Extremely low frequencyElectromagnetic waves in the ELF frequency range (see also SLF) can travel through the oceans and reach submarines anywhere. However, building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system (called Seafarer) operates at 76 hertz, the Soviet/Russian system (called ZEVS) at

82 hertz. The latter, for example, corresponds to a wavelength of 3658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, you cannot build a usual half-wavelength dipole antenna, as it would spread all across a large country.Instead, one has to find an area with very low ground conductivity (a requirement opposite to usual radio transmitter sites) and dig two huge electrodes into the ground at different sites separated by about 60 km, and feed-lines (just wires on poles) reaching them from some station in the middle. Although other separations are possible, 60 km is the

distance used for the ZEVS transmitter which is located near Murmansk. As the ground conductivity is so poor, the current between the electrodes will penetrate deep into the interior of the Earth, basically using a large part of the globe as antenna. The antenna is very inefficient; to drive it, a small dedicated power plant seems to be required although the power actually emitted as radiation is only a few watts. But its transmission can be received virtually anywhere: Even a station at Antarctica noticed when the Russian Navy put their ZEVS antenna into operation for the first time.

Due to the extreme technical difficulty of building an ELF transmitter, only the US and the Russian Navy owned such systems. Until it was dismantled in late September 2004, the American Seafarer system (76 Hz) consisted of two antennas, located at Clam Lake, Wisconsin (since 1977) and at Sawyer Air Force Base near Gwinn, Michigan (since 1980). Before 1977, the Sanguine system was used, placed in the Laurentian Shield in Wisconsin. The Russian antenna (ZEVS, 82 Hz) is installed at the Kola peninsula near Murmansk. It was noticed in the West in the early 1990s. The British Royal Navy once considered building their own transmitter at Glengarry Forest, Scotland, but the project was

cancelled.The method employed was a 64-ary Reed-Solomon, meaning that the alphabet had 64 symbols, each one represented by a very long pseudo-random sequence. The entire transmission, of course, was then encrypted. The advantages of such a technique are first, that by correlating multiple transmissions, a message could be completed even with very

low signal-to-noise ratios, and because only a very few pseudo-random sequences represented actual message characters, there was a very high probability that if a message was successfully received, it was a valid message (anti-spoofing).Two facts should be noted: First, the communication link is obviously one-way. No submarine could have its own ELF transmitter on board, due to the sheer size of such a device. Attempts to design a transmitter which can be immersed into the sea, hanging from an aircraft, were soon given up.

Second, on such low frequency, information can be transmitted only very slowly, on the order of a few characters per minute (see Shannon's coding theorem). Although the actual codes used are of course secret (well, their meaning only—the actual transmissions can be received all over the world and even some radio amateurs do listen) it is reasonable to assume that no specific orders are given but rather only commands like "surface and await orders by satellite radio."

http://en.wikipedia.org/wiki/VLF

http://spaceweather.com/glossary/inspire.html

http://www.qsl.net/sv1xv/lw.htm

http://members.aol.com/trekkspill/VLF.htmlGood photos

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