Editing Decca Navigator System (section)

=== Origins ===
In 1936 William J. O'Brien, an engineer, contracted [[tuberculosis]] that put his career on hold for a period of two years. During this period he had the idea of position fixing by means of phase comparison of continuous wave transmissions. This was not the first such system, but O'Brien apparently developed his version without knowledge of the others, and made several advancements in the art that would prove useful. He initially imagined the system being used for aircraft testing, specifically the accurate calculation of ground speed. Some experiments were carried out in California in 1938, selecting frequencies with harmonic "beats" that would allow for station identification in a network of transmitters. Both the [[U.S. Army]] and [[U.S. Navy|Navy]] considered the idea too complicated and work ended in 1939.{{sfn|Blanchard|1991|p=301}}

O’Brien's friend, Harvey F. Schwarz, was chief engineer of the [[Decca Records|Decca Record company]] in England. In 1939 O’Brien sent him details of the system so it could be put forward to the British military. Initially [[Robert Watson-Watt]] reviewed the system but he did not follow it up, deeming it too easily jammed (and likely due to the existing work on the [[Gee (navigation)|Gee]] system, being carried out by Watt's group).{{sfn|Blanchard|1991|p=302}} However, in October 1941 the British Admiralty Signal Establishment (ASE) became interested in the system, which was then classified as ''Admiralty Outfit QM''. The first marine trials were conducted between [[Anglesey]] and the [[Isle of Man]], at frequencies of 305/610&nbsp;kHz, on 16 September 1942.{{sfn|Blanchard|1991|p=301}}

Further trials were conducted in the northern [[Irish Sea]] in April 1943 at 70/130&nbsp;kHz. It was decided that the original frequencies were not ideal, and a new system using a 14&nbsp;kHz inter-signal spacing was selected. This led to the common 5, 6, 8 and 9''f'' frequencies, used throughout the life of the Decca system. 7''f'' was reserved for a [[Loran-C]]-like extension, but never developed.{{sfn|Blanchard|1991|p=302}}  A follow-up test was carried out in the [[Irish Sea]] in January 1944 to test a wide variety of upgrades and production equipment. By this time the competing Gee system was known to the Admiralty and the two systems were tested head-to-head under the code names QM and QH. QM was found to have better sea-level range and accuracy, which led to its adoption.{{sfn|Blanchard|1991|p=302}}