September 27, 2011: South Korean officials are alarmed after discovering that the navy has only been able to detect 30 percent of the North Korean subs they come across. Moreover, North Korea is using its submarines more frequently in training (for sneaking people into South Korea) exercises. North Korea has a fleet of over 80 mini-subs, plus about 24 older Russian type conventional boats (based on late-World War II German designs, as adapted for Russian service as the Whiskey and Romeo class). China helped North Korea set up its own submarine building operation, which included building some of the large Romeo class subs. North Korea got the idea for minisubs from Russia, which has had them for decades. North Korea has developed several mini-sub designs, most of them available to anyone with the cash to pay. The North Korean minisubs range in size from 76 to 300 tons displacement. Over a dozen of these small subs are equipped to fire torpedoes.
The use of a North Korea midget sub to sink a South Korean corvette in March, 2010, forced the United States, and South Korea, to seriously confront the problems involved in finding these small subs in coastal waters. This was a difficult task, because the target is small, silent (moving using battery power) and in a complex underwater landscape, that makes sonar less effective.
There are some potential solutions. After the Cold War ended in 1991, the U.S. recognized that these coastal operations would become more common. So, in the 1990s, the U.S. developed the Advanced Deployable System (ADS) for detecting non-nuclear submarines in coastal waters. The ADS is portable, and can quickly be flown to where it is needed. ADS is believed to be in South Korea. ADS basically adapts the popular Cold War SOSUS system (many powerful listening devices surrounding the major oceans, and analyzing the noises to locate submarines) developed by the United States.
ADS consists of battery powered passive (they just listen) sensors that are deployed by ship along the sea bottom in coastal waters. A fiber optic cable goes from the sensors (which look like a thick cable) back to shore, where a trailer containing computers and other electronics, and the ADS operators, runs the system. ADS has done well in tests, but it has only recently faced the North Korean mini-subs. There, it was discovered how little capability South Korea warships had to detect the North Korean submarines. Moreover, there is not enough ADS gear to cover all the coastal areas where North Korean subs operate. South Korea is hustling to improve its anti-submarine capabilities. But decades of neglect will take years to recover from.