Communication with submarines

Lore & Background

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 (VLF) radio waves can penetrate seawater just over 100 ft, and many navies use powerful shore 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.

Acoustic transmission is also used: sound travels far in water, and underwater loudspeakers and hydrophones can cover quite a gap. Both the American (SOSUS) 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. An underwater telephone sometimes called Gertrude is also used to communicate with submersibles.

VLF radio waves (3–30 kHz) 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 (relative to a wavelength) 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 (usually a long trailing wire) 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.

Reader's Guide

The significance of submarine communication lies in its role as a critical enabler of strategic deterrence and naval operations. During the Cold War, the development of nuclear-powered submarines that could stay submerged for months created an urgent need for reliable one-way communication links. VLF and ELF systems were developed to meet this need, allowing command authorities to transmit orders to submerged submarines without forcing them to surface and risk detection. The technical challenges are immense: ELF transmitters require huge antennas spanning tens of kilometres and dedicated power plants, yet radiate only a few watts of actual signal. 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, with the entire transmission encrypted. This allowed messages to be completed even with very low signal-to-noise ratios. Because of 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. The legacy of these systems is that they remain the only means to reach deeply submerged submarines, though they are one-way only. A surfaced submarine, or one floating a tethered antenna buoy, can use ordinary radio communications on HF, VHF, and UHF bands, and military communications satellite systems like the U.S. Navy's Submarine Satellite Information Exchange Sub-System (SSIXS) are preferred for long-distance communications to avoid betraying the submarine's location. Recent technology combining acoustic signals and radar has been developed to enable submerged submarines to communicate with airplanes.

Did You Know?

• The Russian ZEVS ELF transmitter operates at 82 Hz, corresponding to a wavelength of 3,656.0 km.
• The U.S. Navy's Project ELF system operated at 76 Hz and used antennas at Clam Lake, Wisconsin and Republic, Michigan.