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The PRC has the largest navy in the world, with an overall battle force of approximately 350 ships and submarines including over 130 major surface combatants. The PLAN remains engaged in a robust shipbuilding and modernization program that includes submarines, surface combatants, amphibious warfare ships, aircraft carriers, and auxiliary ships as well as developing and fielding advanced weapons, sensors, and command and control capabilities.
The PLA is developing the capabilities and operational concepts to conduct offensive operations within the Second Island Chain, in the Pacific and Indian Oceans, and in some cases, globally. In addition to strike, air and missile defense, anti-surface and anti-submarine capabilities improvements, China is focusing on information, cyber, and space and counterspace operations.
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. 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.
China has reportedly constructed a massive extremely low frequency, or ELF, antenna array the size of New York City, as well as a smaller system and associated data processing and signal transmission facilities at various locations throughout the country. Officially, the entire system, known as Project Wireless Electromagnetic Method or Project WEM, will support the Chinese resource extraction industry and provide early warning about potential earthquakes. However, there is significant evidence that its primary function may actually be to provide long-range communication with Chinese submarines, a critical capability to support its growing number of nuclear-armed ballistic missile boats.
ELF radio waves have a proven ability to penetrate deep below water and the ground. In principle, this means that a huge antenna array could be useful in detecting natural resources underground, such as precious metals or fossil fuels. Mining companies already employ ground-penetrating radar and laser imaging systems for similar purposes.
One of the major benefits of modern submarines, especially nuclear-powered types and boats with advanced, non-nuclear air-independent propulsion systems, is their ability to remain largely hidden underwater for extended periods of time. This gives them inherent deterrent qualities. It also makes them well suited to covertly collecting intelligence or, in the case of subs armed with conventional land attack or nuclear-capable missiles, to quietly positioning themselves for short- or no-notice strikes during a crisis.
So, it’s not necessarily surprising that China’s 724 Research Institute, part of the state-run China Shipbuilding Industry Corporation (CSIC), a major supplier of communications and other electronics to the People’s Liberation Army Navy (PLAN), has been responsible for the Project WEM construction. Lu Jianxun, the project’s chief scientist, is also publicly involved in advanced communications work for the PLAN, the Post reported.
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. Therefore China is developing new technologies for submarine communications
Chinese scientists make progress on nuclear submarine communication: BeiDou achieves real-time transmission of deep-sea data
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 .
Researchers replaced batteries on 20 sets of submersible buoys on the network, optimized their positions and installed BeiDou satellite communication modules in them. As the low-volume submersible buoys powered by batteries can only be retrieved once a year, the communication modules were designed to be tiny, power saving and run steadily. “The data collected by the submersible buoys, including the temperature, salinity, flowing speed and direction of seawater, should be transmitted back to the ground lab by satellites. The amount of data was huge,” said Wang Jianing, a researcher at the institute. So they developed multi-module communication and transmission technology, greatly lifting transmission efficiency.
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. “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.
“[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.
The breakthrough research vessel Kexue made changed the situation. Before, real-time observation of marine data had relied on foreign remote sensing satellites. Now there was improved data transmission security and reliability, according to Wang Fan, director of the Institute. China began to establish the real-time scientific observation network in the West Pacific in 2014, and realized real-time transmission of deep-sea data in 6,000-meter depth in this expedition, with the depth range extending from 1,000 meters in 2016 to 3,000 meters in 2017.
Researchers also carried out successful experiments on real-time transmission of deep-sea data of 10,000-meter depth in the Pacific Ocean’s Mariana Trench, laying the technical foundation for achieving further depth. There are 20 sets of submersible buoys, four sets of large floating buoys and some pieces of on-board mobile observation equipment in China’s observation network for scientific research over the tropical West Pacific, which has acquired deep-sea data for five consecutive years. Wang said the data could enhance the precision in ocean climate and environment forecasts.
The tropical West Pacific is home to the world’s most sophisticated waters, with the strongest interchange of energy and materials between earth and the ocean. It has a close connection with the occurrence of El Nino and its duration, which has a significant influence on floods and droughts in China. And the El Nino weather phenomenon, characterized by a warming in the Pacific Ocean, has a profound impact on extreme weather conditions, according to the institute.
With a tonnage of 4,711 tonnes, Kexue is 99.8 meters long and 17.8 meters wide, with a cruising capacity of 15,000 nautical miles. Kexue started its first expedition in April 2014 and has fulfilled several missions in the West Pacific Ocean and the South China Sea, with a more than 250-day voyage every year. China’s BeiDou navigation system started to provide global services in December 2018.
Secure submarine communication using quantumkey cryptography
Submarine communication is restricted by the depth at which vessels can exchange information and the speed at which they can do so through the medium of water. Recently however, researchers have made impressive strides in solving this dilemma using a technique called Quantum Key Distribution (QKD). QKD promises to guarantee secure communication through the principles of quantum mechanics, without sacrificing speed or forcing the submarine to rise nearer the surface.
Dr. Marco Lanzagorta, the director of the Quantum Technologies group in the Information Systems department of ITT Exelis, explains that QKD is a protocol which uses quantum information to generate a pair of perfectly secure keys. “Quantum information is different from classical information, because in classical information the unit is the bit and it can have the value of zero or one,” said Lanzagorta. “The unit of quantum information is the qubit, which is a quantum state of a photon. It can be on zero, one or any superposition of zero and one. It’s more of a concept of information than the classical one.”
Lanzagorta explains that in traditional cryptosystems – such as the public domain system RSA, Diffie-Hellman and ElGamal encryption methods – the security is based on the solution to a very hard mathematical problem. However, there is no formal proof that this mathematical problem, for example prime factorisation in the case of RSA, could not be broken by an advanced algorithm. It has also been conjectured that hypothetical quantum computers could break these types of ciphers exponentially faster. Hence QKD would offer an unbeatably secure solution.
Quantum information has two important properties for securing communications. It cannot be copied which means it cannot be forged, and every time a quantum state is measured by an observer it gets collapsed, which means its properties are very difficult to detect. The technology for QKD already exists and is commercially available but it is currently carried out through an optical fibre, rather than photons travelling freely through air or water. “Some experiments have been done on QKD using photons moving in free space,” said Lanzagorta. “Most recently an experiment was done in the Canary Islands where they did first base QKD at a distance of 144km, showing it is feasible to have this free space quantum communication.
They performed experiments in a 3.3-meter-long tube filled with seawater samples collected in a range of 36 kilometers in Yellow sea. “Although quantum key distribution and quantum teleportation have been achieved via optical fiber installed underneath Geneva Lake and the River Danube respectively, experimental investigation in free-space seawater has never been done so far say authors. Analogous to the existence of transmission window around 800 nm in free-space air there is a “blue-green” optical window at the wavelength regime of 400-500 nm in free-space seawater , wherein photons experience less loss and therefore can penetrate deeper. “We have experimentally demonstrated the distribution of polarization qubits and entangled photons over seawater channel.
The high process fidelity indicate the seawater associated with suspended particulate matter introduces very limited depolarization, which verify the feasibility of quantum communication and quantum cryptography in free-space seawater.” Future explorations include field experiment in open sea, blue-green band quantum repeater and air-sea quantum communication interface.
Chinese scientists use sperm whale sounds to send secret messages for the military
According to a study published last month in IEEE Communication Magazine, the journal of the New York-based Institute of Electrical and Electronics Engineers, civil or military submarines typically have two ways to keep acoustic signals carrying secret messages off the radar: they can either tweak the characteristics of man-made signals to make them harder to crack if picked up by an enemy, or they can send weaker signals to make them harder to discover.
Both methods have their drawbacks, Jiang said in an email. The first one – changing the traits of signals – is based on a process similar to encryption, which makes information harder to decipher by converting data into a code. But the technique tends to alert the enemy because it stands out from natural waveforms. The second approach – sending a weakened signal – is more effective at keeping messages from being detected but is hampered when the message has to travel a long distance, Jiang said.
Since underwater reconnaissance systems generally filter out “ocean noise” such as a whale’s sounds, Chinese scientists developed technique to hide secret messages in the sound pulses that sperm whales emit to evade enemy reconnaissance systems from deciphering them. Jiang and his team built a camouflage system for hiding signals taking inspiration of another technique like steganography, that hides a file, image or audio signal within another one. This breakthrough that could help military submarines avoid scrutiny, researchers said.
While steganography has roots in ancient warfare, and is behind the camouflage pattern featured on modern-day army fatigues, applying the technique in securing underwater communications is a new concept, Han Guangjie, a computer scientist at northwestern China’s Dalian University of Technology, said in an interview.
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The advantage of steganography over encryption and other conventional methods is that the intended secret message does not attract attention to itself, avoiding scrutiny,” said Han, who was not involved in the study. “Plainly visible encrypted messages, no matter how unbreakable they are, arouse interest.”
Although an enemy could still possibly decipher the signals, doing so would be very challenging and require greater resources than would be needed to decode a conventionally encrypted message, Jiang said.
Chinese scientists develop long-distance underwater communication in South China Sea, reported in Sep 2022
Chinese researchers say they have developed and tested in the depths of the South China Sea underwater communication technology that would allow submarines and drones to maintain contact over more than 30,000 sq km (11,600 square miles) – about the area of Belgium.
A listening device picked up sound signals from 105km (65 miles) away at a depth of 200 metres (656 feet) during a field test in an important passageway for submarines, according to the study by the team at northwestern China.
The data transmission rate reached nearly 200 bits per second (bps), on par with the bandwidth of very low-frequency radio sent by a naval command to nuclear submarines using the world’s largest land-based antennas. Despite heavy noises in the background, the encrypted messages contained no mistakes, the researchers said.
Using commercially available technology, acoustic communication of this speed and quality is usually restricted to a distance of less than 10km. In an experiment conducted by South Korean researchers last year using British hydrophones, for instance, a transmission rate of 128bps was recorded over a distance of 20km.
Low-frequency sound – such as calls made by whales – can travel for hundreds, or even thousands, of kilometres in the ocean, but identifying these signals and decoding meaningful information from them is difficult. The sound waves can spread in different directions and speeds as they travel through currents or hit a sea mountain. The longer the distance, the worse the distortion and weakening of the signal when it reaches a receiver.
China conducted an underwater communication experiment in an important passage for submarines in the South China Sea. The results of the South China Sea experiment proved the “effectiveness and good performance” of the new technology in increasing underwater communication range and efficiency, said project lead scientist professor Liu Songzuo, of Harbin Engineering University, one of the Chinese universities sanctioned by the US government, in a paper published in the domestic peer-reviewed journal Acta Acustica on September 6.
