Project Sanguine - How ELF Communication Works

How ELF Communication Works

Submarines are shielded by seawater from all ordinary radio signals, and therefore are cut off from communication with military command authorities while submerged. The lower the frequency, the deeper radio waves are able to penetrate ocean water. Waves in the very low frequency (VLF) range of 3 kHz to 30 kHz are able to penetrate to a depth of about 10 to 30 meters, and since WWII navies have used VLF transmitters to communicate with submarines. To receive VLF signals subs must rise to just under the surface or trail a shallow antenna buoy, making them vulnerable to detection by the enemy.

Radio waves in the extremely low frequency (ELF) band of 30 to 300 Hz can penetrate to a depth of hundreds of meters, allowing them to communicate with submarines at their normal operating depth. The lower the frequency, the longer the wavelength of the radio waves, and transmitters require longer antenna structures to generate them. ELF transmitters use huge antennas called ground dipoles consisting of tens to hundreds of kilometers of overhead cables resembling ordinary power transmission lines. The transmission lines are grounded at the ends, and looping currents deep in the Earth form part of the antenna. ELF transmitters are most efficient when sited over certain low resistivity underground rock formations, which allow the currents to spread deeper forming a larger "antenna". The US system was located in Wisconsin and the Upper Peninsula of Michigan over the Laurentian Shield formation, for that reason.

Such ELF transmitters cannot be installed on submarines, because of the size of the antenna and high power requirements. So ELF communication is one-way, with a receiver in the submarine receiving orders from a shore station, but unable to reply. The low attenuation of ELF waves with distance allows a single ELF station to send messages to submarines all over the world.

Another drawback of ELF is that the ELF band has very small bandwidth, and therefore can only transmit very simple messages, very slowly. ELF signals cannot carry audio (voice) like other types of radio, and can only carry short text messages consisting of a few letters. The US Navy system (above) reportedly uses three-letter code groups and requires 15 minutes to transmit one group. So current systems are not used to transmit detailed orders, but serve a "bell ringer" function, to order a specific vessel to surface and receive further orders by ordinary radio or satellite communication.

The US and Russia are the only nations known to have constructed ELF communication facilities. The Russian Navy reportedly operates an ELF transmitter, ZEVS, located northwest of Murmansk on the Kola Peninsula in northern Russia.