Communication with submarines
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The obvious solution is to surface and raise an antenna above the sea level, then use ordinary radio transmissions. However, a submarine is most vulnerable when on the surface. Early submarines mostly travelled on the surface, diving mainly to evade immediate threats because of their limited underwater speed and endurance. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. To communicate with submerged submarines several techniques are used.
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 traveled by their submarines and connected it by underwater communications cables to their land stations. If a submarine hides near such a device, it can stay in contact with its headquarters. An underwater telephone sometimes called Gertrude is also used to communicate with submersibles.
Very low frequency
VLF radio waves (3–30 kHz) can penetrate seawater to a depth of approximately 20 meters. Hence a submarine at shallow depth can use these frequencies. A vessel more deeply submerged might use a buoy equipped with an antenna on a long cable. The buoy rises to a few meters below the surface, and may be small enough to remain undetected by enemy sonar / radar.
Due to the low frequency a VLF broadcast antenna needs to be quite large. In fact, broadcasting sites are usually a few square kilometres. This prevents such antennas being installed on submarines. Submarines only carry a VLF reception aerial, and do not respond on such low frequencies. So a ground-to-submarine VLF broadcast is always a one-way broadcast, originating on the ground and received aboard the boat. If two-way communication is needed, the boat must ascend to periscope depth (just below the surface) and raise a telescopic mast antenna to communicate on higher frequencies (such as HF, UHF or VHF).
Because of the narrow bandwidth of this band, VLF radio signals cannot carry audio (voice), and only transmit text messages at a slow data rate. VLF data transmission rates are around 300 bit/s - or about 35 8-bit ASCII characters per second (or the equivalent of a sentence every two seconds) - a total of 450 words per minute. Simply shifting to 7-bit ASCII increases the number of transmitted characters per time unit by 14%. An additional shift to a 6-bit or a 5-bit code (such as the baudot code) would result in speeds of more than 600 and 700 words per minute.
Extremely low frequency
Electromagnetic waves in the ELF and SLF frequency ranges (3–300 Hz) can penetrate seawater to depths of hundreds of meters, allowing communication with submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system, Seafarer, which was a variant of a larger system proposed under the codename Project Sanguine, operated at 76 Hertz, the Soviet/Russian system (called ZEVS) at 82 Hertz. The latter corresponds to a wavelength of 3,656.0 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be feasibly constructed.
Instead, one has to find an area with very low ground conductivity (a requirement opposite to usual radio transmitter sites), bury two huge electrodes in the ground at different sites, and then feed lines to them from a station in the middle, in the form of wires on poles. Although other separations are possible, the distance used by the ZEVS transmitter located near Murmansk is 60 kilometers. 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 an antenna. The antenna length in Republic, Michigan, was approximately 52 kilometers (32 mi). 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 at 78° S 167° W detected transmission when the Soviet Navy put their ZEVS antenna into operation.
Due to the technical difficulty of building an ELF transmitter, the U.S., Russia and India are the only nations known to have constructed ELF communication facilities. Until it was dismantled in late September 2004, the American Seafarer, later called Project ELF system (76 Hz), consisted of two antennas, located at Clam Lake, Wisconsin (since 1977), and at Republic, Michigan, in the Upper Peninsula (since 1980). 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 Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines. The Pakistan Navy has commissioned a new naval base in Sindh, PNS Hameed, which will enable the Navy to communicate with its submerged submarines in the VLF band.  The British Royal Navy once considered building their own transmitter at Glengarry Forest, Scotland, but the project was cancelled.
The coding used for US military ELF transmissions employed a Reed–Solomon error correction code using 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).
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.
Due to the limited bandwidth, information can only 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. Submarines may use naval frequencies in the HF, VHF and UHF ranges (i.e. bands), and transmit information via both voice and teleprinter modulation techniques. Where available, dedicated military communications satellite systems are preferred for long distance communications, as HF may betray the location of the submarine. 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).
- Extremely low frequency
- Ground dipole
- Super low frequency
- TACAMO, radio system intended to survive nuclear attack
- Carlos A. Altgeit (2005-10-20). "The World's Largest "Radio" Station" (PDF). Retrieved 2013-09-01.
- "Extremely Low Frequency Transmitter Site Clam Lake, Wisconsin" (PDF). U.S. Navy. 2003-04-08. Retrieved 2009-05-12.
- Trond Jacobsen. "ZEVS, The Russian 82 Hz ELF Transmitter".
- "Navy gets new facility to communicate with nuclear submarines prowling underwater". The Times of India. 31 July 2014.
- Radio Communications of German U-boats in WWI and WWII
- 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)
- Military Communications by Christopher H. Sterling