Signals from the Deep: The Next Wave of Communications with Submarines

Submarines have formed a key part of military arsenals since the start of the 20^th century, using the element of surprise to sink much larger vessels. In the modern day they play a key part in nuclear deterrence, ready to strike at any moment from an unknowable location. As new methods of detecting and attacking submarines emerge, such as radar which can detect the wake of a deep submarine, AI-powered networks of acoustic sensors and torpedoes with depth ranges of hundreds of metres, submarines have been pushed deeper and deeper. However, the deeper a submarine is, the more difficult it becomes to communicate with.

In this article we’ll take a tour of some cutting-edge technologies which could be used to talk to submarines hundreds of metres underwater.

Low-frequency Radio Transmission

Most frequencies of radio waves cannot penetrate deep into saltwater because the conductive salts absorb the energy from the waves. However, frequencies less than 300Hz, known as Extremely Low Frequency (ELF), can penetrate seawater to depths of hundreds of metres and, helpfully, can also diffract around the entire globe by reflecting off the ionosphere. Transmission using such low frequencies is very difficult: not only due to significant noise from lightning strikes, but also because the thousand-kilometre wavelengths make it impossible to construct a traditional half-wave antenna. One solution to this problem is to construct an antenna formed of two electrodes buried kilometres apart, effectively using a conductive path deep through the earth as an antenna. This infrastructure is prohibitively expensive for all but the largest world superpowers to construct, but some recent developments may enable transmitters to be built on much smaller scales.

Researchers at Stanford University have recently developed a low-frequency piezoelectric antenna (that is, built of a material which generates a voltage when subject to mechanical stress). By mechanically vibrating at its resonant frequency, it can transmit signals with 300 times more efficiency than previous technology. Using their apparatus, transmission frequencies can be easily modulated by varying a parallel capacitance, allowing higher rate transmission. Another recent development from the Ham Radio Science Citizen Investigation (HamSCI) achieves similarly efficient transmission of low-frequency signals by replacing a traditional antenna with a spinning permanent magnet.

Problems are also faced on the receiver side, again because ELF wavelengths are orders of magnitude longer than a submarine. Conventionally a submarine would trail a long antenna behind it in the water, but this is not effective at such low frequencies. At the University of Colorado, researchers have proposed detecting electromagnetic signals by directly measuring variations in the magnetic field through the use of extremely high accuracy atomic magnetometers such as the SERF and SQUID magnetometers. These devices work by detecting excitations, induced by the magnetic field, of the outermost electron of group 1 atoms.

Acoustic transmission

If most electromagnetic waves don’t travel well underwater, why not use acoustic waves instead? It turns out that oceanic acoustic channels are extremely difficult to work with. Alongside multipath interference from the seabed and large doppler shift due to the low speed of acoustic signals, steep temperature and pressure gradients strongly affect the speed of sound in water causing signals to refract erratically. Nonetheless, techniques such as AI deep learning channel prediction are enhancing the reliability of underwater acoustic transmission. Scientists at the Northwestern Polytechnical University in China have recently achieved the transmission of data across 375 miles through ocean water, using a self-tuning system which operates without prior knowledge of the ocean topography. One would suspect that this research has drawn the attention of the US Navy, which is currently investing into a network of hidden acoustic transmitters on the seabed to allow high-rate communication with submerged vessels.

Transmission from submarines

We have discussed the issue of sending messages to submarines, but receiving messages back from them presents an even greater challenge. Submarines have limited power capability and carelessly broadcasting signals could reveal their location. Researchers at MIT have developed two very creative solutions to this problem.

Instead of a generating a signal from the submarine itself, the first technique is to use an external transmitter to send acoustic signals to backscatter nodes on the submarine. These backscatter nodes act like a controllable mirror, and can be switched between a reflective state and a non-reflective state. By switching between the two states, the backscatter node can encode a binary signal within the reflection, which can be picked up by an external receiver. On the submarine side, this expends virtually no power.

In distant waters where these external transmitters cannot be built, it would be desirable to communicate between a submerged vessel and an overhead aircraft or even a satellite. This was previously considered improbable, because acoustic signals are almost entirely reflected at the surface of the water. But it was discovered that the ripples caused by this reflection can be detected by bouncing a 60 GHz radar signal off the water’s surface. This method combines the merits of acoustic signals which travel well in water, and radio waves which travel well in air, to establish a communication link.

Future avenues

Looking decades into the future, technologies which currently sit on the border of science-fiction could find their way into practical use.

Beyond light-based and sound-based transmission, some visionary groups propose sending signals using neutrinos , a sub-atomic particle which passes seamlessly through almost all matter, including deep oceans and even the entire earth. Recently, experiments at the Fermilab in Illinois established a neutrino communication link through 240 metres of rock into a cavern about 100m underground. However, the transmission speed was limited to 0.1 bits/s (nowhere near enough to transmit meaningful data!) and hence significant developments in neutrino generator and receiver technology will be required before this becomes suitable for practical uses.

None of this technology prevents the possibility of someone else intercepting and decoding a signal, which represents a significant challenge in top-secret military operations.

This is motivating research into quantum communication, which uses the laws of physics to secure information. One method known as Quantum Key Distribution (QKD) involves sharing a ‘key’, a string of bits used to decode an encoded message, as quantum bits (qubits) rather than 0s and 1s. Any attempt by a third party to read the qubits changes their states, and so before an encoded message is transmitted the participants can compare their keys to look for telltale signs of interception. QKD networks are starting to emerge in a few countries, most impressively as a satellite link between Beijing and Cape Town, but it will be a while before it’s feasible to implement this technology in a submarine.

Submarine communication remains one of the most challenging frontiers in defence technology, balancing the need for secrecy with engineering limitations. From low-frequency radio and advanced acoustic systems to futuristic concepts like neutrino signalling and quantum encryption, researchers are pushing the limits to keep submerged vessels connected without compromising their stealth. While many of these innovations are still emerging, they signal a future where even the deepest oceans are brought into the Information Age.