In 1934 a large-scale Air Defence exercise was held to test the defences of Great Britain and mock raids were carried out on London. Even though the routes and targets were known in advance, well over half the bombers reached their targets without opposition. Prime Minister Baldwin's statement "The bomber will always get through" seemed true.  To give time for their guns to engage enemy aircraft as they came over, the Army was experimenting with the sound detection of aircraft by using massive concrete acoustic mirrors with microphones at their focal points.

Dr H.E. Wimperis, the first Director of Scientific Research for the Air Ministry, and his assistant Mr A.P. Rowe arranged for Air Marshall Dowding to visit the Army site on the Romney Marshes to see a demonstration. On the morning of the test the experiment was completely wrecked by a milk cart rattling by. Rowe was so concerned by this failure that he gathered up all the Air Ministry files on the subject of Air Defence. He was so appalled that he wrote formally to Wimperis to say that if we were involved in a major war we would loose it unless something new could be discovered to change the situation. He suggested that the best advisors obtainable should review the whole situation to see whether any new initiatives could be found. On 12th November Wimperis put this proposal to the Secretary of State and a Committee was set up under Henry Tizard.

The idea of using rays to kill or disable people or machines was very popular, so to start things off Wimperis got Professor Hill to estimate the radio energy needed to cause damage to humans. He sent this to Mr Watson-Watt, Superintendent of the Radio Research Station at Slough for his views on the possibility of developing a radio "Death Ray" to melt metal or incapacitate an aircraft pilot. Watson-Watt passed the letter to A.F. Wilkins who reported that there was no possibility of achieving these destructive effects at a distance but that energy reflected from aircraft should be detectable at useful ranges. This was reported to the first meeting of the Tizard Committee on 28th January and Rowe was instructed to get quantitative estimates for detection.

Wilkins made further calculations from which Watson-Watt wrote a memorandum proposing a system of radio-location using a pulse/echo technique. The Committee gave this a very favourable reception and Wimperis asked Dowding for £10,000 to investigate this new method of detection. Dowding, though very interested, said he must have a simple practical demonstration to show feasibility before committing scarce funds to the project.

For this demonstration Watson-Watt and Wilkins decided to make use of transmissions from the powerful BBC short-wave station at Daventry and measure the power reflected from a Heyford bomber flying up and down at various ranges. Detection was achieved at up to 8 miles and the £10,000 was granted.

A site at Orfordness was chosen to do the detection experiments over the sea. Aerials mounted on three pairs of 75ft wooden lattice masts were installed and detection ranges of 17 miles were obtained. These were rapidly increased to 40 miles by July. Work was done to show how map position and height might be determined and Watson-Watt submitted proposals for a chain of stations to be erected round the coast to provide warning of attack and to tell fighters where to engage the attackers. He suggested that a full-scale station should be built at once, to be followed, if successful, by a group of stations to cover the Thames Estuary and then by a final chain covering the South and East coasts. Construction of 5 stations was authorised and the one at Bawdsey was in operation by August 1936. The others followed shortly after. Plots were to be telephoned to a central operations room and combined with data from the Royal Observer Corps and the radio direction-finding system.

In February 1936 Bawdsey Manor became the centre for the expanding research team and Tizard inspired the RAF at Biggin Hill to investigate fighter control and interception techniques. Their results convinced him that effective interceptions could be obtained against mass raids by day, but not against dispersed attackers at night. He therefore pressed for equipment to go into fighters for them to find and engage targets when positioned within a few miles. Initial tests using a large television transmitter on the ground operating on a wavelength of 6 metres and a receiver in a Heyford Bomber with an aerial between its wheels gave detection ranges of over 10 miles. To get a transmitter into an aircraft and reduce the size of the aerial a much lower wavelength was required. Bowen installed a crude equipment operating at 1 metre in an Anson and in the autumn of 1937 aircraft were detected and also Naval ships several miles away in appalling weather.

From then on Air Interception (AI) and Air to Surface Vessel (ASV) equipments were developed. Further Air Defence Trials showed that better detection of low flying aircraft was needed and Chain Low (CHL) stations were evolved from Coastal Defence (CD) equipments which had been developed for the Army. Gun laying equipments (GL) were developed and also equipments to improve navigation (GEE) and bombing (OBOE) and (H2S).