Communication with submarines
Communication with submarines when they are submerged is a difficult technological task which requires specific techniques and devices. This is because electromagnetic radiation used in radio communication cannot travel through thick conductors such as salt water.
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. However, they were intended to launch ballistic missiles in case of a nuclear war. Communicating with the vessels while they were submerged presented difficulties that were addressed in the following ways:
Acoustic transmission
Sound travels far in water, and underwater loudspeakers and hydrophones can cover quite a gap. Apparently, both the American (SOSUS) and the Russian Navy have placed sonic communication equipment in the seabed of areas frequently travelled by their submarines and connected it by submarine communications cables to their land stations. If a submarine hides near such a device, it can stay in contact with its headquarters.
Very low frequency
VLF radio waves (3–30 kHz) can penetrate sea water to a depth of approximately 20 meters. Hence a submarine at shallow depth can use these frequencies. Even a vessel more deeply submerged might use a buoy on a long cable equipped with an antenna. The buoy rises to a few meters below the surface, and may be small enough to remain undetected by enemy sonar / radar.
Extremely low frequency
Electromagnetic waves in the ELF frequency range (see also SLF) can travel through the oceans and reach submarines anywhere. 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 corresponds to a wavelength of 3658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be constructed, as it would spread 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 bury two huge electrodes in the ground at different sites and feed lines (wires on poles) to them from a station in the middle. Although other separations are possible, 60 km is the distance used by the ZEVS transmitter located near Murmansk. As the ground conductivity is poor, the current between the electrodes will penetrate deep into the Earth, essentially using a large part of the globe as antenna. The antenna is very inefficient. To drive it, a dedicated power plant seems to be required, although the power emitted as radiation is only a few watts. Its transmission can be received virtually anywhere. A station in Antarctica noticed when the Russian Navy put their ZEVS antenna into operation.
Due to the technical difficulty of building an ELF transmitter, only the USA 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.
ELF Transmission
The Extremely low frequency transmission employed was a 64-ary Reed-Solomon, meaning that the alphabet had 64 symbols, each represented by a very long pseudo-random sequence. The entire transmission was then encrypted. The advantages of such a technique are 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 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 in the sea or flown on an aircraft were soon abandoned.
Second, due to the limited bandwidth, information can be transmitted very slowly, on the order of a few characters per minute (see Shannon's coding theorem). Thus it is reasonable to assume that the actual messages were mostly generic instructions or requests to establish a different form of two-way communication with the relevant authority.
Standard radio technology
A surfaced submarine can use ordinary radio communications. Today this usually means not shortwave but rather dedicated military communication satellites. (The US Navy's system is called Submarine Satellite Information Exchange Sub-System (SSIXS), a component of the Navy Ultra High Frequency Satellite Communications System (UHF SATCOM).)
See also
External links
- About the U.S. ELF projects
- ZEVS, The Russian 82 Hz ELF Transmitter By Trond Jacobsen at ALFLAB, Halden in Norway
- Extremely Low Frequency Transmitter Site Clam Lake, Wisconsin, a "Fact File" published by the US Navy (PDF File)
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