested in Headquarters Fighter Command. Tactical control within each Group area would be delegated to Group Headquarters. Once fighters were airborne they would be controlled directly by their Sector. This meant that each Group and Sector, as well as HQ Fighter Command, had to have an Operations Room, in which the air activity within its area of responsibility could be shown. The general situation map (commonly referred to as the plotting table) in a Sector Ops Room needed to show the whole sector area and a large part of the adjacent sector or sectors. In a Group Ops Room, the whole Group area and part of the adjacent Group or Groups had to be displayed. In the Fighter Command Ops Room, a picture of the whole of the British Isles was necessary.
In addition to the CH stations, another source of information had to be integrated: the Observer Corps. This organisation originated in 1914 on the outbreak of World War I. The Royal Naval Air Service being responsible for home defence, the Police were instructed to report to the Admiralty by telephone when enemy aircraft were seen or heard. In 1916, the Army, of which the Royal Flying Corps was an arm, took over from the Admiralty. Cordons of civilian observers were now positioned at a radius of 30 miles (48km) around vulnerable areas, to inform the WarOffice when they saw or heard enemy aircraft and, if possible, to give an estimate of course and height. In 1921 the Observer Corps was restructured into observation posts that reported to observation centres reporting in turn to Fighting Area HQ, which was responsible for the defence of Great Britain.
By the time the RAF was restructured in 1936, the Observer Corps had grown in numbers. It took over from radar the tracking by sound and sight, and reporting, of aircraft when they crossed inland over the coast. The Observer Corps was a body of mostly part-time civilian members. At the end of the war the accuracy of their estimations was assessed most commendably. Height: visual, average error 10 per cent up to 20,000ft (6,060m); sound, 20 per cent. Strength: visual, exact; sound, good.
With three systems (Chain Home, Chain Home Low and the Observer Corps) reporting aircraft movements, a means of resolving disparities and duplication had to be found. Filtering was the name given to this process. At Bawdsey, an experimental filter room was set up in July 1937, to sort out the plots passed by the three CH stations then operating â Bawdsey itself, Dover and Canewdon (Essex) â and telling the filtered plots through to the Ops Room at Fighter Command during the annual exercise in August. On November 8, 1938, this was closed and the Filter Room opened at HQ Fighter Command.
A report from Bawdsey on August 30, after the annual exercise, noted:
âInformation is to be told to Groups and thence broadcast simultaneously to Sectors:
âExperience with Biggin Hill has shown that Sectors require information accurately and speedily at a rate of one plot per minute per
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