Underwater Communication

How do we talk under water where no phone signal can reach? Underwater communication technologies answer this question by using special methods like sound, light, and certain radio waves to send messages below the surface. They help us explore the ocean, keep ships and submarines in touch, and watch over underwater nature. This is important for studying the ocean, military uses, and taking care of the sea environment.

Here are some details to be covered in this note.

• What kind of Technology to use ?
• Hybrid Communication System
• Fresh Water vs Salt Water
• VLF and ELF
• SSIXS

What kind of Technology to use ?

Choosing the right technology for talking underwater is important because each type works best in different situations. Sound wave technology is good for sending messages over long distances underwater. Light technology is faster but works best in clear water and over short distances. For communicating from the surface to deep underwater, we use special buoys that can send microwave signals to satellites. This lets us share information quickly over large areas of the ocean. When picking a technology, we think about how deep the water is, how far the message needs to go, how clear the water is, and how quickly we need the information to be sent and received.

Hybrid Communication System

How do we keep in touch across air, land, and sea, especially when each place has its own communication challenges? The answer lies in a Hybrid Communication System. This smart solution combines different ways of sending messages—like using sound under water, radio waves in the air, and cables on land—to make sure everyone can communicate no matter where they are. It's like building bridges across the sky and ocean, ensuring that no matter how far apart we are, we can always share information and stay connected.

Hybrid Communicatioin System is for scenarios where multiple different underwater communication technologies are combined to form a single communication link, often to leverage the strengths of each technology and mitigate their limitations. This approach is known as a hybrid communication system. Here are a few examples where such integration is beneficial:

• AUVs (Autonomous Underwater Vehicles) Operations: AUVs may use acoustic communication for long-distance, low-bandwidth communication with surface ships or underwater stations, and switch to optical communication for high-bandwidth data transfer when in close proximity to the data collection point or when clarity allows.
• Remote Underwater Monitoring Stations: These stations might use a combination of wired communication for reliable, high-speed data transfer to a surface buoy and then use microwave communication from the buoy to a satellite or surface station for real-time data sharing across global distances.
• Underwater Construction and Research: In operations requiring precise control and monitoring, such as underwater construction or scientific research, a combination of acoustic and optical communications can be used. Acoustic communication provides broad coverage for general commands and monitoring, while optical communication allows for the transfer of high-definition video and large data files over short distances.
• Hybrid Networks for Diverse Environments: In environments with variable conditions (e.g., varying turbidity, depth changes), a hybrid system can adapt by selecting the optimal communication method. For instance, an underwater network could primarily use acoustic communication but switch to optical or wired connections in areas where those methods are more effective.
• Submarine-Airplane Communication:
• Buoyant Wire Antennas (BWA): Submarines can deploy a buoyant antenna to the surface, which allows for radio communication with aircraft. The antenna extends to the surface while the submarine remains submerged at a shallow depth, enabling it to use standard radio frequencies for communication with the airplane.
• Very Low Frequency (VLF) Communication: VLF signals can penetrate sea water to a limited depth, allowing submarines to receive signals without surfacing. However, VLF has a very low data rate and is primarily used for receiving simple commands or alerts. Aircraft equipped with VLF transmitters can communicate directly with submerged submarines, but the submarines typically cannot reply via the same method while deeply submerged.
• Use of Relay Buoys: Submarines may deploy floating communication buoys that can communicate with the submarine via acoustic or other underwater communication methods and with aircraft via radio frequencies. This allows for two-way communication between a submarine and an airplane, with the buoy acting as a relay point.
• Satellite Communication (SATCOM): In some cases, submarines can use satellite communication systems when at periscope depth to communicate with surface ships or aircraft that are also equipped with SATCOM capabilities. This requires the submarine to be close enough to the surface to establish a satellite link.

Fresh Water vs Salt Water

Is there any difference between fresh water and salt water in terms of underwater communication ? The answer is YES. The difference is mainly primarily due to the electrical conductivity of salt water. The impact of differences varies depending on communication technology as well. In practical terms, underwater communication systems, especially those for long-range use, primarily rely on acoustics rather than electromagnetic waves, due to the limitations imposed by water's electrical properties. The differences between fresh and salt water are more pronounced for electromagnetic systems and less so for acoustic systems, but still relevant for designing and deploying underwater communication technologies.

Here goes some more details on this :

• Electrical Conductivity: Salt water is a good conductor of electricity because of the dissolved salts that ionize and allow electric current to flow through it. Fresh water, in contrast, has much lower conductivity because it lacks significant quantities of dissolved ions. This difference in conductivity affects how electromagnetic signals propagate through these mediums. In salt water, electromagnetic signals can be absorbed more quickly, limiting the range and effectiveness of certain types of underwater communication systems, like those based on electromagnetic waves.
• Acoustic Properties: Underwater communication often relies on acoustic (sound) waves because they travel further in water than electromagnetic waves. The speed of sound is slightly higher in salt water than in fresh water due to the increased density and slightly different bulk modulus (elastic properties of the medium). This means that sound can travel further and faster in salt water, potentially affecting communication range and signal timing. However, for acoustic communications, the primary considerations are related to water temperature, pressure, and depth, which affect sound speed profiles and can cause sound to bend or reflect differently in the water column.
• Signal Attenuation: The attenuation (loss of signal strength) of both electromagnetic and acoustic signals can be affected by the medium. For electromagnetic signals, the attenuation is much greater in salt water, making such communication impractical over anything but very short distances. Acoustic signals also experience attenuation, which can be influenced by factors like water temperature, salinity, and particulates, but the effect of salinity (difference between salt and fresh water) is less pronounced compared to electromagnetic attenuation.
• Impedance Matching: The impedance of water (its resistance to the flow of sound or electromagnetic waves) is different between fresh and salt water due to their different physical properties. This affects the efficiency of energy transfer from a transmission device into the water, and thus, devices designed for use in one type of water may not be as efficient or effective in the other.

VLF and ELF

Very Low Frequency (VLF) and Extremely Low Frequency (ELF) are radio frequency bands used in various communication applications, including underwater communication. These frequency bands are particularly suited for underwater communication due to their ability to penetrate deep into the ocean water, which is a challenging medium for electromagnetic signals.

In underwater communication, the choice between VLF and ELF depends on the required communication depth, the distance over which communication needs to occur, and the available infrastructure for generating and receiving the signals. VLF, being easier to generate and requiring relatively smaller antennas, is more commonly used for surface to submarine communication at shallower depths. ELF, despite its ability to reach deeper submarines, is less commonly used due to the significant infrastructure required to generate its signals.

Both VLF and ELF face challenges in underwater communication, such as signal attenuation, noise, and the need for large antennas to transmit and receive these low-frequency signals. Additionally, the low bandwidth of these frequencies limits the amount of data that can be transmitted, resulting in slow data rates compared to higher frequency communication methods.

Despite these challenges, VLF and ELF remain important for applications where deep penetration into water is required, especially for strategic military communications with submerged submarines.

VLF (Very Low Frequency)

Highlights of VLF are as follows :

• Frequency Range: VLF covers the frequency range from 3 kHz to 30 kHz.
• Characteristics: VLF signals can penetrate sea water to a depth of up to 20 meters (about 66 feet), making them useful for communicating with submarines at shallow depths. The range and depth of penetration can vary depending on the salt content and temperature of the water.
• Applications: VLF is primarily used for communicating with submarines while they are near the surface. It is also used for navigational purposes and for broadcasting time signals.
• Antenna Size: VLF antennas are large, often extending over several kilometers. The size is due to the need for the antenna to be a considerable fraction of the wavelength to efficiently radiate energy. For VLF frequencies (3 kHz to 30 kHz), the wavelengths range from 100 kilometers to 10 kilometers, respectively. Therefore, effective VLF antennas are typically very long wire antennas or large ground-based antenna systems.
• Transmitter Power: VLF transmitters operate with high power to ensure that the signal can penetrate seawater and cover great distances. The power levels can range from tens of kilowatts to several megawatts. High transmitter power compensates for the attenuation of signals as they travel through seawater and ensures reliable communication with submerged submarines at shallow depths.

ELF (Extremely Low Frequency)

Highlights of VLF are as follows :

• Frequency Range: ELF encompasses frequencies from 3 Hz to 3 kHz.
• Characteristics: ELF signals have an even greater ability to penetrate seawater, reaching depths of several hundred meters, potentially up to 1000 meters under optimal conditions. However, generating and transmitting ELF signals requires very large antennas and high power, making it more complex and less efficient than VLF for many applications.
• Applications: ELF communication is mainly used for strategic communication with deeply submerged submarines. Due to the technical challenges and costs associated with ELF transmissions, its use is limited and typically employed only by military forces for critical communications.
• Antenna Size: ELF communication requires even larger antennas than VLF due to the even longer wavelengths involved. ELF wavelengths range from 100,000 kilometers (3 Hz) to 100 kilometers (3 kHz), necessitating extremely large antenna installations. Effective ELF antennas might involve large areas of the Earth itself, with systems utilizing long underground cables or large loop antennas spanning significant portions of geographical regions.
• Transmitter Power: ELF transmission systems use extremely high power levels, often on the order of several megawatts, to generate signals that can penetrate deep into the ocean. The power requirement is partly due to the inefficiency of such large wavelength transmissions and the need to overcome the significant attenuation of signals as they pass through seawater.

Why VLF and ELF / Motivation?

The safety and success of submarines rely a lot on staying hidden and being able to talk reliably with their leaders. This need makes it very important to have communication methods that work well for this purpose. That's why using Very Low Frequency (VLF) and Extremely Low Frequency (ELF) radio systems is still important for the military.

The ability of VLF and ELF to reach deep underwater and communicate over long distances is crucial for submarine communication. These technologies allow submarines to keep in touch with their main offices constantly, making sure they can get commands, send back information, and stay informed without being noticed. This feature is especially important for the strategy of deterring nuclear attacks, where submarines equipped with nuclear missiles need to stay hidden to ensure they can respond if needed.

• Submarines are Vulnerable When on the Surface
• Submarines are easiest to find and target when they are close to or at the surface because they can be seen by radar and the naked eye. Therefore, for safety and tactical reasons, it's very important for submarines to stay under the water as much as they can, especially when they are in or near dangerous or watched areas. Good communication systems that work under the sea enable submarines to remain deep below the surface, greatly lowering their chances of being spotted by using the ocean's natural hiding places.
• VLF and ELF Waves Can Penetrate Sea Water to Different Depths
• The deeper electromagnetic waves go into the water, the harder it is for them to get through, especially if they are high-frequency waves. These high-frequency waves get soaked up by seawater very quickly, so they're only good for sending messages to places close to the surface. On the other hand, VLF (3 kHz to 30 kHz) and ELF (3 Hz to 3 kHz) waves can go much deeper, making them great for talking to submarines that are deep down. VLF waves can go as deep as 20-40 meters, perfect for submarines just below the surface. ELF waves can go even deeper, reaching submarines hundreds of meters down. Being able to send messages this deep is very important for keeping in touch with submarines without making them come up to the surface, helping them stay hidden.
• VLF and ELF Waves Can Diffract Around the Earth and the Ionosphere
• Both VLF and ELF waves exhibit excellent diffraction properties, allowing them to follow the Earth's curvature and penetrate the ionosphere, facilitating global communication reach. This characteristic is particularly important for strategic military communications, ensuring that messages can be sent and received regardless of the submarine's location.
• VLF Propagation: VLF signals are reflected between the Earth's surface and the ionosphere, enabling long-distance, beyond-the-horizon communication. This property allows VLF transmitters to communicate with submarines over thousands of kilometers, covering vast oceanic areas.
• ELF Propagation: ELF waves can also travel great distances by penetrating the Earth and sea water, although they require much larger antennas and higher power to generate. Their ability to reach globally is somewhat limited compared to VLF, but their depth penetration is unmatched, making them valuable for deep-submerged communication.

The challenges of VLF and ELF radio systems

The use of Very Low Frequency (VLF) and Extremely Low Frequency (ELF) radio systems for submarine communication comes with several challenges, primarily due to the nature of the frequencies involved and the technical requirements for effective transmission and reception.

Overall, while VLF and ELF radio systems are indispensable for certain military communication needs, especially for strategic submarine communications, they come with significant technical and operational challenges. These challenges include the need for large and powerful transmitters, the limitations of one-way and slow communications, and the inefficiencies related to antenna design and ground conductivity. Addressing these challenges requires careful planning, significant resources, and ongoing technological development to maintain effective and secure communication links with submerged submarines.

Here's a more detailed look at these challenges:

• Large and Powerful Transmitters
• Size: VLF and ELF wavelengths are extremely long, requiring correspondingly large antenna systems to efficiently transmit signals. For VLF, antenna lengths can span several kilometers, while ELF systems might need even larger setups, utilizing significant portions of the Earth's surface or underground cables extending for hundreds of kilometers. This requirement makes the construction and maintenance of such facilities complex and costly.
• Power: Transmitting signals at VLF and ELF frequencies requires a lot of power due to the inefficiencies in the antenna systems at these wavelengths. The transmitters must be capable of generating sufficient power to ensure the signals can penetrate deep into the ocean and cover vast distances. This high power requirement leads to increased operational costs and the need for sophisticated power management systems.
• One-way and Slow Communications
• One-way Communication: Due to the complexity and size of the VLF and ELF transmitting infrastructure, submarines typically cannot send messages back at these frequencies. Instead, they often rely on other communication methods to reply, which may require coming closer to the surface or using different frequency bands, potentially compromising their stealth.
• Slow Data Rates: The bandwidth available at VLF and ELF frequencies is very limited, resulting in slow data transmission rates. This limitation means that only simple, text-based messages can be sent, and the transmission of large amounts of data, such as images or videos, is impractical. This constraint requires messages to be concise and sometimes coded to convey complex instructions efficiently.
• Antenna Efficiency and Ground Conductivity
• Inefficiency: VLF and ELF antennas, due to their size relative to the wavelength they are transmitting, are inherently inefficient. A significant amount of the power used in these systems does not contribute to the effective radiation of the signal but is instead lost as heat or absorbed by the ground. This inefficiency necessitates even higher transmitter powers to achieve the desired signal penetration and range.
• Ground Conductivity: The performance of VLF and ELF systems is also influenced by the electrical conductivity of the ground over which they are built. Low ground conductivity can further reduce the efficiency of the antenna system, complicating site selection and design. Optimal locations for these antennas are often in areas with high ground conductivity, such as near bodies of water or in certain types of soil, which can limit the feasible locations for constructing these large-scale facilities.

NOTE : If VLF and ELF is unidirectionaly only, how can the listener respond ?

If communication via Very Low Frequency (VLF) and Extremely Low Frequency (ELF) is unidirectional, meaning the signal can only be sent from the transmitter to the receiver without a direct path for return communication, the listener (in this context, usually a submarine) has to use alternative methods to respond or to initiate contact.

It's important to note that the choice of response method depends on operational requirements, the submarine's current mission, and the need to maintain stealth. Modern military submarines are equipped with a range of communication systems to ensure they can maintain contact with command under various circumstances, balancing the need for stealth with the need for effective communication.

• Use of Different Frequency Bands for Response
• Submarines can respond using higher frequency bands that do not require the large infrastructure of VLF or ELF for transmission. These higher frequencies, however, have their limitations, such as shorter range and the need for the submarine to be at or near the surface to achieve effective transmission.
• Deploying Buoys
• A submarine may deploy a buoy to the surface, which is connected to the submarine by a cable. This buoy can then use satellite communication or other radio frequencies to transmit messages back to command. This method allows the submarine to remain submerged while still enabling two-way communication.
• Coming to Periscope Depth
• For satellite communications, submarines might come to periscope depth to use antennas that communicate with satellites. This method allows for high-bandwidth communication but requires the submarine to be relatively close to the surface, increasing its vulnerability to detection.
• Using Other Platforms or Assets
• Military operations often involve multiple assets that can communicate with each other. A submarine might relay its response or information to another platform, such as a surface ship or an aircraft, which then communicates the message back to command using other frequencies or communication methods.
• Pre-arranged Protocols
• In some cases, especially when using ELF and VLF for strategic or emergency communication, submarines and command may operate under pre-arranged protocols. These protocols can dictate specific actions in response to certain messages, eliminating the immediate need for back-and-forth communication.
• Short Burst Data Systems
• Some submarines are equipped with systems capable of sending short burst data transmissions. These systems can quickly rise to periscope depth, send a burst of data via satellite or other high-frequency methods, and then immediately return to deeper waters. This minimizes the risk of detection.

The impacts of VLF and ELF radio systems

The deployment and operation of Very Low Frequency (VLF) and Extremely Low Frequency (ELF) radio systems, while critical for military communications, especially with submerged submarines, have raised concerns about environmental and health impacts, as well as legal and regulatory issues.

While VLF and ELF radio systems play an indispensable role in strategic military communications, their operation raises several health, environmental, and legal concerns that require careful consideration, ongoing research, and regulatory oversight to address effectively.

• Health and Environmental Concerns
• Cell Damage and Interference: There have been studies and debates regarding the potential health effects of prolonged exposure to electromagnetic fields (EMFs), including those generated by VLF and ELF transmissions. Some research suggests that exposure to these low-frequency waves could potentially cause cellular damage or interfere with the biological processes of living organisms. However, the evidence is not conclusive, and the subject remains a topic of ongoing research and discussion. The World Health Organization and other health bodies continue to assess the risks associated with EMF exposure.
• Impact on Marine Life: The ocean is a complex acoustic environment where many marine species, including whales, dolphins, and fish, rely on sound for communication, navigation, and hunting. There is concern that VLF and ELF waves, which can penetrate deep into the ocean, might interfere with these natural communication channels, potentially affecting the behavior, migration patterns, and reproductive cycles of marine life. While some studies have indicated possible impacts, the extent and significance of these effects are still being researched, and there is a call for more comprehensive studies to understand the implications fully.
• Legal and Regulatory Issues
• Violation of International Agreements: The operation of VLF and ELF stations, especially those involving large antenna systems and high-power transmissions, may come into conflict with international agreements related to environmental protection, the use of international waters, and electromagnetic pollution. For instance, concerns have been raised about the potential for these systems to affect neighboring countries or marine environments beyond national jurisdictions, leading to diplomatic and legal challenges. Countries operating such systems must navigate these international norms and agreements, ensuring compliance while meeting their strategic communication needs.
• Addressing the Concerns
• Regulatory Oversight: To mitigate potential negative impacts, regulatory bodies and international organizations have established guidelines and limits on electromagnetic field exposure, both for the general public and occupational settings. These guidelines aim to protect human health and minimize environmental impacts.
• Research and Monitoring: Ongoing research into the effects of electromagnetic fields on health and marine life is crucial. This research informs policy decisions and operational practices, ensuring that the benefits of VLF and ELF communications are balanced against the need to protect human health and the environment.
• Technological Innovations: Advances in technology may offer ways to reduce the potential impacts of VLF and ELF transmissions. This could include more efficient antenna designs, lower power transmissions, or alternative communication technologies that reduce reliance on these low-frequency waves.

The alternatives of VLF and ELF radio systems

Exploring alternatives to Very Low Frequency (VLF) and Extremely Low Frequency (ELF) radio systems is crucial for submarine communications, especially as technology evolves and the need for more versatile and less intrusive communication methods becomes apparent. These alternatives aim to address some of the limitations and challenges associated with VLF and ELF systems, such as their large infrastructure requirements, slow data rates, and potential environmental impacts.

While newer technologies offer improved communication capabilities for submarines, the unique advantages of VLF and ELF systems in certain scenarios ensure their continued relevance. The choice of communication system is dictated by operational requirements, including depth, stealth needs, and the type of information being transmitted. As technology advances, integrating these various communication methods will likely continue to evolve, ensuring effective and secure submarine communications across a wide range of operational scenarios.

• Underwater Docking Stations
• Underwater docking stations represent a technological advancement in submarine communication, serving as relay points for communication between submerged submarines and surface or shore-based command centers. These stations can be anchored to the seafloor or be mobile platforms. Submarines can either physically dock to these stations or communicate via short-range, high-frequency acoustic or optical communication systems when in proximity. This method allows for higher data rates compared to VLF and ELF, supporting the transmission of complex data, including video and large datasets, undersea.
• Satellite-based Systems
• Satellite communication (SATCOM) provides a high-bandwidth, global communication solution for submarines when they are at periscope depth or deployed with a communication buoy. This approach leverages the extensive coverage of military and commercial satellite networks, enabling real-time, secure communication links with command structures. SATCOM systems can support a wide range of communication needs, from voice and text messages to data and video transmission, offering a significant improvement in speed and capacity over VLF and ELF systems.
• Use of VLF and ELF for Emergency or Backup Purposes
• Despite the development of alternative communication methods, VLF and ELF systems may still be used for emergency or backup communication purposes. Their ability to penetrate deep into the ocean makes them uniquely suited for scenarios where other communication methods are unavailable or compromised, such as in deep-diving operations or when a submarine is operating in stealth mode and cannot surface or deploy buoys for satellite communication.
• Emergency Communication: In situations where a submarine is cut off from other communication methods due to equipment failure, jamming, or other operational challenges, VLF and ELF can provide a critical link to command authorities, ensuring that submarines can still send and receive essential commands and distress signals.
• Backup Systems: As a backup, VLF and ELF systems offer a reliable, albeit slower, alternative to ensure continuous communication capability under various operational conditions. This redundancy is crucial for maintaining operational security and effectiveness, particularly for strategic assets like ballistic missile submarines.

Examples and Implications

• VLF Example: The U.S. Navy operates a VLF transmitter facility at Cutler, Maine, which uses an antenna system stretching over 2.4 kilometers (1.5 miles). This facility is capable of transmitting signals at power levels up to several hundred kilowatts.
• ELF Example: The former U.S. Navy ELF communication system (which was decommissioned in the early 2000s) used transmission sites in Michigan and Wisconsin, with antenna systems that effectively consisted of long underground cables stretching over hundreds of kilometers. The transmitter power for these systems was in the range of several megawatts to ensure signal penetration to submarines at operational depths.

SSIXS

The Submarine Satellite Information Exchange System (SSIXS) is a communication system designed to enable submarines to exchange information with command centers, other submarines, or surface ships using satellite technology. SSIXS allows for the secure and rapid transmission of a wide range of data, including but not limited to, text messages, orders, intelligence information, and reconnaissance data.

SSIXS represents an essential component of modern naval operations, enabling strategic communication between submarines and command structures without compromising their operational security. The system reflects the evolving nature of military communications, incorporating advanced technology to meet the demands of contemporary and future underwater warfare and strategic deterrence.

• Key features of SSIXS include:
• Secure Communications: SSIXS provides encrypted communication channels to ensure the confidentiality and integrity of the transmitted information, crucial for military operations.
• Global Coverage: Leveraging satellite networks, SSIXS offers global communication capabilities, allowing submarines to communicate from virtually anywhere in the world's oceans.
• Versatility: It supports various types of data exchange, enhancing operational flexibility and the ability to adapt to different mission requirements.
• Low Probability of Detection: By using techniques like burst transmissions, SSIXS minimizes the risk of detecting the communicating submarine, helping to preserve its stealth.

Radio frequency and reacheability to underwater of SSIXS

The Submarine Satellite Information Exchange System (SSIXS) primarily utilizes satellite communication technologies, which operate at higher radio frequencies compared to Very Low Frequency (VLF) or Extremely Low Frequency (ELF) systems traditionally used for underwater communications. Satellite communication systems generally operate in the GHz range, such as the C-band (4–8 GHz), X-band (8–12 GHz), and higher frequency bands. These higher frequencies allow for the transmission of large amounts of data at high speeds but have limited ability to penetrate seawater.

• Reachability to Underwater
• Due to the high-frequency nature of satellite communications, direct communication between satellites and submerged submarines is not feasible, as these frequencies cannot penetrate deep into seawater. Here’s how SSIXS addresses this limitation to ensure reachability to submarines:
• Periscope Depth: Submarines typically need to ascend to periscope depth to establish communication with satellites. At this shallow depth, a submarine can deploy an antenna above the water surface to transmit and receive signals.
• Buoyant Wire Antennas: Some submarines may deploy a buoyant antenna or communication buoy to the surface while remaining at a deeper depth. This buoy then communicates with satellites, relaying information back to the submarine.
• Use of Intermediate Systems: In some cases, submarines can communicate indirectly with satellites through intermediate systems. For example, a submarine could send information to a surface ship or an aircraft, which then relays the information to the satellite.
• Implications
• The need for submarines to approach the surface or use intermediate systems for satellite communication introduces operational considerations, particularly regarding stealth and detectability. While SSIXS provides a powerful tool for high-speed, global communication, its use must be carefully balanced with the tactical situation and the need to minimize the risk of detection.
• For long-range, secure, and stealthy communication while fully submerged, submarines still rely on VLF and ELF systems, despite their slower data rates and lower bandwidth. These low-frequency systems enable basic command and control messages to be sent to submarines deep underwater, complementing the capabilities of satellite communication systems like SSIXS for comprehensive operational effectiveness.

YouTube

• Underwater Visible Light Communications Device - TishoYanchev (2016)
• How to communicate with a submarine? - Prof Simon - Professor Simon Holland (2018)
• The US Secret Underwater Spy Technology – The US Navy's SOSUS - The Infographics Show (2019)
• Project E.L.F. - The history of communicating with submarines underwater - KB9VBR Antennas (2021)
• Personal underwater data communication via acoustics | TNO - (2022)
• SALSA: Smart Adaptive Long- and Short-range Acoustic network - TNO (2022)
• Estimation and control of underwater optical wireless communication systems - KAUST RISC Lab (2023)

Refernces

• Underwater Communication - Science Direct
• Wireless communication breaks through water-air barrier - MIT News (2018)
• Introduction to Underwater Communications - Amrine Technology News (Feb 2024)