December 31, 2024: China has much to fear from west Pacific nations South Korea and Japan combining their submarine fleets with the 25 or more U.S. Navy submarines based in the region. Japan has 22 and South Korea has 21. Some subs are laid up for repairs or upgrades, so the three navies have about sixty submarines available to block shipping from entering or leaving China.

There are also some additional weapons. South Korea has become the seventh nation to use Sea Launched Ballistic Missiles or SLBMs. The second version of the KSS-III submarines featured a number of firsts. They were non-nuclear, the first non-nuclear ballistic missile submarines that not only use Air Independent Propulsion or AIP but do so using lithium ion instead of lead-acid batteries. With this AIP system the KSS-III submarines can operate submerged for about three weeks.

These subs also have ten Vertical Launch Cells or VLS filled by Hyunmoo 4-4 SLBMs with a range of 800 kilometers. North Korea is developing similar technology, but the north can only manage to build crude imitations of what the South Korean created. South Korea is far wealthier and technically advanced than the north. Both Koreas produce weapons that work often enough to do some damage and kill people. The South Korean weapons do this more reliably and effectively.

South Korea joins six other nations, the United States, Russia, France, Britain, China, and India that also have SLBMs and submarines to launch them from. The United States was the first nation to develop SLBMs and install them in nuclear powered submarines. The South Korean SLBMs have conventional warheads and are launched from non-nuclear submarines. South Korea is finally developing nuclear weapons, after not doing so for many years to appease the United States.

China has been aware of this submarine threat for decades but only in the last few years have they improved their ASW/Anti-Submarine Warfare efforts. For over a decade China has had underwater passive sonar systems in its coastal waters. This enables China to monitor submarines operating off its coasts and, presumably, in the South China Sea. South Korea did the same in 2011 when it announced that it was installing underwater submarine sensors off its coasts and this was apparently completed in 2013. The South Korean effort was in response to North Korea using a small submarine to torpedo a South Korea patrol ship in 2010. China simply wants to keep foreign warships as far away as possible, even if it means trying to force them out of international waters.

The Chinese navy expanded considerably since the 1990s, but little of the expansion involved ASW equipment for new ships or developing maritime patrol aircraft, China still maintains nearly a hundred small ASW vessels to patrol coastal waters. China has also been developing and putting into service maritime patrol aircraft like the KQ-200, which is similar to the American P-8. . Most of the new Chinese ships lack any ASW capability and that will take decades to rectify, including the time it will take to train Chinese crews to use ASW effectively. Western navies, especially the Americans, have a huge lead in this area.

China has begun building a multi-sensor system for obtaining constant data on the precise location of surface and submarine vessels in the South China Sea. Surface surveillance would be carried out by a constellation of ten remote sensing satellites so that the South China Sea is under constant surveillance. Satellites at an altitude of 600 kilometers would be equipped with SAR/synthetic aperture radar and digital cameras. A typical SAR can produce photo quality images at different resolutions. A medium resolution of 3 meters covers an area 40 x 40 kilometers. Low resolution of 20 meters covers 100 x 100 kilometers. This takes care of surface ships, including diesel-electric subs when surfaced. Since 2010 China has been experimenting with such an array, using three satellites moving in formation over the western Pacific. China announced that the first South China Sea satellite will be launched in 2019 and all will be in orbit by 2025

Technical details were not revealed by China or South Korea, but this sort of thing is similar to the system of passive/just listen sonars the United States deployed on the sea bottom in key areas during the Cold War. SOSUS (SOund Surveillance System) consisted of several different networks. On the continental shelf areas bordering the North Atlantic was the CAESAR network. In the North Pacific there was COLOSSUS plus a few sensors in the Indian Ocean and a few other places that no one would talk about. The underwater passive sonars listened to everything and sent their data via cable to land stations. From there it was sent back to a central processing facility, often via satellite link.

SOSUS was accurate enough to locate a submarine within a circle no wider than 100 kilometers. That's a large area, but depending on the quality of the contact, the circle might be reduced to ten kilometers. The major drawback of the system was that it did not cover deep water areas more than 500 kilometers from the edge of the continental shelf. This was not a problem for the South Korean or Chinese systems, as both only cover coastal waters or shallow offshore areas like the South China Sea.

SOSUS systems are very expensive to maintain. SOSUS managed to survive the end of the Cold War by making its sensors available for civilian research and by using cheaper and more powerful electronic and communications technology. While many parts of the SOSUS have been shut down, additional portable SOSUS gear has been put in service, to be deployed as needed.

South Korea had the advantage of being able to get help from the United States about SOSUS and how to collect and process the sound signatures of submarines operating in the area. The U.S. was also able to help South Korea obtain more sensitive passive sonar systems that can identify submarine location more accurately. The U.S. has been doing research in this area and knows that such cooperation would result in American access to the South Korean SOSUS. South Korea also has the design and manufacturing capability for this sort of device. The first South Korea SOSUS system was placed off the west coast, near the North Korean border. North Korean submarines, travelling underwater, using battery power and near the coast, are very hard to detect. The South Korean SOSUS helps even the odds. For China, preventing a submarine based blockade of Chinese ports is still a threat they have not developed an effective antidote for.