9

Radio

Radar and the rapidly expanding Observer Corps network gave the R.A.F., from 1936 onwards, the tools on which to base an air defence system. The service could not, however, operate without certain other vital links, the most important of which was fighter control.

In 1934 Fighting Area headquarters, in its wooden shack at Uxbridge, had no direct telephone lines to its fighter sectors. The communications problem loomed large and it had to be solved.

Information received, whether from electronic or visual sources, was of little use if the fighter controllers were unable to communicate with the aircraft and know their position and other relevant information at any time.

The signals directorate at the Air Ministry was far sighted and industrious. As far back as 1928 experiments were initiated into shortwave direction finders which would accurately fix an aircraft’s position.

Squadron Leader Chandler, a signals technical officer, worked on direction finding from its earliest days. With the Marconi Wireless Telegraph Company he evolved the Chandler-Adcock short-wave direction finder. This was designed to operate on the fifty to seventy-metres waveband with a range of up to fifty miles covering the radius of action of fighters of the time.

The set was first used during combined short-wave tests with the Army in August 1932, although it had only just completed experimental trials and no service experience had been obtained. No hut was available for the second stage of this exercise, so the finder was positioned in the middle of a field at Hornchurch, Essex. The Officers and airmen using the equipment had only a few days training and during the tests had to contend with wind, rain and thunderstorms. Nevertheless the results achieved were promising. The report on the exercise recorded that:

‘The direction finder can give the direction of the aircraft with an accuracy at least as good as the pilot normally gives his position when he sees the ground.’

Tests continued at Hornchurch through 1933 using Bristol Bulldog and Hawker Hart aircraft and on a long-range sortie with a Vickers Virginia. In addition, foreign short-wave stations in a number of European countries, including Germany, were received to determine the possibility of obtaining bearings.

After painstaking analysis of all the trials, a comprehensive report was prepared by Chandler in August 1933 in which it was recommended that high-frequency direction finders should be set up in each Air Defence of Great Britain fighter sector. The bearings obtained from three-minute transmissions by the aircraft were to be plotted continuously from a central plotting station which would give the pilot on patrol his position by radio telephone. Chandler ended his report by forecasting that ways might be found of obtaining automatic transmissions from the fighters without action by the pilot. This idea was to become the well-known ‘Pip-Squeak’ apparatus which surprised the Luftwaffe in 1940.

Thus in 1933 the basic system of fighter direction and control, which was to be used seven years later in the Battle of Britain, was already devised.

The next step was to turn over high-frequency direction finding (H.F./D.F.) to the Air Defence of Great Britain so that the organisation could be developed under operational conditions.

Various improvements were made to the direction finder in the ensuing months and by the late summer of 1935 four stations were operating, at Biggin Hill, Kent, Hornchurch and North Weald, Essex, and Northolt, Middlesex. In January 1935 basic trials with H.F./D.F. for fighters were started and these were followed by night tests in May. ‘Homing’ to base was tried first and then position ‘fixing’ using two or more H.F. direction finders. During the 1935 annual air exercises one fighter pilot arranged for unofficial fixes to be relayed to him and succeeded in intercepting a bomber above the clouds at night. On September 30th Hart had been posted to H.Q., Fighting Area, Uxbridge, as deputy chief signals officer with a brief from Air Vice-Marshal Joubert de la Ferté, now Air Chief Marshal Sir Philip Joubert, to assess the efficiency of the warning and control system by a series of co-ordinated tests using H.F./D.F., airborne radio and the Observer Corps.

The Fighting Area control room was contained in a wooden hut at Uxbridge. A gridded map table was installed and Warrant Officer R. M. Woodley worked out a system of reference counters and standardised the sequence of incoming information.

Raids were simulated and fighters scrambled to intercept on information supplied by the Observer centres. Positioning and homing were done by means of H.F./D.F., although because of the shortage of suitable aircraft radio sets and mechanics the precious radios had to be switched from one station to another.

The exercises showed clearly that H.F./D.F. was the answer to control, but far more serious was the report compiled by Hart, which stated that Fighting Area could not hope to intercept any enemy bomber and stop it dropping its load on London unless warning could be given when it was at least thirty-five miles from the Kent coast—and as aircraft speeds increased this minimum distance would rise.

At this time Hart and his colleagues were not fully aware of the existence of radar, but the reports they compiled gave added impetus to the work of Bawdsey and to the setting up of the preliminary chain of five radar stations. Meanwhile on March 17th, 1936, at a meeting at H.Q. Fighting Area, it was decided to fit H.F./D.F. at all sector airfields.

The main task of developing sector operational control devolved on Biggin Hill aerodrome where projects had been undertaken since World War I.

Tizard, whose fertile mind had for many months been occupied with the techniques of using radar information, suggested that full-dress interception experiments should be put in hand. At a conference in August 1936 with the new Fighter Command A.O.C., Dowding, in the chair, it was agreed that experiments should continue to 1937 using Hawker Harts as bombers and No. 32 Squadron Gloster Gauntlet fighters, both based at Biggin Hill. Tizard accordingly set up a team at Biggin Hill in the summer of 1936 for operational research. The members were Squadron Leader Ragg, a navigation expert, Flight Lieutenant W. P. C. Pretty (now Air Marshal), the sector signals officer, and Dr. B. G. Dickens, an Air Ministry scientist. Dickens became what was the world’s first operational research civilian and his appointment set a pattern for the future.

Tizard was unable, for security reasons, to tell the team about radar. He therefore requested that they investigate fighter direction and control techniques assuming that bearing, distance and altitude of enemy aircraft could be given to them at guaranteed time intervals from some mysterious source. Even with this information ‘elipses of accuracy’ had to be drawn which resembled a rugby football 8 in. by 4 in. on the plotting table and within which area the enemy was presumed to be.

The essence of the problem was to take the radar or Observer Corps information in ‘filtered’ form and convert it into interception courses for the fighters to follow. Three Hawker Harts simulated bomber attacks, while the Gauntlets of No. 32 Squadron undertook the interceptions using radio-telephone information from the ground.

Because no genuine information from radar was available, the bombers’ position was fixed by high-frequency radio using the four new H.F. direction-finding stations. Only one channel on one frequency was available, so that the bombers had to be sent out over the North Sea with a given flight plan. As they returned, the wireless operator in the leading aircraft screwed down his morse key to give a continuous wireless transmission. This was picked up by the direction-finding stations, who plotted it and telephoned the information to the Biggin Hill operations room.

There a large horizontal backboard was provided for the actual plots using the range and bearing method. The plotter employed a long wooden ruler with mile notches on it attached at one end to a compass rose which represented Biggin Hill itself With dividers and protractors suspended on elastic, chalk marks were made on the board to position the bombers and an interception course worked out for the fighters.

The inaccuracies in the direction-finding information and in converting track and ground speed into course and airspeed gave numerous apparent changes of course which confused the plotters and complicated interception. Added to this the fighter pilots had to do their own navigation on dead reckoning.

To bring two fast formations of aircraft together as quickly as possible proved exceptionally difficult. The small group at Biggin Hill laboured for weeks to resolve the ‘four vector interception problem’ which was the key to the application of radar and visual information. All sorts of weird instruments were devised to do the job including a complex instrument known as the Simmonds-Goudine computer. A large number were produced at £ 5 each, but were hardly used.

Weeks ran into months but still the solution to successful interceptions was missing. To one man at Biggin Hill trigonometry and computers were distinctly unpleasant subjects. The station commander, Wing Commander E. O. Grenfell, had many tasks to perform but he found time to watch the calculations, exasperation and arguments of the team in the small operations room. As he studied the lines and bearings on the blackboard he became convinced that interceptions could be made by eye based on the D.F. information and the fighter position. To the team worn out with figures and computations this was like a red rag to a bull. To prove the stupidity of the suggestion they suggested that Grenfell try it ‘by eye’. Accepting the challenge, he called ‘steer seventy degrees’ and made further alterations to the fighters’ course as he judged necessary.

To the amazement of the onlookers he completed a perfect interception. Away to the east the Gauntlets and Harts met.

Tizard realised that here was the immediate solution. He discarded the computers and the pages of mathematics and evolved a simple rule of thumb known as the ‘Principle of Equal Angles’, or more colloquially by the staff as the ‘Tizzy Angle’. By drawing a line from the bombers to the fighters and making this the base of an isosceles triangle with the fighter angle always equal to the bomber angle, the two formations would meet at the apex of the triangle.

If the bombers altered course the controller could visualise a new triangle on the plotting table and tell the fighters to alter heading accordingly. If the fighters outpaced the bombers and reached the apex first they would be ordered to circle and wait.

This simple system evolved by improvisation proved an extraordinary success. In 100 practical experiments in 1936 using the method Biggin Hill recorded 93 per cent interceptions. Sector control and vectoring procedure later used in the Battle of Britain was evolved on this basis. With it the great importance of the post of sector controller was established. In August 1937 Dowding asked for three H.F./D.F. stations in each fighter sector.

The next step was to link Bawdsey radar station in 1937 with Biggin Hill. The radar picture was transmitted to the airfield operations room from the experimental Bawdsey fighter centre. The H.F. direction-finding network was then free to keep a continuous check on the fighters alone, relaying their positions to the sector controller who could alter his interception triangles accordingly. The fighters were able to disperse with dead reckoning and concentrate on following the controller’s instructions.

The Sector Operations room at Duxford with the Controller’s position, Ops ‘A’, ‘B’ and ‘B 1’ and the gun control officer. On the walls are sector maps squadron call signs, a map of R.A.F. airfields in eastern England, recognition silhouettes and the inevitable poster pointing out that ‘Careless talk costs lives’

The blackboard in the operations room proved messy and made plotting difficult. Instead, a vertical ground-glass screen was erected on which the aircrafts’ positions were plotted using counters with rubber suction pads. Biggin Hill continued for some years to be the guinea-pig for Fighter Command. Improvements constantly made in equipment and techniques were incorporated at other sector operations rooms. New controllers were trained at Biggin Hill where they were given synthetic as well as ‘live’ instruction.

While the web of sector operations rooms grew, the H.F. direction-finding system was expanded at a tremendous rate. Co-operation between the experts on radar, fighter control and H.F./D.F. was excellent and there was a continuous exchange of ideas.

Watson-Watt, Rowe and other civilians attended the R.A.F.’s direction-finding conferences. Through Watson-Watt and Bawdsey the service was able to obtain new types of aerials with higher efficiency and the loan of a cathode ray direction finder in 1936 for Northolt. When standardised this speeded up plotting and allowed more fighters to be dealt with by one operator.

By September 1936 plans were in hand for twenty experimental stations to be built for cathode ray installations.

The final arrangement for Fighter Command’s direction-finding organisation was already on paper in September 1936, following No. 11 Group’s ‘Final sector training exercise’ on July 27th, 29th and 30th. Eight fighter squadrons operating from Biggin Hill and Hornchurch, the former concentrating upon ‘positioning’ for interception, and the latter on ‘homing’ back to base. By September 1937 long-term plans were in hand for a complete H.F./D.F. network for the three R.A.F. commands, to be completed by 1939.

Squadron Leader Chandler’s original suggestion in 1933 that some means might be found for automatic transmission from aircraft of signals for direction finding were echoed in the signals report on No. 11 Group’s 1936 exercise:

The heart of radar operations in 1940—the filter room at Bentley Priory. The operations map shows the East and south coasts of England and the network of all-important C.H. and C.H.L. radar stations. On the standard throughout all R.A.F. and Observer Corps operations rooms

The Operations Room at Fighter Command headquarters, Bentley Priory, where the final overall picture of the air situation could be seen with filtered radar and the Observer Corps tracks. The officers on the dais look down on the ops table and the W.A.A.F.s with their ‘croupier’ rakes

‘It is understood that some thought has been given by the Air Ministry to the idea of incorporating an automatic periodic D.F. transmission in aircraft transmitters. It is thought that such transmissions would enable sector D.F. stations to keep constant check on aircraft positions and so speed up and facilitate the work of interception.’

Chandler, the brain behind the H.F./D.F. network, set to work to evolve a system. This device, ‘Pip-Squeak’, automatically switched on the H.F. transmitter in the aircraft for fourteen seconds in every minute. During the remainder of the minute the radio set was switched for normal reception and transmission by the pilot. A control was provided so that the pilot could cut out the automatic transmission when necessary.

A clock, similar in principle to the contactor unit in the aircraft, was available in the control room and, where necessary, in the D.F. station. The face was divided into four coloured sectors. A hand rotating once a minute indicated which aircraft should be transmitting at that time.

The ‘Pip-Squeak’ in the aircraft was arranged so that it could be set to transmit in any quarter-minute period. The pilot was told which period he was to set his control. Four aircraft could transmit in rotation automatically and be identified on the ground by the quarter-minute period in which they transmitted. Each of four aircraft therefore had its position plotted once every minute, the pilots being relieved of the onus of navigation and position fixing. Cockerel was the code word for the device. If a pilot forgot to switch on ‘Pip-Squeak’ the controller might ask him: ‘Is your cockerel crowing?’

The aircraft transmissions enabled a bearing to be taken by each of three D.F. stations in one sector. The bearings were plotted in the sector control room and interception courses passed by radio to the fighters on the ‘equal angles’ principle as in the early tests at Biggin Hill.

The D.F. network using ‘Pip-Squeak’ was in existence when war started. Training had been in progress for some time. This system and radar became the keys to all the interceptions made in the Battle of Britain.

One further improvement was considered necessary, however. High-frequency wavebands were becoming overcrowded and were subject to interference and distortion which affected the speed and accuracy with which pilots could talk to sector control.

In January 1937 a requirement was issued for a very high frequency (V.H.F.) radio-telephony set with 100 miles range. The Royal Aircraft Establishment were entrusted with development following its research work over the two previous years. The demand presented problems and by mid-1938 it was reported that it might still take four years to perfect fighter V.H.F. In January 1939 the Director of Communications Development, Air Ministry, decided that a set could be produced more speedily but that it would not be up to the original specification.

The Chief of Air Staff approved revised plans and orders were given for the re-equipping of four sectors each in Nos. 11 and 12 Groups. Hornchurch, North Weald and Debden were to operate both H.F. and V.H.F. simultaneously.

The whole V.H.F. programme became a race against time, which was lost. By August 1939 the first sets, designated TR 1133, were ready for delivery and in October trials began with six Spitfires of No. 66 squadron based at Duxford. The results were excellent. For the second stage of the programme it was anticipated that by May 1940 all Fighter Command aircraft would be equipped with an improved V.H.F. radio, TR 1143.

Serious delays occurred in deliveries of both sets and at the time of Dunkirk V.H.F. was held in reserve. Aircraft continued with the TR 9 H.F. set. In the main the Battle of Britain was fought using the old and well-tried H.F. network. It was not until the end of September 1940 that sixteen day fighter squadrons had been re-equipped with V.H.F. It was a case of too little too late.