Unseen, largely unknown and, until recently, highly classified, the US Navy’s vast network of underwater sonars is one of its greatest advantages over rival fleets. The United States can detect many, if not most, enemy submarines through much of the world’s oceans.
A sub that can be detected can also be killed. It’s a profound problem for the Chinese navy as it eyes a possible amphibious assault across the Taiwan Strait. Its growing fleet of quiet attack submarines could protect the landing force—but only if they themselves can avoid detection.
The problem for the US is that the Chinese fully appreciate how vulnerable they are underwater—and they’re actively thinking about ways to end that vulnerability. The US fleet needs new and better ways of defending its underwater sensors during a seabed battle that could get very nasty, very quickly in the months and weeks leading up to a possible Chinese move against Taiwan.
In particular, the US needs more ships that can repair the sensor network at sea.
"There are only a small handful of vessels capable of such at-sea repairs, fewer than 10 globally, and they are easy targets when on station," warned Chris O’Flaherty, a retired Royal Navy captain with deep experience in undersea warfare.
The US Navy was a pioneer in seabed surveillance. In 1950, the service launched the then highly secret Project Jezebel, a generational effort to lay thousands of miles of undersea cable connecting sensitive acoustic sensors to shore stations staffed by sonar analysts.
By the time the navy declassified the Integrated Undersea Surveillance System in 1991, it also included catamaran surveillance ships towing additional acoustic arrays.
Today, the system—again largely cloaked in secrecy—probably also features many small drones on the surface and under the waves. Shore sensors, ocean surveillance satellites and reconnaissance aircraft, crewed and uncrewed, also complement the undersea equipment.
The authors argue that the location of individual 'nodes' in the US undersea surveillance system can be located and 'removed'.
It’s not a totally global, totally comprehensive surveillance system—but it’s close. And it vexes naval planners of the People’s Republic of China.
"The probability that PRC submarines are discovered when leaving port is extremely high," Senior Captain Zhang Ning, a faculty member at China’s Naval University of Engineering, wrote along with coauthors in a November 2023 journal article translated by Ryan Martinson, a professor at the US Naval War College.
"There is a fairly high probability that PRC submarines will be detected and intercepted while operating in the Near Seas’ along the First Island Chain between the Philippines and Japan," Zhang and his coauthors warned, according to Martinson.
Cued by surveillance, US and allied anti-submarine forces—submarines, ships and aircraft—can cut off Chinese attack boats from the deep water where they could best perform their missions.
But the US surveillance system isn’t invulnerable, Zhang and his coauthors stressed.
"The authors further argue that the location of individual 'nodes' … in the US undersea surveillance system can be located and 'removed'," Martinson wrote in an analysis of Zhang and company’s own analysis, published by the Center for International Maritime Security.
O’Flaherty listed the ways Chinese forces could disable US undersea sensors. They ranged from the "relatively overt"’—the deployment of remotely operated vehicles from unhidden surface motherships, "to go down to almost any depth and to uncover and sever cables"—to "semi-covert" methods.
One semi-covert method would be sending autonomous submarines equipped with sonars to find the cables and plant explosive charges to cut them.
For several years now, the US Navy has been studying a possible replacement for the ageing cable-repair ship USNS Zeus, but the service is still years away from signing a contract and cutting steel.
The Chinese may want to undertake a covert counter-cable effort far in advance of any attempt to invade Taiwan. Long-range underwater vehicles could, "leave an explosive charge in the immediate vicinity of a cable, ready for actuation at a time of the owner’s choosing—which could be years hence," O’Flaherty said. "Actuation of such an explosive can be via a coded acoustic signal, which is very easy to achieve."
Cables can be repaired, of course—usually by highly trained crew aboard specialised auxiliary vessels. The US Navy operates just one cable-repair ship, the 14,600-ton USNS Zeus, delivered to US Military Sealift Command in 1984. For several years now, the navy has been studying a possible replacement for the ageing Zeus, but the service is still years away from signing a contract and cutting steel.
And anyway, there’s no way Zeus and its crew could perform their hard, precise work in contested waters during wartime. As O’Flaherty said, cable ships are "easy targets".
It’s possible the US fleet’s single special-mission submarine—the heavily modified, 12,000-ton USS Jimmy Carter—could covertly deploy divers for select cable repairs. After all, finding, tapping and eavesdropping on the enemy’s cables is reportedly among the boat’s secret missions.
Even with Jimmy Carter on cable duty, the US Navy would be stretched thin trying to safeguard the surveillance system that lends it one of its greatest advantages in wartime.
And forget hiring private companies to help. Even if they were willing to risk ships during open conflict, there’s a global shortage of commercial cable vessels. Fewer than 10 are in use, but one trade group claimed the world needs 20.
The US fleet still spends most of its nearly US$40 billion annual shipbuilding budget on aircraft carriers, amphibious ships, destroyers, logistics ships and submarines. It had better start prioritising cable vessels, too—and figure out how to protect them after the shooting starts.
Article reprinted with permission from the Australian Strategic Policy Institute's analysis and commentary site The Strategist.