How Do Submarines Communicate Underwater?

Submarines are among the most sophisticated vehicles that humanity has ever developed. Their capability to navigate both the ocean surface and deep waters, their capacity to operate silently, and their resilience against the immense pressure found in the deep sea make them some of the most technologically advanced vessels today. Some, such as the U.S.'s Ohio-class submarines, are considered among the best weapons in the U.S. military's arsenal, capable of striking any craft or location while beneath the water surface.

Submarines are important vessels in both military and non-military contexts. Therefore, finding ways to receive instructions from surface bases or ships while underwater is essential, making communication equipment crucial. However, most forms of communication, like internet-based and cell phone-based communication, become ineffective at any depth. Therefore, submarines employ unique methods to exchange information with one another and with their associated bases or surface ships. One of the primary methods used is very low and extremely low-frequency radio waves, which, despite having their own challenges, have proven reliable over the years. Here's how submerged submarines keep in touch with the surface world and how a key advancement in technology could completely transform these vital systems.

Unlike the high-frequency radio waves used by devices such as cell phones, which typically operate within the 800 to 1900 megahertz range, the preferred frequency bands for submarine communication fall between 0 and 3 kilohertz or 3 and 30 kilohertz. The former refers to extremely low-frequency waves (ELF), which are used during deep dives because they can penetrate hundreds of feet below the water's surface. On the other hand, the latter are very low-frequency waves (VLF), which are better suited for when the submarine is near the surface due to their limited ability to penetrate water. Submarines are equipped with receivers designed to decode these waves into understandable transmissions, enabling them to communicate with the world above the water's surface. 

VLF and ELF present several challenges that hinder their effectiveness in submarine communication. First, they operate on a one-way system, meaning a submerged submarine can receive transmissions but cannot send them. This is mainly because large antennas, which cannot be fitted on the submarine, are necessary for sending ELF and VLF data. Since these antennas, like the ones in Lualualei, Hawaii, and Cutler, Maine, can cover vast distances, submarines are equipped only with receivers that allow them to receive messages. 

Second, ELF and VLF have limited bandwidth, resulting in painfully slow data transfer. Transfer speeds can drop as low as a few bits per minute and don't generally exceed a few hundred bits of data per second. ELF can only allow short-coded messages to be sent to submarines, while VLF can transmit about 450 words per minute. Fortunately, other forms of communication available for submarines exist. 

Submarines can also communicate with each other or their corresponding bases using communication buoys, acoustic modems, underwater docking stations, satellites, and underwater optical communication. Buoys are handy when a submarine is deeply submerged but needs to connect to a VLF channel. The buoys are connected to a submarine via a physical link, like a feed cable, have an antenna, and can be small enough to go undetected by enemy vessels while closer to the water's surface. 

Submarines will also use acoustic modems to communicate with each other or surface ships. These modems convert data, like words and pictures, into sound waves transmitted through the ocean or seawater from one vessel to another. Depending on environmental conditions and the modems in use, the acoustic range can be as low as 100 meters or as high as 23 miles. The sound waves are received by another modem that decodes them back into digital data.

Underwater docking stations are another essential method for submarines to communicate. While most are used for charging unmanned and autonomous underwater vehicles, some serve as communication relay points for submarines and their bases or surface ships. These stations allow submarines to either dock or simply come close enough to the docking station to communicate. Of course, more complex data can be sent or received with this method than with VLF and ELF.

Satellites are another crucial method for submarines to communicate. The submarine must be at periscope depth to deploy an antenna that can breach the water's surface or use communication buoys or a buoyant antenna to send and receive information via satellite networks. This method can accommodate a broader range of communication needs, including the ability to send voice messages, as more frequencies of radio waves can be used.

Submarines can also use underwater optical communication to transmit and receive data while submerged. This technology includes light-emitting diodes (LEDs) and laser diodes (LDs), which can be used as a medium to fire information from a ship or satellite to a receiver in a submerged submarine. Color has always played a vital role in watercraft operations. You may have noticed that many militaries around the world paint their ships gray, and numerous submarines feature black hulls. Every hue and shade serves a purpose, and this is especially true for the wavelengths used in underwater optical communication. Blue and green are usually the ideal choices here as they are not easily absorbed by water molecules and can penetrate deeper into the water.

More advanced submarine communication methods are being developed at world-class research centers and institutions. One of the most notable is MIT's Translational Acoustic-RF Communication (TARF), which, while still in its early stages, has the potential to transform submarine communication. TARF uses sonar and radio signals to create a seamless communication link between an aircraft in the sky and a submerged submarine. For some time, submarines and airplanes have struggled to communicate effectively because submarines in the deep oceans rely on options like sonar, which are less effective in the air than in water, while airplanes use high and very high-frequency radio signals that cannot penetrate deeply into the water where submarines operate. TARF technology aims to address this issue by placing a transmitter in a submarine that emits an information-rich sonar signal toward the water's surface. A TARF receiver on an aircraft captures and analyzes the vibrations generated by the sonar signal at the water surface, decoding them into actual information.