Watch the skies


01 September 2020
The four steel 364ft RDF transmitting towers erected on the coast at Swingate Dover, which formed part of the Chain-Home radar stations along Britain’s coast (Dover Museum) The four steel 364ft RDF transmitting towers erected on the coast at Swingate Dover, which formed part of the Chain-Home radar stations along Britain’s coast (Dover Museum)
Graham Caldwell explains how the highly developed RAF air defence network was indispensable in winning the Battle of Britain.
Watch the skies Images

Nothing more sums up what Air Chief Marshal Sir Hugh Dowding was up against as Air Officer Commander-in-Chief of Fighter Command, when he advocated for the latest technology to defeat a potential Luftwaffe air bombardment in the likely event that Britain would soon be at war with Germany, than when attending an Air Ministry conference in 1938. Dowding later related, “At the meeting I asked that bullet-proof glass should be provided for the windscreens of the Hurricane and Spitfire. To my astonishment those around the table dissolved into gusts of laughter, making my request sound as if I’d asked for something grotesquely impossible.” Dowding retorted sharply, “If Chicago gangsters can have bullet-proof glass in their cars I can’t see any reason why my pilots should not have the same!”

Such was Dowding’s passion, since his appointment in July 1936, to prepare the air defence of Great Britain for war. Ultimately, with the help of forward-thinking scientists and engineers, Dowding put in place a system combining existing and new technology communication methods that would provide an early alert for the defence forces, enabling them to successfully defeat a sustained enemy air bombardment of the homeland. The name given to this all-encompassing United Kingdom air defence was Ground-controlled interception, code-named GCI, but it later became more commonly known as the Dowding System.

WAAF Radar Operators were based at the RDF Chain-Home stations around Britain’s coast and communicated directly with both their respective regional Group Controller and HQ Fighter Command at Bentley Priory (MoD/AHB/RAF)Huff-duff and pip-squeak
Since the end of WWI and certainly until at least 1937, the RAF had continuously advised the government that the invention of aerial bombardment was the new dominant factor in any future war pronouncing: ‘The bomber will always get through’. Retaliation, by bombing the enemy’s homeland harder and more frequently, was believed to be the only counter-measure. Dowding, in his then capacity as Air Member for Supply and Research since 1930, did not agree, believing that bombers could be prevented in getting through provided there were enough fighter planes. Whilst fighter production was stepped up, what was still lacking in 1937 was a means to detect enemy bombers’ range and height in sufficient time to scramble fighter squadrons to intercept and destroy them. In early 1935 the government scientist, Robert Alexander Watson-Watt, developed the ability of radio waves to detect aircraft, which he named RAdio Direction-finding And Ranging (RADAR). Dowding leapt upon the findings and, by 1939, the system, code-named Radio Direction Finding (RDF) was able to detect bomber-sized targets at ranges of 100 miles, whilst also measuring their height and direction of travel. This proved decisive in 1940 because it was able to detect enemy aircraft while they were still forming up over France, giving RAF commanders ample time to marshal their entire force directly in the path of the raid. This had the effect of multiplying the effectiveness of Fighter Command as if they had three times as many fighters, allowing them to defeat ever larger German formations.

Once enemy bombers passed behind coastal Radar the Observer Corps took over to track them inland. Here an observer sights an aircraft with his Post Instrument Plotter to determine its position and heightWork was put in hand to build a chain of RDF stations facing out to sea at 25 mile intervals along the English coast in a network code-named Chain Home or CH for short. During the battle, RDF towers were virtually left alone, except for targeted attacks during a few days in August 1940, but because the towers survived, owing to their open steel girder construction, the Luftwaffe concluded the stations were too difficult to damage by bombing and left them alone for the rest of the war, without realising just how essential the radar stations were to British air defence!
However, once enemy formations crossed the coastline and were behind the RDF towers, the only means of tracking their position was by the Observer Corps (OC) using a Post Plotting Instrument, an optical sighting system mounted on a gridded map that determined the location and height of overhead aircraft. The Observer Corps was manned entirely by volunteers and was placed under the direct control of Fighter Command. This enabled OC stations to report sightings, using the existing General Post Office (GPO) landline network, directly back to RAF Sector Stations via OC filtering centers.

During the period from July to October 1940 the Observer Corps was at full stretch operating 24 hours a day, 7 days a week, plotting enemy aircraft passing overhead. Dowding was also successful in securing approval for the Army’s Anti-Aircraft Command to be integrated into the RAF communications system, which enabled its guns and searchlights to be ready well in advance of enemy formations arriving overhead. Communication with the fighters was solved by utilising the Spitfire and Hurricane installed twin-channel TR.9D high-frequency (HF) radio telephone. New technology HF Direction Finding sets (HF/DF) nicknamed huff-duff, were linked to the fighter’s spare radio channel, which was compatible with the pip-squeak radio navigation system (the name derives from a contemporary comic strip called Pip, Squeak and Wilfred). Pip-squeak sent out a 1kHz tone picked up by ground-based huff-duff receivers and then, by using the triangulation of three HF/DF measurements, observers could determine the location of friendly aircraft, the primary means of locating friendly forces.

This RAF Sergeant, using direct observation, reports the landing of a Halifax bomber to the control tower via the airfield telephone system, a method frequently used for security purposes to minimise pilot radio chatter (MoD/AHB/RAF)  Additionally added to all RAF aircraft was an invention called Identification Friend or Foe (IFF) a radar-based identification system. This provided the primary means of differentiating between friendly and hostile aircraft by using a transponder that listens for an interrogation signal, which then sends back a response signal, in which case the aircraft must be friendly, because IFF can only positively identify friendly targets, not hostile ones. Whilst all these communication systems were useful in their own way, it meant that there were now four sources of information, RDF, OC, huff-duff and IFF, none of which had a complete picture of the airspace. This is where Dowding’s input and organisational skills provided a vital contribution towards ensuring everything worked in harmony by 1940.

WAAF Telephonists were employed at all the main Group and Sector Stations, which ensured internal RAF communications worked smoothly throughout Fighter Command (MoD/AHB/RAF) The national GPO telephone system
The introduction of direct dial telephones and services, such as the 999 emergency number and the speaking clock, helped drive private uptake of phones in the 1930s, but with the onset of the war from 1939 onwards, military demands took priority. The Frequency Changer Telephone, or Scrambler, was a secure telephone system developed in 1939 by the Post Office Research Division by adapting the standard black 394 and 396 GPO models with a Type-164 handset. It was ready by early 1940, just in time for the Battle of Britain, plus there was also a 4-wire variant for use over radio. The scrambler was housed in a wooden or metal enclosure under the table or in an adjacent room, whilst the actual telephone, with its three selector buttons, was placed on the desk. For identification purposes, scrambler telephones always had a bright green handset. The responsibility for manning the RAF air defence telephone land-lines, which cables were buried sufficiently deep enough underground to provide protection against bombing, fell to the Woman’s Auxiliary Air Force, including WAAF radar operators at the forward RDF stations along the coast.  

The airfield sector control room at Box in Wiltshire, which was part of RAF 10 Group. Note the Sector Controller sitting in the centre on the first upper levelCommunication and control
Dowding’s Fighter Command headquarters was based at RAF Bentley Priory, a converted country house on the outskirts of London. A key feature at Bentley Prior was the Filter Room, to which all RDF stations and the Observer Corps were linked by direct landlines. This enabled all enemy air movement data to be sorted and tracked using the latest cathode ray (oscilloscope) screens. In 1940 this technology was the most advanced in the world and one of Britain’s most carefully guarded secrets. Once the tracks of incoming bombers had been established, the information was passed to the relevant RAF Group headquarters, then downwards to the Operation Rooms of the Sector Stations based at the main airfields. A contemporary account of the work inside a Battle of Britain operations room is described in the 1942 publication Britain’s Wonderful Fighting Forces by Captain E. Hawkes R.A.: ‘The Operations Room is the nerve centre, setting in motion the whole complex organisation of Home Defence. The job of the radio operator, who is in personal touch with the pilots and is himself a fighter pilot, is of first-class importance. As soon as any order is received from a Group Headquarters, administrative staff work out the course the fighters are to take, so that they will contact the bombers in the shortest possible time and instructions issued by telephone direct to the aerodrome. Because of the two-way radio contact, the pilots can report progress immediately and be given update information, such as a change in the bombers’ course or height. The Operations Room is a centre of tremendous activity, the rushing in and out of messages and the ringing of telephone bells, the raucous voices from loudspeakers. Maps too are to be seen everywhere; on the walls and on the plotting tables and a special staff of expert (WAAF) map plotters is always on hand’.

A typically busy plotting staff, seen here controlling over 20 different friend and foe aircraft plots across the operations room plotting tableObviously, because this was written during the war, many features, such as RDF, were still secret. The book certainly didn’t describe the methodology of an Operations Room, which consisted of a giant lamp-system Tote Board that indicated the strength and readiness of each fighter squadron. Information about both hostile and friendly aircraft movement was passed to a large number of WAAF plotters stationed around a giant table bearing a map of the sector. Details about the number of aircraft, their position, height and bearings (plus Squadron number if friendly) were transferred to wooden blocks called plots that were positioned and moved along the map table in a similar way to a croupier at a roulette table, using plotting rods that were adjustable in length and magnetised to pick up the plots.

An exact replica of a 1940’s RAF Sector Clock, which tells the time perfectly, available from IWM Duxford for £80 (  This allowed the whole picture of a raid to be monitored by the Sector Group Controller, who was stationed in a gallery above the plotting table. A large Sector Clock was mounted on the wall, which was the mechanical forerunner of a computerised system for airspace control. The face was marked with five-minute red, yellow and blue triangular segments. It had an outer 12-hour ring and an inner 24-hour dial. Red, yellow and blue arrow markers were placed on the plotting table next to the relevant plots to indicate each raid’s direction and were used in concert with the Sector Clock to indicate time. As the plotter received the position of a raid, they selected an arrow marker matching the colour of the relevant segment on the clock face. Communication flowed in both directions, ensuring that everyone was making decisions based on the most up to date information.

Whilst the German’s were aware that Britain had radar, they had no inkling what incredible detail the RDF stations passed onto the RAF fighter pilots in the air, nor the elaborate communications system in place, consistently underestimating its importance. After the war German fighter ace Adolf Galland admitted the oversight by saying: ‘From the first the British Royal Air Force had an extraordinary advantage, never to be balanced out at any time during the whole war, which was their radar and fighter control network and organisation. It was for us a very bitter surprise. We had nothing like it. We could do no other than knock frontally against the outstandingly well-organised and resolute direct defence of the British Isles.’

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Acknowledgements: Stuart Hadaway, John Rice and Sam Loverso.       

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