The Dowding system was the world's first wide-area ground-controlled interception network, controlling the airspace across the United Kingdom from northern Scotland to the southern coast of England. It used a widespread dedicated land-line telephone network to rapidly collect information from Chain Home (CH) radar stations and the Royal Observer Corps (ROC) in order to build a single image of the entire UK airspace and then direct defensive interceptor aircraft and anti-aircraft artillery against enemy targets. The system was built by the Royal Air Force just before the start of World War II, and proved decisive in the Battle of Britain.
The Dowding system was developed after tests demonstrated problems relaying information to the fighters before it was out of date. Air Chief Marshal Hugh Dowding, commander of RAF Fighter Command, solved the problem through the use of hierarchical reporting chains. Information was sent to Fighter Command Headquarters (FCHQ) central filter room at Bentley Priory and used to prepare a map of the battle. Details of the map were then relayed to the Group and Sector headquarters, where operators re-created the map at a scale covering their area of operations. Looking at the maps, commanders could make decisions on how to employ their forces quickly and without clutter. Instructions were relayed to the pilots only from the squadron's sector control rooms, normally co-located at the fighters' operating bases.
The Dowding system is considered key to the success of the RAF against the German air force (Luftwaffe) during the Battle of Britain. The combination of early detection and rapid dissemination of that information acted as a force multiplier, allowing the fighter force to be used at extremely high rates of effectiveness. In the pre-war period, interception rates of 30% to 50% were considered excellent; that meant that over half the sorties sent out would return without having encountered the enemy. During the Battle, average rates were around 90%, and several raids were met with 100% success rates. Lacking their own direction system, Luftwaffe fighters had little information on the location of their RAF counterparts, and often returned to base having never seen them. When they did, the RAF fighters were almost always in an advantageous position.
Although many histories of the Battle of Britain comment on the role of radar, it was in conjunction with the Dowding system that radar was truly effective. This was not lost on Winston Churchill, who noted that:
Development
Previous systems
To counter air raids on London during World War I, Edward Ashmore constructed a system known as the London Air Defence Area (LADA). Ashmore put defensive weapons into three rings around the city, searchlights and anti-aircraft artillery in the outer ring, fighter aircraft in the middle ring, and the innermost ring in the city contained more anti-aircraft guns. Ashmore set up a large plotting table at Horse Guards in London. Information from spotters was relayed to this central room where wooden blocks were placed on a large map to indicate the location of aircraft and other information. Observers around the map could relay this information to one of 25 regional control rooms, who re-created the portions of the map relevant to them and passed the information to the various weapons in their region.
After the war, LADA was run by a War Office department known as Air Defence of Great Britain (ADGB). ADGB was responsible for the defence of the entire British Isles during the 1920s and 30s. LADA became the Metropolitan Area, and ADGB expanded on Ashmore's system to deal with longer ranged and faster moving aircraft. Coloured markers matched coloured areas on a sector clock marking off five-minute intervals. As plots were called in, the plotters used markers with the colour pointed to by the clock, which produced multi-coloured paths of markers on the table making easily followed tracks; the freshness of the information could be determined from the colour of the latest marker. An identical system was later set up in The Midlands.
The system lacked early detection, which was deemed essential as the performance of aircraft improved. Experiments with acoustic mirrors and similar devices were carried out but these always proved unsatisfactory, with detection ranges often as low as even in good conditions. Lacking an alternative, in December 1934, the Air Ministry (AM) made plans to deploy sound detection devices around London as part of the Thames Estuary plan.
RDF
A month after the Thames Estuary plan was approved, the Committee for the Scientific Survey of Air Defence (known also as the "Tizard Committee" after its chairman, Henry Tizard) was formed to consider newspaper stories and other claims made over the years of electronic "rays" that could stop an airplane engine and even kill a pilot at long range. The committee consulted well-known radio expert Robert Watt for a judgement on the reports. Watt asked his assistant, Arnold Wilkins, to carry out the necessary calculations. Wilkins quickly concluded that it was impossible; the amount of radio energy needed would be well beyond the state of the art of existing electronics. When Watt asked about alternatives, Wilkins recalled a General Post Office (GPO) report on aircraft causing fading in radio reception, and suggested that this effect might be used to detect aircraft at long range. The two wrote a joint memo on the concept and returned it to the Tizard Committee in time for their first formal meeting in late January 1935.
The committee seized upon the concept as a potential solution to the problem of bomber detection and the fear that "the bomber will always get through". A second memo with more detailed calculations arrived in February and was shown to the Air Member for Supply and Research, Hugh Dowding. He was impressed with the concept but wanted a practical demonstration. This was carried out on 26 February 1935 in what is now known as the Daventry Experiment, a lash-up using existing equipment clearly indicated the presence of a nearby aircraft. Dowding immediately released funding for development. By the summer of 1935 the system, code-named "RDF", was able to detect bomber-sized targets at ranges of . Plans were made to build a chain of RDF stations at about intervals along the English coast in a system called Chain Home (CH).
On 27 July, Henry Tizard suggested running a series of experiments of fighter interception, based on an estimated fifteen-minute warning time that RDF would provide. A seven-month-long series of tests began in the summer of 1936 at RAF Biggin Hill, under the direction of Wing Commander Eustace Grenfell (the officer commanding the station) aided by a navigation expert, Squadron Leader Robert Linton Ragg. Gloster Gauntlet fighters intercepted virtual aircraft, civilian airliners and then Bristol Blenheim light bombers.
The first interceptions were calculated using trigonometry and mechanical calculators but eventually Grenfell had enough of the "confounded machines" and directed a perfect interception by eye. Tizard introduced the equal angles method of rapidly estimating an interception point, by imagining the fighters and bombers at the opposite corners on the base of an isosceles triangle. The bombers were flying at a particular angle to the base and the controller sent the fighters along the opposite angle, to converge with the target at the apex of the triangle. With this "Tizzy Angle", interception rates shot up and by the end of 1936 they were consistently above 90 percent when the altitude was known and did not change. If the bombers changed their altitude, or the fighters arrived below them, manoeuvring for the attack reduced the success rate to about 60 percent.
Teething trouble
ADGB was disbanded in 1936, its duties were handed to the Air Ministry, and divided into Fighter Command and Bomber Command. Dowding was promoted to Air Officer Commanding-in-Chief of Fighter Command on its creation on 6 July 1936.
The first operational CH station was set up at the radar researchers' new laboratory at Bawdsey Manor. In spite of rapid progress, RDF still had teething problems. On one occasion, Dowding was watching the displays of the test system for any sign of the attackers when he heard them pass overhead, a complete failure. These problems were remedied over the next year, and in April 1937, tests using the prototype CH radar at Bawdsey demonstrated great promise. This version also allowed the controllers to measure the altitude of the aircraft to within .
To increase warning time, the CH systems were built close to the shore facing out to sea, placing them as close to the enemy as possible. This meant they provided no information on the location of friendly fighters on the way to the attack, or of enemy aircraft once they crossed the coast. For these areas, Dowding planned to rely on the existing Observer Corps (OC) system that Fighter Command inherited from ADGB.
Using different equipment, the reports from CH and the OC were sometimes contradictory, and the volume of information supplied by the national network was overwhelming. An added problem was the lack of identification of the aircraft; CH could not discriminate between friendly and hostile aircraft, and the OC could not discriminate between friendly and enemy fighters when they were flying at high altitude.
To address the problem of identification, Dowding pressed for a supply of high-frequency direction finding (huff-duff) sets, which could locate the fighters using signals broadcast from their existing radio sets. This led to the introduction of the "pip-squeak" system that was widely available from early 1940, along with the first identification friend or foe (IFF) transponders which were available in some quantity by October 1940. This solved the identification problem, but meant that there were now three sources of information, RDF, OC and huff-duff, none of which had a complete picture of the airspace.
Building the system
Dowding recognized that the main problem was not technical but too many sources of information, none of them with complete coverage, and none able to report useful information to the fighters on their own. Dowding, Tizard, and mathematician Patrick Blackett, another founding member of the committee, began evolving a new system that inherited concepts from ADGB.
To handle the mass of data and the potential for overlap, Dowding instituted a policy of sending all location reports from the radars to a new fighter direction centre at Fighter Command headquarters. This mimicked the system the OC had already set up for coordinating the reports from many individual spotters into a map covering a larger area. The new system did the same at a far larger scale, all of the UK. Telephone operators in contact with the CH and OC centres relayed reports to workers around the map, who used these reports to place small coloured markers on the map. The markers formed lines which, as they elongated, indicated the location and direction of the targets. As they were identified, each cluster was assigned a number, its "track".
As the paths of the targets became clear over time, operators observing the map picked tracks heading towards the various fighter groups and forwarded the information to the group headquarters. The group HQs used these reports to recreate a portion of the master map containing just the tracks relevant to them. Commanders at the group headquarters could easily picture the battle in their area and assign fighters to targets. Targets assigned to squadrons within a sector were forwarded in this same fashion to the sector headquarters, which recreated the map at an even larger scale, using this map to guide their fighters.