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Stephen Chen

A new submarine detection technology that can track even the quietest of subs has shown potential during testing in the South China Sea, its developers say.
Scientists from Shanghai Jiao Tong University said their detector – about the size of a pickup truck – was tested on a deep ocean seabed.
It was able to capture faint electromagnetic waves generated by a rotating propeller nearly 20km (12 miles) away.
By analysing these low-frequency signals, the scientists said they could potentially position and track military targets. They published their findings in the Chinese peer-reviewed Journal of Vibration and Shock last month.
The detection range is about 10 times greater than anything previously reported, meaning the technology could potentially give China an advantage in an intensifying underwater arms race.
That distance had been considered impossible to achieve since detectable electromagnetic signals emitted by submarines do not transmit that far in seawater. But they can be detected from further away by monitoring the seabed – a technique not previously used to detect submarines.


Electric currents are used on the hulls of navy vessels to reduce corrosion from seawater. When their propellers rotate, they cut through the electric field and generate electromagnetic radiation.
But those electromagnetic waves weaken in conductive seawater, and while some weapons can detect these signals they must be close to the target.
According to publicly available records, the furthest away that submarines have previously been detected was 2.5km-plus, and that was also achieved by Chinese scientists.
“Although a 3km detection and positioning range is already a significant breakthrough, it still falls short of meeting the requirements for detecting and positioning targets in the open sea,” Jiang Weikang, a professor with the Shanghai university’s State Key Laboratory of Mechanical System and Vibration, wrote in the paper.
“The new technological approach has demonstrated tremendous potential in sea trials,” he said, and it could potentially increase the detection distance of low-frequency electromagnetic signals to “over 50km”.
That distance is comparable to the range at which a modern torpedo can destroy a target.
The Shanghai scientists achieved this by using a detector to receive signals not from the seawater but from the seabed.
Some of the electromagnetic waves emitted by a submarine’s propeller travel down into the seabed and spread along the rock layers.
As the vessel moves, its propeller rotates about once per second. And as it does this, the intensity of the extremely low-frequency electromagnetic waves in the rock layers diminishes at a rate of about 0.173 decibels per km.
That is about 0.5 per cent of the rate it diminishes in seawater, meaning the signal detection range is much longer on the seabed than in seawater.
Marine geologists have long used this method to explore for undersea oil and gas, but it has yet to be used to monitor submarines.
In the paper, Jiang and his team noted that the challenge of using this technology is that the electromagnetic radiation from the propellers found in the seabed is extremely weak. The seabed also contains a significant amount of electromagnetic noise from natural or non-military sources.
To get around this, the scientists developed an algorithm to detect extremely weak target signals in complex and open environments.
They said positioning the target was also difficult since the detector needs to link multiple high-precision sensors spaced over 1km apart on the seabed. Their clocks must be synchronised and have a margin of error of one-billionth of a second – a level of precision that is difficult to maintain in the harsh deep-sea environment.
Low-frequency electromagnetic waves also carry limited physical data, and researchers have yet to find a way to maximise its use for target identification and positioning.
The paper included a photograph of the detector, which has instruments and battery compartments to support long-term underwater operations and a surface buoy.
It can also be used to track surface ships by detecting the wake of propellers.
“We have achieved accurate detection of targets in the open sea,” the paper said.
Submarines have become quieter – the sounds they emit are now comparable to background ocean noise, making it increasingly difficult to detect them using traditional acoustic methods.
Rivals China and the United States have both stepped up efforts to improve anti-submarine warfare in hotspots such as the South China Sea and the Taiwan Strait in recent years.
While the US has the edge on submarine technology, China has made progress in areas including gravity anomaly detectors, terahertz radars, unmanned underwater vehicle swarms, and AI-powered anti-submarine satellites that can detect invisible wakes on the high seas.


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China

Shanghai team uses seabed to monitor low-frequency signals in new approach that captured electromagnetic waves 20km away


Shanghai team uses seabed to monitor low-frequency signals in new approach that captured electromagnetic waves 20km away.


South China Sea

China’s submarines