Extremely Low Frequency (ELF) for Anti-Submarine Warfare (ASW)

A submarine immersed in seawater behaves like a short-circuited galvanic cell which, because of variations in resistance (as a function of propeller shaft angle) between the rotating propeller shaft and its bearings, may produce an electric current that is modulated at the shaft rate. Corrosion currents arise from differences in electrical potential among the interconnected dissimilar metals on the outside of a submarine: copper (in the bronze propeller), iron (in the steel hull), and zinc (in the hull-protecting sacrificial electrodes). Generally, the largest currents flow between the propeller and the nearest zincs giving rise to a current which flows from the propeller through the submarine into the water back to the propeller. Modulation of the submarine’s corrosion currents gives rise to ELF emissions. A fixed-site ELF sensor offers the relative advantages of freedom from platform and geological noise, much longer integration times, the ability to exploit the vector nature of the signature, and a much larger separation in frequency between the Extremely Low Frequency Emission (ELFE) signal and the ferromagnetic signal. During Phase 1, Navmar identified a design concept for the ELF buoy. During phase 2, design details will be refined, and units will be fabricated for concept demonstration testing.