D-Day and Decca
06 June 2019
Posted by: Thomas Wakelin
Decca Navigator - History
David S. Jones, a former Decca employee, provides this very fitting introduction to the Decca Navigator system
"I have been struck by one fact that is perhaps unique to Decca amongst the hyperbolic systems featured in this web page. The fact is that Decca was proposed, developed, designed, promulgated, manufactured, sold-by, operated, maintained and supported by one company during its entire fifty year life.
All of the other systems were developed as part of a quasi-government research program and subsequently farmed out to various contractors for manufacture; leaving their deployment and support in the hands of some government department. Typically an armed forces agency or the like was tasked with system operation and staffing. Since these agencies just looked upon their task as budgetary administration, no continuous bond developed between the product and it’s keepers. Routine changes in personnel and re-assignment of government agency responsibilities would only serve to break any continuity bond. All of these agency staff would only consider their time spent with these systems as part of a routine assignment, however dedicated the individual. Hardware manufacturers would likewise have a similar transitory relationship with the system.
Not so with Decca, which maintained a closely bonded group of developers and engineers for many years, all of who played a part in the history of the system. Even Bill O’Brien, the systems founder stayed active in the company for almost forty years after he first joined. This common pool of knowledge, stories and pioneering spirit is something which I believe sets Decca apart when compared with others."
The difficulty of navigating a minesweeper across the English Channel and making a precise landfall at night was considered impossible without some form of precise radio navigation. Established on the south coast of England, Decca began transmitting on the day before the D-Day invasion force landed. Had this radio navigation aid not been available, it is now believed that D-Day would have followed a completely different plan. How did this all came about?
The Decca Navigator system found its origins in the United States but was later developed into an operational system by Decca Radio and Television Ltd. of London. Originally it was conceived by an American, W (Bill). J. O'Brien as a method of measuring the ground speed of aircraft undergoing trials and was simply named 'Aircraft Position Indicator'. Without knowledge of the patents of Harms or Honore and without even being aware of developments in the US by Shanklin, Donnelly, and Holmes, O'Brien worked on the system independently from 1936 to 1939. He was unsuccessful in raising any interest in the US Armed Forces or the civil authorities so the system lapsed until the outbreak of war in 1939. Thereupon he offered the idea to the British Air Ministry through his friend H. F. Schwarz, an American working in London for the Decca Record Company. Neither was aware of the work which was proceeding on pulsed navigation systems in Britain so the idea was rejected by Watson-Watt as being prone to jamming and subject to interference.
O'Brien and Schwarz, with support from Decca, then tested a prototype system in California using a master transmitter at 300 kHz and a slave at 600 kHz. Comparison was made at 1200 kHz and the accuracy of the system was demonstrated in a car. It proved the basic viability of the system and was a major departure from earlier proposals by using harmonically related radio frequencies for transmission. This solved the problems of identification and phase comparison at the lowest common multiple of the carrier waves without needing any sort of modulation. It was a neat solution and had the additional advantages of occupying a very narrow bandwidth and only using low power for the transmissions. It did not however, eliminate the problem of "ambiguity".
The British Admiralty, which started planning the eventual landings in France, had a requirement for an accurate navigational system so it started taking an interest in Decca in 1941. There was also a need for a stand-by navigational system to guard against any possibility of the existing "Gee" system being jammed hence further impetus was given to the development of Decca. Trials were organized off Anglesey in mid-1942 using the same frequencies and equipment that were used in California. This trial was highly successful and resulted in further research with assistance from the Admiralty Signals Establishment (ASE).
Early in March 1943, Decca was given the order to produce 27 receivers plus the driver and phase control units needed for the transmitters. All equipment was delivered by mid-May when the Royal Navy began its training and preparations in earnest. In January 1944, a test of Decca (or QM as it was then known) on new frequencies was carried out in the Irish Sea and it was also compared with the Royal Air Force Gee system for accuracy.
It is important to note that the Gee system was in widespread use at the time. The naval version of Gee (Outfit QH) was first used by the Royal Navy in the Dieppe raid in August 1942 and was subsequently established as a standard system for surface navigation. For Operation Neptune (D-Day), the initial legs of swept channels were planned to coincide with the same lines as the Gee lattice maps. So important was accuracy that some 860 invasion ships were outfitted with Gee Outfit QH.