Acknowledging today's anniversary of Pearl Harbor with some notes from Day of Deceit, by Robert B. Stinnett:
In 1940 the Navy already had a vast network for tracking ships in the Pacific Ocean using their radio signals, and breaking their cryptographic codes. For example, they "traced the Pacific Ocean routes taken by eight of Japan's tankers from October 1 to December 6, 1940." Most of Japan's oil was being obtained in California despite an alleged American oil embargo.  Two of the tankers so traced, the Kyokuto Maru and Shiriya, later became part of the Pearl Harbor strike force, and were radio-IDed again at that time.