The Chinese have completed the first of three 5,000 ton Type 927 acoustic surveillance ships. These twin-hull ships are designed to deploy an underwater SURTASS (Surveillance Towed Array Sensor System) LFA (Low-Frequency Active) sonar for detecting submarines in coastal waters and on the high seas, especially very quiet nuclear or non-nuclear subs. Two more Type 927 ships are under construction. These ships, if they operate like their American counterparts, with ship crews of about 25 plus a dozen or more navy personnel to operate the SURTASS gear.
The U.S. has had five similar ships in service since 2003. These ships were built during the 1990s. One of them, USNS Victorious, was aggressively harassed by Chinese naval warships in 2009 while it was more than 200 kilometers off the Chinese coast in the Yellow Sea. The Chinese harassed other navy surveillance ships as well. The Americans threatened retaliation and the Chinese backed off but did not halt the harassment.
The U.S. Navy put the first SURTASS into service during 2003. This was despite a problem with lawsuits seeking to ban the new sonar because of possible injury to whales and other seagoing mammals. There were also problems with budget cuts to this Cold War era technology in the decade after the Cold War ended in 1991. At the time SURTASS was seen as necessary to deal with increasingly quiet non-nuclear submarines operating in coastal waters. Initially, the main threat was Russian Kilo class diesel-electric subs equipped with AIP (air-independent propulsion system). Western subs were the first to receive AIP so the U.S. Navy knew how effective AIP subs could be. Diesel engines, while reliable, are noisy, and making noise is a good way to attract the not-so-friendly attention of opposing naval forces. The fuel cell and air-independent propulsion systems are much quieter, and, most importantly, enable the sub to stay underwater for days or weeks at a time. This allows them to get closer to potential targets, most likely carriers and amphibious vessels before they are detected.
In anti-submarine warfare, particularly against submarines, detection range is important. The newer, quieter subs have the potential to get close enough to launch anti-ship missiles which can, depending on design, be launched from a torpedo tube, head for the surface and then the missile takes to the air for ranges up to several hundred kilometers before they are detected. Those missiles could ruin any surface ship's day. Particularly when those ships have to enter coastal (littoral) waters. These sub-launched missiles were already in use at the end of the Cold War.
Now Chinese Type 39 subs with AIP are beginning to appear. The first Chinese AIP had less power and reliability than Western designs, and appear to be less capable than Russian or Western AIP. But the Chinese have kept improving their AIP, just as they have done with so much other military technology. Western AIP allows diesel-electric subs to remain underwater for several weeks.
The American SURTASS (WQT-2) was initially limited by a legal settlement that allowed limited training. The new active sonar adjunct to the passive UQQ-2 will be deployed on a transducer lowered to a depth of 100 meters (330 feet) from a vessel moving at six kilometers an hour, operating for short periods. WQT-2 is an upgrade to the existing SURTASS system, which is a passive system. This is UQQ-2, a large number of microphones attached placed to 1,830-meter cable. For shallow water using two shorter cables with microphones are used. Ships using the passive system used the SURTASS Block Upgrade, which was mainly about new microphone technology plus new satellite communications so that ocean surveillance ships can more easily and effectively communicate with destroyers and other warships that can act on submarine location information.
The WQT-2 is intended to increase detection range, particularly in shallow waters near the coastline, and thus regain the reaction time that passive systems can no longer provide reliably. The increased reaction time could be used to evade the submarine, forcing it to either attack from an unfavorable position, or speed up to get into a good attack position and thus make noise. The latter would make the submarine easier to detect. Once a hostile submarine is detected, you can go after it. This could involve using anti-submarine aircraft (fixed-wing or helicopter) to use their own sensors and weapons (Lightweight Torpedo) and destroy the sub before it can launch an attack. Another would be to allow a friendly submarine (probably nuclear-powered) to deal with it.
After testing on a leased vessel, SURTASS (WQT-2) was installed in the USNS Impeccable (T-AGOS 23). Originally there were to be six new twin-hull T-AGOS ships built so they could operate two in the Atlantic and two in the Pacific. Only one 5,000 ton T-AGOS was built and a compact version of SURTASS (WQT-2) was developed for the smaller 3,300 ton Victorious class T-AGOS ships.
India has also recently completed a ship similar to the Chinese and American acoustic surveillance ships. India is concerned about Pakistani and Chinese AIP subs in the Indian Ocean.
SURTASS was originally developed to complement the much more expensive SOSUS (SOund Surveillance System) networks. These were Cold War era systems that were largely abandoned after the Cold War ended in 1991. Now SOSUS is back. China began installing underwater passive sonar systems in its coastal waters back in 2011. 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 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.
Technical details were not revealed by China or South Korea, but this sort of thing is similar to the system of passive (they just listen) sonars the United States deployed on the sea bottom in key areas during the Cold War. SOSUS 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 some 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 up to ten kilometers. The major drawback of the system was that it did not cover deepwater areas more than 500 kilometers from the edge of the continental shelf. This is 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. Some 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 electronics and communications technology. While many parts of the SOSUS have been shut down, additional portable SOSUS gear (SURTASS) 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 locations 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, traveling underwater, using battery power and near the coast, are very hard to detect. The South Korean SOSUS will help even the odds.
China’s Internet-based espionage efforts have probably already stolen a lot of American SOSUS secrets and that helped them a lot.