The Spitfire Site

A Tribute to Britain's Finest Fighter

Early Radar Memories

From Sgt. Jean (Sally) Semple, one of Britain’s pioneer Radar Operators
Retold by Katherine Reynolds

"Working under the closest secrecy since 1939, over 4,000 WAAF personnel have played an important part in the air victories achieved by radiolocation (Radar). They tracked hostile and friendly aircraft, flying bombs and rockets, German E Boats and Allied Merchant vessels, and have guided British and Allied fighter pilots on to enemy aircraft. Trained to use and service some of the most delicate and complicated instruments ever invented, they have carried out their duties with enthusiasm, often under uncomfortable conditions and sometimes under enemy fire."
Air Ministry communiqué revealing the existence of the British Radar, 8 August 1945

“All the ascendancy of the Hurricanes and Spitfires would have been fruitless but for this system which had been devised and built before the war. It had been shaped and refined in constant action, and all was now fused together into a most elaborate instrument of war”
Winston Churchill, The Second World War

Seventy-four years ago on February 26, 1935, experimental work conducted in Britain demonstrated that aircraft could be detected by radio. The history of this discovery and its vital role in Britain’s defence in 1940 and the following years has been told many times. But what of the observers, working calmly with evolving equipment, and always in danger of enemy attack? My sister, Jean Semple (nicknamed Sally), was one of the first service-women in Radio Detection and Ranging, as Radar Technology was originally known.

After joining the WAAF on March 20th, 1940, Jean embarked on the long training on the RF5 at Bawdsey Research Station at Bawdsey Manor. Bawdsey Manor was home to the very first Chain Home station .This is where the first RAF air exercises using radar were conducted in 1936-1937. By the outbreak of the war, the station was developed to the RDF training school. Poppa Jennings gave Jean’s group members their very first lectures on Course 6, courses 1 to 5 having been given before the war.

Two photos of Jean Semple in WAAF uniform
[via Katherine Reynolds]

With training complete, Jean received the ID: – 890144 - and was stationed firstly at Pevensey, secondly at Rye and her postings after that included a return to Bawdsey as an observer. She also served on the Isle of Man and at Douglas Wood. Her initial job-description was WAAF Special Duties. At the outset of hostilities the UK shortened the term ‘Radio Detection and Ranging’ to ‘Radio Location.’ When America came into the war they required the acronym ‘Radar’ to be used instead. At this point Jean and her colleagues became known as Radar Operators.

The type of Radar Jean worked on was called CH which stood for Chain Home. Chain Home was literally a chain of radio detection stations spaced twenty miles apart around the coast. At the outbreak of war in September, CH had eighteen stations covering the eastern and half of the southern coast of Britain, more stations being added later. Information from the stations was passed to RAF Fighter Command Headquarters located in Stanmore in North London.

The initial Chain Home used High Frequency (HF) wavelength meaning that it required large antenna masts to radiate sufficient power. Its four, 360-foot-high transmitter aerials were made of steel, while the four, 240-foot-high receiver aerials were of wood. The transmitter aerials sent pulses of radio energy, at varying heights, which, when reflected from an aircraft, were picked up by the receiver aerial.

Three of the four transmitter aerials of the Chain Home RDF are visible to the left, while the four receiver masts are grouped to the right

In the receiver room, the distance of the ‘echo’ could be measured on the face of the cathode ray tube. Establishing the position of the aircraft was a more complex matter. The device used for this purpose was a Goniometer, known as a Gonio, with coaxial cables leading to four coils that could be sensitised. With the help of this device, the position of the target could be ascertained through triangulation from other stations.

When a signal was transmitted, if there was an aircraft in the area, the body of that aircraft would bounce that signal back. If the signal stayed on the same bearing that would be great but if it kept changing all the time the observer had to react to the plane’s every move. In the view of Jean and her colleagues, the Spitfire returned the clearest signal of all aircraft being tracked at the time.

Jean touches on the complexities of her work:

‘Visualise the cathode ray tube. Running across its screen was a thin, horizontal green line. This was your time base. Fixed to the outside of the screen was a strip of Perspex, looking rather-like a ruler. This ‘ruler’ measured distances from 0 to 200 miles.

On the screen there’d be a whole lot of blips, looking a real mess. The left side of the thin, green line showed blips like planes beating but which were usually permanent echoes including those of church steeples and of our actual radar station. The first five miles of signals received were, therefore, usually ignored.’

Using the DF (Direction Finder) Jean would go to-and-fro through the time base, swinging the Goniometer to determine mileage and find bearings. The further away the blips, the wider she needed to swing. She’d be aiming to reduce the blips in length, until she could hold them right on the line.

One of the problems of the early Chain Home operations was the system’s inability to direct the transmitter signals, which made it impossible to establish if the tracked aircraft was flying at the calculated distance in front the radar chain, meaning over the sea, or behind it - inland. Early in the war, this caused a lot of confusion and unfortunate incidents. Jean describes the solution to this problem.

‘You needed to know whether, having sent a signal out, the returning blip was coming from inland or from out to sea. Only blips from out to sea needed to be passed to Stanmore. This is where the sensor button came in. You pushed the sensor button and watched. If the blip was coming from inland the signal on the screen would grow smaller. If it was coming from out to sea it would intensify.’

There was also a lot of working out on paper and there was what eventually proved to be a hit or miss formula for determining the height of a plane that you were plotting.

‘You established its range, multiplied that by 100, and then added two-thirds of the actual range squared. This calculation was meant to take into account the curvature of the earth.’

Radar operations were conducted under the veil of strict secrecy. In the case of invasion, operators were given revolvers. Should the station be endangered by the enemy, they were to fire the revolver at the screens, rendering them useless. In the beginning, each radar station also had an acid bath – a lead, acid filled tank kept close to operations. In the event of invasion all maps and other sensitive material was to be thrown into it.

On one occasion, Jean decided to try the system out. She threw some obsolete documents into the tank. The papers went in white with black lines on and came out dark-greyish with clear white lines on. The system was found wanting by testers, and subsequently removed from all stations.

The huge CH aerial masts were clearly visible even from large distances, attracting the attention of German intelligence already in the years preceding the war. Surprisingly, the Germans did not discover their true purpose until 1940 but once they did, several attempts we made to bomb the RDF stations from the air. Pevensey was attacked this way on 12 August 1940 and temporarily put out of action. Fortunately, it took only 6 hours to bring it back on the air again.
[Tc7 via Flickr]

Tracking the Luftwaffe

Jean had to watch the early type RF5 screen with a hairy RAF blanket thrown over her head as the cathode ray tube was weak and therefore hard to see.

On her very first day of operation, in 1940, at Pevensey, she saw on her screen a large number of what were probably Heinkels and Dorniers. She informed her (male) supervisor and suggested he take over but he said: ‘It’s Your Operation, you do it.'

At this early time there was no pressing of buttons. You worked out your own plots. It was hard to count the exact number of planes as they were all coming together and going up and down – which is called beating. You don’t see a separate blip for each one when there’s a lot bunched together.

Jean counted the number of aircraft as an unprecedented 200 plus. She and her supervisor reported the sighting to Stanmore, which could hardly believe Jean’s figures. Could they be dealing with migrating birds?

The plotter in the filter room at Stanmore said she would have to seek verification from her scientific observer who was looking down on the plotting from the window above. Jean was proved to be right. Not birds but planes. Nearly 200 enemy planes.

In those early days at Pevensey you were able to speak to the pilot. Both the operatives and the pilots used nicknames. One character who was often called upon at that time was Blood Orange Smith. Blood Orange was meant to be a reconnaissance pilot but often got bored and went after the enemy. He always came back safely.

You had to plot outgoing friendly planes as well as incoming enemy ones. Distinguishing between the two was another difficulty inherent of the early CH operations, but by the time of the Battle of Britain, RAF combat aircraft carried special ID system called IFF. Jean explains:

‘If we were sending up fighters into the heart of the battle you could be plotting one of your own planes believing it was the enemy, so you needed an ID to establish that they were friendly. This was called IFF – Identification, Friend or Foe.’

At a later point, Jean returned to Bawdsey in an operational capacity. At this time Stanmore decided that their own personnel would benefit from knowing where their plots were coming from and arranged a two-week exchange. Thus my sister found herself seated at the famous board in Stanmore’s filter room. She remembers:

’There was a Halma Clock on the wall. Its face was divided into colours. Time was divided into colours. There was never more than 2-1/2 minutes of plotting on the table at one time because everything was moving so quickly.

A number of people looked down at the plotting from the windows above. The scientific observer, of course, plus VIPs, the BBC, representatives of the Army and Navy, members of the Observer Corp and, frequently, Lord Dowding himself wearing full uniform and, I was told, his carpet slippers.’

Jean still has a photograph of Betty Smith, the girl she made the exchange with, who she believes went on to become a film actress.

Radar improvements

As the war progressed, many modifications and improvements were made to the original equipment, although its basic modus operandi remained the same. In due course Jean moved onto the more sophisticated RF7. This had a vastly improved cathode ray tube that allowed her to throw off the hairy blanket and watch her screen in a more convenient way. Jean still has a drawing that she made of an RF7.

Schematic drawing of the RF7 radar receiver from Jean's personal notebook
[Jean Semple]

Illustration of the RF7 receiver unit from the contemporary RAF handbook makes for an interesting comparison with Jean's drawing.
[Crown Copyright]

With the newer equipment you could provide a signal that an Allied pilot could switch onto if need be. For instance, in the event that he had broken away from the squadron and lost his bearings. This signal could only be used as a last resort as it could be picked up by the Germans.

In early RF7 days, at one station, 11 and 12 Group operated a system called Spotted Dog. Set next to the observer was a large console about four-foot-square covered with Perspex. Jean remembers:

‘Laid out on the console under the Perspex was our section of a map of Britain. The Perspex itself was marked with an ordinance grid reference. The console had a gadget, connected to a range finder. This gadget was an illuminated spot the size of a farthing.

Once the operator had an optimum reading they would stop the gadget. A person standing beside the operator with a clipboard wrote down time, seconds and plot. Another person turned to the console and marked the position on its ordinance grid with a grease pencil. Meanwhile, the observer made speech contact with the filter room at Stanmore.

All the plots recorded on the clipboard were later transferred to another map. The idea was to see where, in principal, all the dots were collecting and where they were not. In the end you had big patches of dots and other areas where there was nothing. You could draw a line round the big patches of dots, like a balloon. These were called lobes. The areas remaining would be the blind spots that Jerry could take advantage of. '

Connected to the Radar Station’s four aerials was the Parent Unit, the Plan Position Indicator. The PPI’s parabolic reflector resembled our modern satellite dishes. There was a little tube in the centre of it, which was hollow. No one seemed to know why it worked, but it did. The PPI was connected to a cathode ray tube that produced not a thin line but a wide sweep. This system adequately covered the lobes and the vulnerable gaps between the lobes. In due course a curtain array was, including 100-foot dipoles that could be energised at will.

Another procedure to ensure accurate observing and plotting was by use of a theodolite. A rigger would climb the 340-foot steel aerial with the theodolite. Meanwhile, an autogiro would be flying at a pre-determined height and bearing. The theodolite then took the true reading of the height and bearing of the autogiro. Its results would be picked up on the CH cathode ray tube and compared with the less accurate Goniometer plot. The discrepancy would be declared a percentage of a pure rose, and would be drawn as, for instance, a 90 to 95% distorted rose and allowances could then be made for the differential. After a while, this work became automated with the help of RAF General Post Office equipment and personnel working from a sub-station.

By 1943, the bomber offensive was in full swing and with it, the aerial operations intensified beyond recognition. Jean remembers:

‘As war progressed we were sending up 1000 bombers at any one time and they all had to be plotted. The plotting was enormous for 1000 bombers so we had to divide our maps into four. Taking each quarter you tried to give four readings, an inner and an outer reading both of bearing and of range. We’d start with a mean range, appearing like a range of mountains, with inner and outer edges and four corners. Sometimes the planes were miles apart.’

Jean (Sally) Semple achieved the rank of Sergeant and had the chance to go further, having been twice recommended for a commission. Marriage intervened, however, and she left the service with a Clause 11 discharge – pregnancy. Her friendships with fellow Radar Operators continued into civilian life with her closest colleague, Joan Pile, dying only a short time ago. Jean and her husband have recently moved from the Welsh countryside to a pleasant town house close to leisure amenities.

Jean would like me to end this account with a question: ‘On every watch, a civilian male would sit in the corner of the operations room wearing a set of headphones. He never spoke or wrote anything down. He was known as “The Headache.” Does anyone know what he was doing?’

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