Communication with submarines
Communication with submarines is a field within military communications that presents technical challenges and requires specialized technology. Because radio waves do not travel well through good electrical conductors like salt water, submerged submarines are cut off from radio communication with their command authorities at ordinary radio frequencies. Submarines can surface and raise an antenna above the sea level, or float a tethered buoy carrying an antenna, then use ordinary radio transmissions; however, this makes them vulnerable to detection by anti-submarine warfare forces.
Early submarines during World War II mostly traveled on the surface because of their limited underwater speed and endurance, and dived mainly to evade immediate threats or for stealthy approach to their targets. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months.
In the event of a nuclear war, submerged ballistic missile submarines have to be ordered quickly to launch their missiles. Transmitting messages to these submarines is an active area of research. Very low frequency radio waves can penetrate seawater just over, and many navies use powerful shore VLF transmitters for submarine communications. A few nations have built transmitters which use extremely low frequency radio waves, which can penetrate seawater to reach submarines at operating depths, but these require huge antennas. Other techniques that have been used include sonar and blue lasers.
Acoustic transmission
travels far in water, and underwater loudspeakers and hydrophones can cover quite a gap. Apparently, both the American and the Russian navies have placed sonic communication equipment in the seabed of areas frequently travelled 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
radio waves can penetrate seawater to a few tens of metres and a submarine at shallow depth can use them to communicate. A deeper vessel can use a buoy equipped with an antenna on a long cable. The buoy rises to a few metres below the surface, and may be small enough to remain undetected by enemy sonar and radar. However, these depth requirements restrict submarines to short reception periods, and anti-submarine warfare technology may be capable of detecting the sub or antenna buoy at these shallow depths.Natural background noise increases as frequency decreases, so a lot of radiated power is required to overcome it. Worse, small antennas are inherently inefficient. This implies high transmitter powers and very large antennas covering square kilometres. This precludes submarines from transmitting VLF, but a relatively simple antenna will suffice for reception. Hence, VLF is always one-way, from land to boat. If two-way communication is needed, the boat must ascend nearer to the surface and raise an antenna mast to communicate on higher frequencies, usually HF and above.
Because of the narrow bandwidths available, voice transmission is impossible; only slow data is supported. VLF data transmission rates are around 300 bit/s, so data compression is essential.
Only a few countries operate VLF facilities for communicating with their submarines — Norway, France, the United States, Russia, the United Kingdom, Germany, Australia, Pakistan, and India.
Extremely low frequency
Electromagnetic waves in the ELF and SLF frequency ranges can penetrate seawater to depths of hundreds of metres, allowing signals to be sent to submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The U.S. Navy's Project ELF system, a variant of a larger system proposed under codename Project Sanguine, operated at 76 hertz, and the Soviet/Russian system at 82 Hertz. The latter corresponds to a wavelength of 3,656.0 kilometres. That is more than a quarter of the Earth's diameter. The usual half-wavelength dipole antenna cannot be feasibly constructed, as that would require a long antenna.Instead, someone who wishes to construct such a facility has to find an area with very low ground conductivity, 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. 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. 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.
Owing to the technical difficulty of building an ELF transmitter, the U.S., China, 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, consisted of two antennas, located at Clam Lake, Wisconsin, and at Republic, Michigan, in the Upper Peninsula.
- The Russian antenna is installed at the Kola Peninsula, near Murmansk. It was noticed by the West in the early 1990s.
- The Indian Navy has an operational VLF communication facility at the INS Kattabomman naval base, to communicate with its Arihant class and Akula class submarines. Beginning in 2012, this facility was being upgraded to also transmit ELF communications.
- China on the other hand has recently constructed the world's largest ELF facility – roughly the size of New York City – in order to communicate with its submarine forces without them having to surface.
ELF transmissions
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.
Owing to the limited bandwidth, information can only be transmitted very slowly, on the order of a few characters per minute. Thus it was only ever used by the U.S. Navy to give instructions to establish another form of communication and 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.