The Dowding system

Victory by information

One of the Group Operations Rooms used during the Battle of Britain
(Photo: Author’s own)

The Battle of Britain was the first major victory for the Allies in World War II. After the fall of France and the evacuation of the British Expeditionary Force at Dunkirk (Read our earlier article), Germany’s obvious next step was the invasion of the United Kingdom (Read our earlier article). This, however, required the Luftwaffe to gain air superiority over Britain. German leadership was confident they could achieve that, while the British populace knew it was facing a sore trial. And yet, Britain came out victorious. The Luftwaffe would never fully recover, and the invasion of Britain was shelved forever.
What was the reason behind the victory? It was certainly not numerical superiority: at the height of the battle, the Royal Air Force (RAF) had 749 fighters against 2,550 German planes. It also wasn’t just the courage and skill of pilots, nor the quality of their aircraft. One decisive factor was Britain’s secret weapon: the first integrated air defense system in the world. Simply called “the reporting system” or “the system” at the time, today it’s known by the name of its inventor: the Dowding system, developed by Air Chief Marshall Hugh Downing, the commander-in-chief of Fighter Command, RAF.

Air Chief Marshall Hugh Dowding
(Photo: Imperial War Museums)

The fundamental purpose of the system was to enable a smaller force of fighters to intercept and destroy a larger number of incoming attackers. The central problem was knowing where and when the attackers were going to arrive. If the attacks cannot be predicted in advance, the defenders need to run constant air patrols and hope to catch the bombers by chance, something the RAF did not nearly have enough planes and pilots for. The Dowding system used air defense methods developed during World War I combined with the cutting-edge technology of the time to give the defenders the needed information.
One cornerstone of the system was Chain Home, the world’s first radar defense system. Built at great cost and with no guarantees that it would work as well as hoped, Chain Home was a network of radar stations located along the southern, southeastern and eastern coast of Great Britain. Placed as close to the European mainland as possible, Chain Home could often detect German attacks even as they were forming up above their airfields in occupied France.

Transmitter towers of a Chain Home station
(Photo: Imperial War Museums)

However, the Radio Direction Finding (RDF) system, as it was called, did have its weaknesses. One was that it couldn’t detect low-flying aircraft. A second network, Chain Home Low, was built with a lower detection floor, but even this was not perfectly reliable.

A Chain Home Low station
(Photo: Imperial War Museums)
A second weakness was the unlike modern radar, Chain Home registered reflected radio waves not as discrete dots, but rather a spike in an otherwise straight line on a display screen. Several planes in proximity showed up as a single spike with a few small irregularities. Radar operators were taught how to guess at the shape and size of a bomber formation from these blips, but determining the number of incoming planes remained a guessing game. In fact, different radar towers could detect the same group as having a different size: a tower detecting the formation head-on would only detect the first wave, while another tower “looking” at the formation from its flank, could distinguish between separate waves. Determining altitude was also difficult.
A sketch showing what different formations would look like on the trace

Chain Home’s third major problem was that it could only “see” in one direction, toward the English Channel. Once German formations were over Britain, they were invisible to radar. Dowding turned to the Royal Observer Corps, a civil defense organization, to provide eyes over Great Britain. Established in 1925 but tracing its roots back to World War I, the corps consisted of 30,000 volunteer observers manning 1,000 stations across the country. Members were trained in recognizing various types of enemy and friendly aircraft by sight, and used simple methods such as binoculars and sextants to determine the altitude, speed and direction of planes up in the air.

Two members of the Royal Observer Corps working their plotting instrument
(Photo: Imperial War Museums)

Dowding’s early experiments before the war revealed that the system’s single greatest fault was information overload. All the radar towers and observer stations simultaneously reporting their findings created a flood of information that simply could not be handled. A series of exercises held with 350 aircraft in 1934 showed dismal results: 70% of the planes playing the role of bombers reached London without ever seeing a defender – and that was after the defenders were given bombers’ location in advance. There was a clear and pressing need for some way to transform the torrent of information from radar and observers to data which could be actually used – and it was this method of data management that became the heart of the Dowding system.
The territory of Great Britain (and Northern Ireland) was split up between four active Groups, numbered 11 to 13. 11 Group, which did most of the fighting, was responsible for the smallest area, in the southeast quarter of Great Britain. The territory of each Group was further divided into 4 to 7 Sectors, with each Sector at least one airfield, and possibly several satellite airfields.

Keith Park, commander of 11 Group, with the Hurricane he regularly flew during the battle
(Photo: unknown photographer)

The ballroom of Bentley Priory, a stately home to the north of London, was transformed into the Filter Room, which received most of the incoming information. All RDF stations reported their detections to the Filter Room directly. Coastal Observer Corps stations reported their sightings to their regional headquarters, which then passed on the collated information to the Filter Room, so the latter wouldn’t be overloaded by all the direct reports. Inland Observer Corps stations also reported to their own headquarters, but that information was passed on to the Group Operations Rooms and their subordinate Sector Operations Room, rather than the Filter Room, so as not to overwhelm the latter with information they didn’t need for a large-scale overview.

The Filter Room at Bentley Priory
(Photo: Bentley Priory Museum)
The Filter Room had a large map of Great Britain with a grid overlaid on it. Plotters stood around the table wearing headsets, each connected to a different RDF station. As a report came in, a plotter would place tokens on the map. A circular token represented a detection, identified by a number if it came from radar, or by several dots if it came from the observers. A triangle token displayed a number representing the estimated number of aircraft, and a square token marked the estimated altitude.
Tokens used in the Filter Room
(Image: screenshot from contemporary film footage)
The Filterer had to decide whether two signals were two different groups, or a single one detected by two different radars – if it was the latter, its location could be triangulated more accurately. The Filterer also had to reconcile different estimates of altitude and aircraft number. In order to do this, they had to be familiar with the individual quirks and weaknesses of each RDF tower and each shift manning them. Once an incoming group was spotted twice, the Filterer could finally also determine its course, and lay down an arrow token marking that.
The Controller set on a dais overlooking the map and worked with the Liaison Section to make sure that no friendly flights from Coastal Command, Bomber Command or other service was misidentified as hostile. Controllers were recruited from the London Stock Exchange, whose brokers were accustomed to making quick decisions under pressure.
The Headquarters Operations Room at Bentley Priory
(Photo: Bentley Priory Museum)
Finally, once a raid was identified with an arrow token, three Tellers passed on the information to the Headquarters Operations Room also located in Bentley Priory, the four Group Operations Rooms, and the Sector Operations Rooms all over the country.  The HQ Operations Room was only used to follow the big picture, and to make strategic decisions, but did not give individual orders to intercept flights. Such decisions were made in the Group Operations Rooms and passed down to the Sector Operations Rooms. The only exception was when a raid happened right along the boundary line between two Groups, in which case HQ decided which Group to assign it to.
Plotters from the Women’s Auxiliary Air Force at the table in the Group Operations Room of 11 Group
(Photo: Imperial War Museums)
Group Operations Rooms had their own similar maps, only showing their part of the country, but in greater detail. Once a raid was reported from Bentley, a wooden “raid block” was placed on the map, holding one-inch placards with numbers marking enemy strength and altitude. Once one or several squadrons were launched to intercept the bombers, tiny flags bearing the squadron numbers were added to the block. However, not all raids were answered. One of the tasks of the Group Controller was to decide if an incoming flight was a real attack or a feint designed to draw out the defenders and make them unable to respond to the real bomber force that would show up later. The Group Controller had to rely on his experience, his understanding of German tactics, and his knowledge of what happened in the past. For example, real attacks against the southeast of England were always preceded by a feint toward the city of Kent 30-40 minutes earlier.
A raid block representing a German force of 25 aircraft flying and 20,000 ft (6070 m), being intercepted by No. 72 and 92 Squadrons
(Photo: Daniel Stirland)
Group Operations Rooms also displayed a set of tall “tote boards,” each assigned to one squadron of that Group. Each squadron comprised four flights (Red, Yellow, Blue and Green), and each flight was always in one of several statuses, such as “Released” (and not available to take orders), “Available in 30 minutes,” “Enemy sighted, “Ordered to Land” and others. Lights would turn on and off to show which flight was in which phase of operation. This helped the Group Controller to decide which squadron (and how many squadrons) to send up for an interception. Group Operations Rooms, along with Sector Operations Rooms, also informed anti-aircraft guns, barrage balloons and searchlights in their area of ongoing developments.
Tote boards at a Group Operations Room
(Photo: Warbird Tails)
Once the decision to intercept was made, it was passed down to the Sector Operations Room located in the sector of the squadron’s airfield. It was the task of the Sector Operations Room to not only keep track of German formations but also the interceptors, and to guide the latter to their targets. Sector Commanders were normally former pilots, either retired or on medical leave, since anyone else giving orders would have been likely ignored as a “paper pusher” by the pilots up in the air. Guiding interception squadrons was far from trivial, since a squadron up in the air had to rely on their eyes and radio messages, and the information on the map was always a few minutes out of date. Keeping track of the interceptors’ exact position was made easier by the “pip-squeak” transmitter located in each plane. The transmitter’s signal was received by several radio towers in the sector, allowing for the position to be triangulated.
Sector ‘G’ Operations Room in Duxford, Cambridgeshire
(Photo: Imperial War Museums)
Another challenge in the Sector Operations Room was determining an intercept path. Before the war, complex calculations were used to determine the optimal flight path, but these proved cumbersome to use. During a 1936 exercise, one Wing Commander Eustace Grenfell became so frustrated with the method that he decided to just eyeball it, and ended up directing a perfect interception. His instinctive method was later formalized as the Tizzy Angle, named after Henry Tizzard, head of the Committee for the Scientific Survey of Air Defence. It was a simple method: the line between the interceptors and the raiders is the base of an isosceles triangle. The path of the raiders is one of the legs. Draw in the other leg, and have the interceptors follow that until the two legs meet. This practice allowed for a 90% intercept rate as long as the bombers’ altitude was known and constant.
A visual explanation of the Tizzy Angle
One feature present in all Operations Rooms at all levels was the sector clock, a clock whose circumference was divided up into five-minute sections by red, yellow and blue triangles. Whenever a token in the Filter Room or a block in an Operations Room was placed on the table, its color matched the color of the current five-minute interval. This allowed observers to quickly determine when a token or block was placed. The sequence of colors repeated every fifteen minutes; old markers whose color just came into use again were removed to avoid clutter and out-of-date information.
A sector clock with color-coded five-minute segments
(Photo: Martinvl / Wikipedia)
The Dowding system not only allowed for the collation and compartmentalization of information, but also did it quickly. This was an absolute necessity, since every wasted minute meant that interceptors could travel 4 miles (6.4 km) and climb 2,000-2,800 ft (610-853 m) less before the bombers dropped their load.
Two Canadian pilots scrambling for their Hurricanes during the Battle of Britain
(Photo: National Air Force Museum of Canada)
The system didn’t only employ the newest technology available, but was also excellent at sorting and organizing the information provided by that system: every level of command only received the information it needed, and tactical decisions were pushed as far down on the chain of command as possible. Interception rates between 30 and 50% were considered excellent before the war. During the Battle of Britain, the system achieved an average rate of 90%, previously undreamed-of. It even went down in the annals of science: physicist Patrick Blackett’s work on the Dowding system became the foundation of a field of mathematics called operations research.
Patrick Blackett, whose work with Dowding created a new field of mathematics
(Photo: Nobel Foundation)
Sir Hugh Dowding’s work on the system was undoubtedly the greatest contribution to the Battle of Britain by a single person. However, he was denied the acclaim he deserved, as he was forced to relinquish command in late 1940. This was ostensibly due to his inability to counter German nighttime bombing raids in the Blitz that followed the Battle of Britain, but most historians agree that this was only a pretext; airborne radar, the technology needed intercept night raids, was simply not available at the time. Dowding was a sincere and strong-willed man who ruffled many feathers during his tenure at the head of Fighter Command. One particularly bitter disagreement was between him and proponents of the “Big Wing” strategy, which called for a small number or large interceptor groups rather than the dispatch of small squadrons piecemeal. It was perhaps no coincidence that his replacement was a Big Wing advocate. It’s also been suggested that even though Dowding did everything possible against night raids, somebody still had to take the fall to appease the public. It is telling that a book published about the Battle of Britain in 1949 literally did not mention Dowding’s name at all.

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