Approximately 70% of the Earth’s surface is covered by water, yet almost 95% of the underwater world remains unexplored. Nearly 4000 robots are swimming up and down in the world’s oceans, which allow scientists to measure and understand ocean dynamics, like the directions and speeds of currents, as well as physical characteristics like temperature and salinity, yet scientists can only recover the collected sensor data and track the position of the robots only when they rise to the surface or when the robots are retrieved at the end of a mission.
Real-time data retrieval of frequent measurements, continuous tracking of underwater robots and increased spatial coverage and sensing from a network of submerged robots/sensors is hindered by the limited communication speed and absence of GPS underwater. This hampers a wide range of activities, including real-time underwater sensing, sea-life monitoring, port surveillance, ocean mapping, subsea infrastructure inspection, wireless diver-to-diver communication, wireless diver/underwater vehicle communication, untethered sea exploration, subsea search-and-rescue operations, underwater wireless video feeds, and off- shore drilling monitoring.
Underwater communication is vital for undersea exploitation and modern communication. Conventional ways which employ acoustical technique for underwater communication have their drawbacks including high path loss, narrow bandwidth, high bit error rate, among which unconditional security is more demanding due to commercial and secure interest.
Submarine communications have always been a challenge because radio waves can’t penetrate sea water. The obvious solution is to surface and raise an antenna above the sea level, then use ordinary radio transmissions. However, a submarine is most vulnerable when on the surface. Early submarines mostly travelled 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. To communicate with submerged submarines several techniques are used.
Submarine communications are currently carried out while submerged using ELF or VLF radio waves because only very low or extremely low frequencies can penetrate the water at those depths.
Using ELF and VLF presents a number of disadvantages, however. These frequencies offer a very high path loss, narrow bandwidth, and high bit error rate. The VLF and ELF frequencies only offer a very low bandwidth: VLF supports a few hundred bits a second while ELF sustains just a few bits each minute which is too low to support high bandwidth data such as video.
These systems also impose severe operational limitations: these are extremely low bandwidth one-way systems that require towed antennas or buoys, and submarines need to steer specific courses and reduce their speed. The transmission sites have to be very large, meaning the submarine must tow cumbersome antenna cables, plus it usually has to align on a specific orientation and reduce speed to obtain optimal reception. The options such as submarines briefly surfacing or the use of towed antennae compromise the ability of the vessel to remain stealthy. For a submarine to retain all its tactical advantage, it must remain submerged in the mixed layer, which is around 60 to 100 metres deep, below which surface sonars cannot detect them.
Sound travels far in water, and underwater loudspeakers and hydrophones can cover quite a gap. Apparently, both the American (SOSUS) and the Russian Navy 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.
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. The low frequency of the acoustic waves limits communication speed to around a kilobit per second. To put that in perspective, average broadband speeds are around 50 megabits per second (50,000 times as fast).
Recent developments in adaptive underwater communications, robust direction finding for GPS-less underwater localization, software-defined underwater acoustic modems, and soft robotics for low-cost macro/micro autonomous underwater vehicles (AUVs) are notable enabling technologies to achieve faster communication speeds, accurate positioning, and low-cost testbed deployments underwater.
Submarine communication requirements
Submarines communicate via multiple, complementary RF systems, covering nearly all the military communications frequencies. No one communications system or frequency band can support all submarine communications requirements. Submarine shipboard communications systems consist of RF antennas and radio room equipment, both RF transmitters/receivers and baseband suites.
Submarine communications were once limited to those necessary to communicate mission support information and the minimal command and control that a submarine previously required. The Navy continues to implement the principles of Network Centric Warfare, where the capability of the total force is made greater than the contributions of individual platforms through networking of sensors, weapons control systems, and information systems.
As submarines continue to conduct a variety of missions to include intelligence collection, Indications and Warning (I & W), anti-submarine warfare, anti-surface warfare, strike warfare, and mine warfare, they will have to be an integral part of networked sensors and platforms.
Submarines’ future missions will require a revolution in communications connectivity and supporting bandwidth. The vision is to allow submarines to communicate without the current restrictions of depth and speed and with sufficient bandwidth to maximize the effectiveness of data and intelligence collected by the submarine, such that real-time connectivity and reach-back is achieved
The US Navy is investing in new and previously demonstrated techniques for communicating with submarines at speed and depth for coordinated ASW operations. These techniques most commonly use either trailing wires or towed buoys for submarine communications, which impose limitations on the submarine’s maneuverability and stealth, and therefore negatively impact the submarine’s ability to fully conduct ASW operations. An airborne laser which could penetrate shallow water would permit submarine communications without the restrictions of floating wires or buoys
The development of these advanced communications has already begun with the incorporation of Narrowband based systems that are IP architecture based. Following this is development of a higher data rate antenna and wideband based communications and ultimately a buoyant cable antenna that allows two-way communications at depth and speed.
Ultimately submerged data exchange and communications capabilities will be a key enabler for employing off-board vehicles, sensors, and distributed networks of UUVs, sensors and other payloads.
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).
UHF SATCOM provides a relatively high data rate but requires the submarine to expose a detectable mast-mounted antenna, degrading its primary attribute – stealth. Conversely, extremely low frequency (ELF) and VLF broadcast communications provide submarines a high degree of stealth and flexibility in speed and depth, but are low data rate, submarine-unique and shore-to-submarine only.
ELF [Extremely Low Frequency 30 Hz – 300 Hz 10,000 Km – 1,000 Km wavelength] – This is the only band that can penetrate hundreds of meters below the surface of the ocean. The US Navy transmits ELF messages using a huge antenna in Wisconsin and Michigan created by several miles of cable on towers in conjunction with the underlying bedrock.
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 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.
This band is used to send short coded “phonetic letter spelled out” (PLSO) messages to deeply submerged submarines that are trailing long antenna wires. The communication is only one way, therefore it is used primarily for prearranged signals or to direct the submarine to come closer to the surface for faster communications. Environmental factors do not have a strong influence on changing the signal and therefore it is quite reliable.
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).
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 might 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.
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, UHF, or VHF).
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.
High-speed acoustic communication by multiplexing orbital angular momentum
Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a method of packing more channels onto a single acoustic frequency — massively increasing the quantity of data that can be transmitted underwater. With their new setup, Berkeley researchers were able to simultaneously pack eight channels onto an acoustic frequency, rather than just the one. This means sending 8 bits at the same time, or an increase of 8x the current rate.
Authors demonstrate a high-throughput communication approach using the orbital angular momentum (OAM) of acoustic vortex beams with one order enhancement of the data transmission rate at a single frequency.
The topological charges of OAM provide intrinsically orthogonal channels, offering a unique ability to multiplex data transmission within a single acoustic beam generated by a transducer array, drastically increasing the information channels and capacity of acoustic communication. A high spectral efficiency of 8.0 ± 0.4 (bit/s)/Hz in acoustic communication has been achieved using topological charges between −4 and +4 without applying other communication modulation techniques.
Such OAM is a completely independent degree of freedom which can be readily integrated with other state-of-the-art communication modulation techniques like quadrature amplitude modulation (QAM) and phase-shift keying (PSK). Information multiplexing through OAM opens a dimension for acoustic communication, providing a data transmission rate that is critical for underwater applications.
Combining acoustic and radio transmissions
MIT system could enable submarine-to-aircraft communications
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 sound signals, which travel as pressure waves.
When these waves hit the surface, they cause tiny vibrations. Above the water, a radar 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 tested in relatively controlled environments with small surface ripples. The team has tested it at depths of 11.5 feet in swimming pools and with circulation currents to mimic some ocean conditions. Next, the researchers plan to test TARF at greater depths and higher altitudes, along with making the technology more robust to large ocean waves.
“The radar reflection is going to vary a little bit whenever you have any form of displacement like on the surface of the water,” said Adib. “By picking up these tiny angle changes, we can pick up these variations that correspond to the sonar signal.” To filter the tiny sonar vibrations from the much larger waves surrounding them, the researchers developed signal-processing algorithms. Natural waves occur at about 1 or 2Hz — or, a wave or two moving over the signal area every second. The sonar vibrations of 100 to 200Hz, however, are a hundred times faster. Because of this frequency differential, the algorithm zeroes in on the fast-moving waves while ignoring the slower ones.
The communications system was tested in a water tank and two swimming pools on the MIT campus, with the sonar signals sent from depths of between 5cm and 3.5m below the surface, and the radar at heights of 20-40cm above the surface. TARF was able to successfully transmit data in all scenarios, even when confronted with small waves created by swimmers in the pool. In waves higher than 16cm, however, the system wasn’t able to decode signals. The next steps will involve refining the system to work in rougher waters. “It can deal with calm days and deal with certain water disturbances,” said Adib. “But [to make it practical] we need this to work on all days and all weathers.”
Chinese scientists make progress on nuclear submarine communication
People’s Daily reported in Feb 2019 that a successful test transmission of real-time high-capacity data between deep ocean transponders and the Beidou navigation satellite system had been carried out. Marine research ship Kexue, or “Science”, conducted the test in the western Pacific a long with several other missions on a 74-day trip before returning to its home base of Qingdao, Shandong .
“This technology … significantly increases the safety, independence and reliability of deep ocean data transmission,” Wang said, adding that using China’s Beidou system meant the submarines no longer had to rely on foreign satellites for such communication. “The transponder with Beidou, at a depth of 6,000 metres, has been safely in operation for more than a month now and it is working well,” Wang said.
Real-time underwater transmission of temperature, salinity and currents data at the 6,000 metres depth – with transponders relaying signals every 100 or 500 metres – was “another big breakthrough” for the team, Wang added. They did this using a combination of inductive coupling and underwater acoustic communication technologies, the scientist said.
“[A submarine] usually can’t transmit on its own unless it raises a communications mast or buoy to the surface,” said Collin Koh, a research fellow with the Maritime Security Programme at Nanyang Technological University in Singapore. But doing so increases the risk of the submarine being detected, so a satellite link makes for stealthier and more efficient communication.
Adam Ni, a researcher with Macquarie University in Sydney, said the development was the latest in China’s drive to modernise its submarine fleet. “Along with advances in submarine stealth technology, strong surface fleet [to complement] infrastructure, and space-based information support, the latest breakthrough is another element of China’s modernising submarine power, especially its SSBN force, which is increasingly important for nuclear deterrence,” Ni said.
Optical Communications LiFi
Another technology the Navy is interested in using Li-Fi to improve submarine communications, since radio waves travel poorly under water and current acoustic communications are slow. The underwater VLC in the blue/green spectral range (450 nm-550 nm) is able to achieve data speeds of hundreds of Mbps for short ranges (less than a hundred meter) complementing long range acoustic communication. However optical waves also has limitations as light has a tiny wavelength that can be easily scattered by micro-particles and marine life in the ocean and the information gets lost.
One potential solution is to carry out optical communications using a laser, a concept which has been around since the 1980s when experiments were carried out to demonstrate that it is possible to maintain an optical channel between a submarine and an airborne platform.