In 1936 William J. O'Brien, an engineer, contracted tuberculosis which 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 Navy considered the idea too complicated and work ended in 1939.