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In many cases, the obvious solution is to surface and raise an antenna above the water surface to use standard technology. This is not sufficient, however, for nuclear-powered submarines. These vessels, developed during the Cold War by the major military powers, are capable of staying submerged and hidden for weeks or months. Yet, they are supposed to surface and launch ballistic missiles in case of a nuclear war. How could such an order be sent to a submarine which is well hidden but also out of communication reach?
Several technologies have been developed and deployed:
Electromagnetic waves in the ELFExtremely low frequency (ELF is the band of radio frequencies from 3 to 300 Hz. ELF was used by the US Navy to communicate with submerged submarines. Because of the electrical conductivity of salt water, submarines are shielded from most electromagnetic c frequency range (see also SLFSuper Low Frequency (SLF) is the frequency range between 30 Hertz and 300 Hertz. In this frequency range there are the frequencies of the AC power grids (50 Hertz and 60 Hertz). The radio services Saguine on 76 Hertz and ZEVS on 82 Hertz operate in this r) 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 wavelengthThe wavelength is the distance between repeating units of a wave pattern. It is commonly designated by the greek letter lambda (λ). In a sine wave, the wavelength is the distance between peaks: The x axis represents distance, and I would be some vas: The US Navy's system (called Seafarer) operates at 76 HertzHertz is also the name of a car rental company. See The Hertz Corporation''. The hertz (symbol Hz is the SI unit of frequency. It is named in honour of the German physicist Heinrich Rudolf Hertz who made some important contributions to science in the fiel, 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 Republic, 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.
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."