During the summer of 1940, Bentley Priory was probably the single most important building in the whole of the United Kingdom. A former country house in Stanmore, north London, it was the headquarters of the Royal Air Force’s Fighter Command, which was led by Air Chief Marshal Sir Hugh Dowding, who since 1936 had been refining a system to protect the country from aerial attack.

The Battle of Britain from July to October 1940 proved that his secret air defence strategy, known as the Dowding System, worked. Improved and refined, it continued to thwart the enemy during the Blitz of 1940-41 and the V1 and V2 rocket attacks in 1944-45.

The principles used in this radar-based early warning system are employed in our defence systems today but now computers do the analysis that in WW2 was handled mainly by a small group of young women in the Women’s Auxiliary Air Force, which had been formed in 1939.

A stained glass window at Bentley Priory celebrates their previously overlooked role by showing a WAAF in the Filter Centre, which was the pivotal heart in the Dowding System. Among the WAAF veterans who served in the Filter Centre at Bentley Priory and elsewhere during the war are Patricia Clark (nee Robins) and Eileen Young husband (nee Le Croissette), who put on the WAAF uniform in 1940 and 1941 respectively, when both were just 19.

Virtually every movie about the Battle of Britain or the Blitz includes images of young WAAF personnel pushing around markers in RAF Operation Rooms, but until very recently it was almost unknown to the public that the information used by the Ops staff had been received, analysed and collated at great speed in neighbouring rooms called Filter Centres.

“If it wasn’t for the girls in the Filter Centres, the Ops Room wouldn’t have known where the German planes were and wouldn’t have been able to tell the pilots,” insists the sprightly and engaging Patricia Clark. “The main reason people don’t know about what we did is that we all had to sign the Official Secrets Act and that restriction was not lifted until 1975.”

More than seven decades on, Eileen Young husband, who has become an eloquent figurehead for her WAAF colleagues, is equally indignant. “What we did in the Filter Centres has been a bigger secret than what was done at Bletchley Park,” she maintains.

Victory in the Battle of Britain and other air battles over the United Kingdom during World War Two immediately conjures images of the bravery and fortitude of The Few, but the reality is that victory was delivered by an extensive system that integrated new technology, processes communications and a band of highly skilled RAF and WAAF personnel. In 1940 the system enabled The Few in the air and the Anti-Aircraft Artillery on the ground to overcome a numerically much stronger enemy. The success of the Dowding System during the Blitz meant that Hitler’s Operation Sea lion, his planned invasion of Britain, came to nothing.

In 1932 the British Prime Minister Stanley Baldwin infamously declared that “the bomber will always get through”. This widely held view of the indefensible threat of modern aerial attack was challenged by Sir Hugh Dowding, who in the three years before World War II conceived and organised the development of a unique integrated air defence system. Born in 1882, he was approaching retirement age, but he adopted a maverick stance against many of his superiors and political masters, certain in his view that Britain’s only hope was to keep its meagre air force concentrated on the home front to defeat the expected German onslaught from the skies.

Observation, communication of information, analysis and speed of response were the vital elements of his plan. He was one of the first to recognise the potential of radar detection. His system relied on a ring of radar stations on the British coast that had been set up in the mid-1930s by Robert Watt-Watson, a radar pioneer working for the British Air Ministry. These stations were the starting points of what became known as CH or Chain Home.

Radar detections from a single position were imprecise 70 years ago. By coordinating the overlapping readings from neighbouring stations, a much more accurate assessment of the position and altitude of incoming aircraft — and, importantly, of the number of aircraft -could be made. It was this sort of plotting analysis at which Patricia, Eileen and their WAAF colleagues in the Filter Centres became very skilled.

Human observation, especially by the Observer Corps, was important to fill in the gaps between radar stations, especially in the early part of WW2 when radar coverage was directed mainly out to sea. (The Observer Corps was awarded the prefix “Royal” by King George VI in April 1941 in recognition of its contribution during the Battle of Britain.) As the war progressed, systems using different radio frequencies were better at tracking aircraft over land. Airborne RAF planes also contributed information and, on the ground, bilingual personnel, including WAAFs, listened in to the wireless chatter of German aircrew.

This mass of information needed to be sent, received, analysed and processed very quickly if it was to be useful. Dowding put hundreds of telephone lines underground (to be safe from bombing) to enable the information to flow between the radar chain and observer groups, RAF Bentley Priory and the other Fighter Command units.

Under the Dowding System, the country was divided into geographical areas, each of which was covered by a Fighter Command Group. These areas in turn were subdivided into several Sectors with a Sector station controlling two or three airfields.

During an enemy attack, details of incoming aircraft — and, importantly, of friendly aircraft — were sent by the Chain Home stations to the Filter Centre at Group HQs. Once collated, assessed and plotted on a large table-top map, this information was passed to the Group Operations Rooms and thence to Sector Ops Rooms, where the Controller made the decision when to scramble the fighters. He and his deputy controllers provided pilots with coded courses to vector, or guide, the fighters to the target. The order to “scramble”, together with the codename for the area to be patrolled, was relayed by telephone to dispersal huts at the fighter airfields.

Under the integrated Dowding System, the information from the Filter Centre was disseminated to other key parties involved in the aerial combat, such as anti-aircraft batteries, air raid authorities, Balloon Command, and after it was formed later in the war, the Air Sea Rescue service.

In the early days of WW2, it was not thought the WAAF personnel would be able to handle as efficiently as the men of the RAF the calculations and pressure of the Filter Centre operations. The pool of trained RAF staff was small and there were increasing demands for personnel to operate systems overseas. It was decided to give women a chance and this proved to be the correct decision because they acquitted themselves wonderfully.

“With our smaller hands, we were able to place the small counters that represented hostile planes and friendly aircraft much more dexterously than men. We learnt very fast to be quick,” says Patricia.

On the recommendation of a friend who was working in a Filter Centre, Eileen Young husband told her recruiting officer that she was good at mathematics and wanted to be a Clerk Special Duties, the code used for the WAAF plotters. The recruiting officer was astonished that she had even heard of the term, as it was so hush-hush. Seventy years on, Eileen explains why the analysis done at speed in the Filter Centre was so important to achieve the effective use of the RAF’s very limited resources: “Quite simply, during the Battle of Britain and the Blitz, we didn’t have enough planes, pilots or fuel. The Spitfires and Hurricanes had fuel to keep them in the air for just under an hour, so the very earliest they could intercept was about 30 minutes from their airfield. The trick was not to get the planes up too early, but to get them to the right area at the last minute. That’s why all our calculations, our intersections of the arcs of incoming planes’ paths, our predictions of where they were going to be and when, were so important.”

The central work area of a Filter Centre was the table on which was a map covering the Group’s area. Most of the map, Eileen recalls, showed the area of the sea as it was over the water that the early radar located airborne activity. Each radar station in the area was assigned to a WAAF known as a plotter, who communicated with the station through a headset and large mouthpiece that curved upward from the chest. As the information — type of aircraft, number of aircraft, direction of aircraft, altitude, friend or foe — came over the telephone lines the plotters laid down markers on the table. The markers, of different shapes and colours, carried different letters and numbers that represented different information.

This information was simultaneously assessed and collated into simple arrowed tracks on the map by a Filterer Officer, who was free to walk round the table and to «filter» the information being laid down by the plotters. The often-frantic activity on the table — Eileen recalls that it was normal that 15 or more plotters would be working shoulder-to-shoulder — was supervised from a balcony on which sat a Filter Officer (who from 1941 was usually WAAF), a Filter Room Controller (who was always RAF) and WAAF «tellers», who passed on by phone the key elements of the plotting to the nearby Ops Room.

During the Battle of Britain, the Filter Centre at Bentley Priory was the only one in the country — the success of the system during the summer of 1940 led to its expansion. A recreation of the Filter Centre will be a feature of the new museum.

By coincidence, although at separate times, Patricia and Eileen were both first posted to 10 Group, which covered the south-west of England and south Wales. Their paths crossed later in the war. At RAF Rudloe, 10 Group’s HQ near Bath, Pat’s first Filter Centre in late 1940 was in a former cowshed — it was later relocated underground.

This was symptomatic of the make-do-and-mend reality of the early years of the Second World War. The Dowding System used the most advanced technology of the day — radar — yet even at Bentley Priory to modern eyes the construction of the balconies in the Filter and Ops Centres and the technical infrastructure was somewhat Heath Robinson.

As well as analysing and plotting enemy aircraft, Filter Centres had to identify all aircraft operating out to sea to prevent fratricide and enemy forces creeping in under cover of returning friendly aircraft. This was achieved electronically by an aircraft sending a signal to the CH radar -although not all aircraft were fitted with this equipment early in the war — and by all friendly movements being notified to Filter Centres. Identification in itself was a complex task. Early in the war it was realised that an organisation to rescue downed aircrew at sea was necessary, so the Air Sea Rescue Service was formed in 1941 following the experiences of the Battle of Britain. The Filter Centres supplied much of its information.

“This was very important because quite a few planes went down in the drink,” Patricia Clark recalls. “If we tracked one losing height, we were able to alert the maritime rescue services and often pilots were surprised to find a boat waiting for them when they ditched.”

The seriousness of their work throughout the war was made painfully obvious to the WAAF personnel by the alarming fatality rates among aircrew.

Eileen Young husband joined up after a favourite cousin, who was in the RAF, was killed on a training mission. In 1943 the crew of a stricken Lancaster bomber who had been fished out of the Channel was brought to a Filter Centre to discover how their plane had been pinpointed so accurately. Patricia Clark became engaged to one of the visitors, an Australian navigator called Ken Lyons. He was killed on an air raid over Germany in 1944, aged 26. She had already lost two cousins who were pilots in the Battle of Britain. “I wouldn’t say you got used to it, but death was very commonplace,” she says today.

Talking to impressive women like Patricia and Eileen today, it is difficult to imagine the sexist prejudices of the times that they and their fellow WAAF personnel overcame. Patricia recalls all her comrades were her age: “I can’t remember any of the plotters or Filterer Officers being older than their mid-20s at the most.”

They were determined to show that they could handle the technical tasks that RAF men were doing. Although the WAAF had been established in June 1939 when war seemed likely, it was not until 1941 that women working in technical roles, such as in the Filter Rooms, were commissioned. Patricia Robins finished the war as a Flight Officer and Eileen Le Croissette as a Section Officer.

The Filter Centres were “manned” by WAAF personnel 24 hours a day, with the telephone lines to the radar stations kept open permanently. Shifts were eight hours long. At times of heavy enemy activity, such as the Blitz or during the V1 and V2 raids, Filter Centres were places of intense activity. At other times, however, when bad weather prevented flying, the WAAFs caught up with their reading, knitting and sewing. Patricia began writing stories for women’s magazines during these quiet shifts.

Winston Churchill’s comment that: “Never in the field of human conflict was so much owed by so many to so few” has passed into legend, but he also wrote in his six-volume history, The Second World War, that: “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 the like of which existed nowhere in the world.”.

This “most elaborate instrument of war” was the Dowding System.

Says Patricia Clark: “I am now 92 years of age and my fellow WAAFs have waited a very long time for recognition of the vital part we Filter Centre Plotters, Filterers and Filter Officers played. We were totally dedicated to our work and proud to be considered more than capable and well able, as was doubted, to replace the men who were needed for active service overseas.”

Eileen Young husband adds: “It is my firm belief that the Filter Centre was the linchpin of the air defence of Britain and it is time this was celebrated. Many Filter Centre personnel have been puzzled and sometimes upset that their vital work under difficult conditions was never acknowledged. At last the part they played in the Dowding System is now being recognised.”

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