Why Beijing is speeding up underwater drone tests in the South China Sea

China is testing large-scale deployment of underwater drones in the South China Sea with real-time data transmission technology, a breakthrough that could help reveal and track the location of foreign submarines.

A government research vessel dropped a dozen underwater gliders at an unspecified location in the South China Sea earlier this month, Xinhua reported on Saturday. It was the biggest joint operation conducted by Chinese unmanned gliders, according to the state news agency, and comes as the US vows to step up patrols in the disputed waters.

This latest effort by China to speed up and improve collection of deep-sea data in the South China Sea for its submarine fleet operation, coincides with US President Donald Trump’s reported approval of a plan to give the United States Navy more freedom to carry out patrols in the South China Sea – a move analysts say will add to uncertainties over Sino-US relations and regional security issues.

The plan, submitted to the White House in April by US defence secretary Jim Mattis, outlines a full-year schedule of when US navy ships will sail through contested waters in the South China Sea, the far-right Breitbart News website cited a US official as ­saying on Friday.

Such a move could be seen as a challenge to China’s maritime claims in the disputed waters.

Yu Jiancheng, chief scientist of the expedition commissioned by the Chinese Academy of Sciences, said the 12 Haiyi (or “Sea Wing”) autonomous underwater vehicles would roam for one month and collect detailed information in the ocean on a host of topics including temperature, salinity, the cleanness of water, oxygen level and the speed and direction of sea currents.

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“The data is being transmitted back to a land-based laboratory in real time,” meaning the information is sent out the moment it is collected under the water, Yu was quoted by Xinhua.

Yin Jingwei, dean of the college of underwater acoustic engineering at Harbin Engineering University, said that if the endeavour works as promised, “it is definitely a breakthrough”.

The university, formerly known as the PLA Military Engineering Institute, developed China’s first submarine. Yin was lead scientist in several military research projects on underwater communications.

“Real-time data transmission is extremely difficult for underwater gliders,” he said.

Drones of this type have been used in the past year on US Navy destroyers to locate submarines, according to Western media reports. They are called gliders because they use small wings and a buoyancy control mechanism to glide down and up in the water, and wave energy to propel themselves forward.

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These machines can travel long distances without needing to recharge their batteries for weeks or even months. Equipped with multiple sensors, they not only can monitor the natural environment but also can pick up data of interest to military forces, such as the propeller noise or magnetic anomaly – meaning the disturbance in the magnetic field – caused by a nuclear submarine. And because the glider produces virtually no sound, its existence can be unknown to the sub.

But American gliding drones have one weakness, according to Yin.

“They can transmit data to a mother vessel or satellite, but only when they come up to the surface,” he said. This limitation can cause a time lag and discontinuity in the data stream, which can affect a military operation such as submarine tracking.

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Although it would be surprising if China solved the problem ahead of the US, “I cannot rule out the possibility,” Yin said.

In January, China said it had built a deep-sea communications network in the western Pacific Ocean. The system allows sensors operating more than 400 metres below the surface to continuously transmit data to satellites through a grid of solar-powered buoys. Underwater data transmission can be carried out via cable or wirelessly by sound.

The Xinhua report did not say how communications among the gliders was achieved, or how far apart the drones were from one another.

Having effective communication among the gliders is important as it allows them to exchange location information that is vital for planning their movements in a region and avoiding collisions and other accidents.

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“I think it could be very hard for them to spread out over a long distance,” Yin said. “If they do, each must carry a powerful data transmission device. It will take the underwater communication technology to its limit.”

Radio waves cannot travel in water. Long-distance underwater communication, therefore, relies almost exclusively on sound waves. But sound travels slowly, and can carry only a small amount of information.

Professor Zhu Min, a researcher with the Institute of Acoustics at the Chinese Academy of Sciences, who developed a long distance communication system for the Jiaolong, China’s most powerful operating submersible capable than can take three people down to a depth of 7,000 metres, said that in water, sound travels hundreds of times slower than electromagnetic waves can in the air.

And radio waves can be sent out over high-frequency bandwidths to transmit an enormous amount of data, often in megabits or gigabits per second.

That large volume of data can be reduced to a few kilobits once in water. “The fastest sound communication in water is slower than the dial-up modem in the earliest days of the Internet,” he said.

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The small battery that gliders usually carry has limited power for long-distance data transmission, Zhu said. Because there is no satellite navigation system such as the Global Positioning System (GPS) or Beidou under the ocean, gliders need different technology to determine and inform one another of their whereabouts.

These technological challenges make massive deployment and coordination of underwater gliders very difficult, he said.

Thus, the Chinese underwater glider group must operate with tactics and strategy “quite different” from those of the large scale drone operation in the sky, according to the researchers.

“The underwater operation may give each glider more freedom to determine its own action due to the limited communication within the group,” Zhu said. “This means the individual unit needs to be equipped with a smarter brain to deal with various situations.”

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Yin said that in the air, a large number of drones could be deployed to search for and zero in on one specific target. But in the water, the gliders more likely would be sent out to survey and monitor random targets within a region.

“These are different kind of approaches requiring different kind of strategic thinking,” he said.