The report said the PLA, which was conducting a naval exercise in the area, responded swiftly by sending out a counter force, the size and nature of which remains classified.

The two forces were so close that the US military “self-destroyed” its floating sonars to prevent the sensitive devices from falling into China’s hands, said the report, published by the Chinese-language journal Shipboard Electronic Countermeasures on April 25.

The team, led by Liu Dongqing with PLA Unit 95510, said the US activities were a severe challenge to China’s national security. “In a time of war, this could pose a detrimental threat to our submarines carrying out critical missions,” the study said.

Sensitive location, timing

According to the report, US spy planes deployed sensors in waters near the Dongsha islands – also known as the Pratas islands – a group of atolls and reefs under Taiwanese control.

The incident was dangerous because any perceived support for Taipei’s administration of the islands could be seen as strongly provocative by Beijing, which considers Taiwan as a renegade province to be brought under mainland control, by force if necessary.

Unlike other contested islands in the South China Sea – where the US has been conducting freedom of navigation operations to challenge what it regards as excessive Chinese claims – only Taipei and Beijing claim the Dongsha islands.

Adding to the matter’s sensitivity, the US does not officially recognise Taiwan, in common with most countries. The military engagement near the Dongsha grouping could be interpreted by China as a deliberate attempt to escalate tensions in the region.

On January 8, three days after the incident is said to have taken place, General Mark Milley, head of the US Joint Chiefs of Staff, made an unusual phone call to a Chinese general and promised to give China advance warning if it really intended to launch a war.

Top US military official defends phone calls to Chinese counterpart

It is unclear if Milley’s call was related to the incident outlined in the report, but if war had broken out during a presidential transition, there may have been significant implications for the transfer of power from Trump, who lost the election in late 2020.

The US Constitution grants broad powers to the president as commander-in-chief of the military and a major conflict could have delayed or disrupted the transition process.

In testimony to the Senate Armed Services Committee, Milley said the US had “concerning intelligence” that China was worried about an attack by the US. The controversial call aimed to “prevent war between great powers armed with nuclear weapons”.

“My task at that time was to de-escalate,” Milley said. He added that he was “certain” Trump did not intend an attack on China. “It is my directed responsibility to convey presidential orders and intent,” he said, according to a Politico report in September 2021.

Rising tensions

Liu and his colleagues from the PLA’s electronic warfare unit said the US has made a substantial effort to target China’s submarine forces in recent years, as part of a significant increase in its military activities in the South China Sea.

This has included the use of advanced technology to detect and track Chinese submarines, with sonar buoys and a variety of sensors that can locate vessels even when they are deeply submerged, the team said.

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The PLA scientists said the US techniques pose a “severe threat” to China’s submarines, making it much more difficult for them to operate undetected in the region.

Liu’s team said the US spy planes “typically fly at an altitude of 60 metres (196 feet)”, noting this is a lower cruising altitude than most aircraft and can pose safety risks because it is relatively close to ground level.

The scientists said the low altitude is a deliberate tactic used by anti-submarine patrol aircraft, such as the US Navy’s P-8A, to improve the ability to detect and track submarines.

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Several aircraft were involved in the hunt for Chinese submarines and they worked together in a coordinated manner to achieve their objectives, the report said. “The purpose is to monitor, block and contain us,” Liu and his team said.

By operating in multiple flights over a period of time, the US military is able to maintain a persistent presence in the region and gather more information about Chinese submarine activity, the researchers found.

China’s response

The researchers said intense confrontations similar to the Dongsha incident are likely to become more frequent as Beijing adopts more decisive countermeasures to protect its marine rights in the South China Sea.

China’s submarine force would work increasingly with the air force to provide support during submarine operations, they said. This could involve using aircraft to distract or mislead US surveillance planes, or to provide cover for submarines.

Chinese team says new stealth tech for submarines can ‘cancel out’ US sonar

Liu’s team said China’s electronic warfare capabilities could be used to disrupt or jam US floating sonar systems, making it more difficult for them to detect submarines.

The scientists noted that China is developing new decoys that are more realistic and effective at fooling US sonar systems, emitting sounds and movements that are similar to a real submarine.

The Chinese military is also working increasingly with private companies to develop new stealth technology for submarines, the report said.

Civilian firms with expertise in signal processing and machine learning, for instance, can develop advanced algorithms and signal processing techniques to analyse incoming sound waves and generate counter-waves that cancel them out and avoid detection by active or passive sonars.

Private companies specialising in electronics and magnetism could also be helpful in developing demagnetisation technologies for submarines, making it harder for them to be detected by magnetic sensors.