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, 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; they dived mainly to evade immediate threats. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. Transmitting messages to these submarines is an active area of research. Very low frequency (VLF) radio waves can penetrate seawater a few hundred feet, and many navies use powerful VLF transmitters for submarine communications. A few nations have built transmitters which use extremely low frequency (ELF) 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.
Sound travels far in water, and underwater loudspeakers and hydrophones can cover quite a gap. Apparently, both the American (SOSUS) and the Russian navies 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 can 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 and radar. However these depth requirements restrict submarines to short reception periods, and antisubmarine warfare technology may be capable of detecting the sub or antenna buoy at these shallow depths.
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 carry only 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, VHF, or UHF).
Because of the narrow bandwidth of this band, VLF radio signals cannot carry audio (voice), and can transmit text messages only at a slow data rate. VLF data transmission rates are around 300 bit/s – or about 35 eight-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.
Low frequency communications are generally at 10kHz or lower. The penetration depth in seawater is only a few meters, and a very long antenna wire is required to float near the surface.
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 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 system, Seafarer, which was a variant of a larger system proposed under 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 kilometres. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be feasibly constructed.
Instead, someone who wishes to construct such a facility 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 kilometres (37 miles). 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., 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 (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 VLF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines. China on the other hand has recently constructed the world's largest ELF facility roughly the size of NYC in order to communicate with its submarine forces without having them to surface.
The coding used for U.S. 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 was only ever used by the US 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.
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).
Combining acoustic and radio transmissions
A recent technology developed by a team at MIT combines acoustic signals and radar to enable submerged submarines to communicate with airplanes. An underwater transmitter uses an acoustic speaker pointed upward to the surface. The transmitter sends multichannel sound signals, which travel as pressure waves. When these waves hit the surface, they cause tiny vibrations. Above the water, a radar, in the 300 GHz range, continuously bounces a radio signal off the water surface. When the surface vibrates slightly thanks to the sound signal, the radar can detect the vibrations, completing the signal's journey from the underwater speaker to an in-air receiver. The technology is called TARF (Translational Acoustic-RF) communication since it uses a translation between acoustic and RF signals. While promising, this technology is still in its infancy and has only been successfully tested in relatively controlled environments with small, up to approximately 200 mm, surface ripples, while larger waves prevented successful data communication.
In April 2017, NATO's Centre for Maritime Research and Experimentation announced the approval of JANUS, a standardized protocol to transmit digital information underwater using acoustic sound (like modems and fax machines did over analog telephone lines). Documented in STANAG 4748, it uses 900Hz to 60kHz frequencies at distances of up to 28 kilometres (17 mi). It is available for use with military and civilian, NATO and non-NATO devices; it was named after the Roman god of gateways, openings, etc.
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- 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
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- Military Communications by Christopher H. Sterling
- TARF, Wireless Communication from Underwater to the Air By Francesco Tonolini and Fadel Adib