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Mini Submarine

I'm not clear on submarine stealth (N.Korean/S. Korean incident)?

A comment from an expert as to how the North Korean mini-sub can slip in undetected in shallow waters;

"The North Korean version of stealth is old-school diesel-battery operated subs that evade modern detection methods."

Now how can this be so? During WWII and later hadn't ASDIC advanced to a point that stealth (especially diesel boats) became difficult unless below the thermal layer? I was under the impression anti-sub detection of today would have no difficulty in locating and tracking a sub....especially a diesel boat to boot.

Any Navy/former Navy sonar people care to put this in layman's terms?

Sonars work in one of 2 ways. Active sonar acts a lot like radar - you send out a sound wave and listen for it to bounce back. Active sonar can detect a submarine that is making no noise at all. This works very well in deep water but in shallow water the sound waves bounce off the bottom, off rocks, old wrecks and so on, so you end up with a lot of false targets. There is also the problem of varying salinity and temperature in coastal waters with warmer fresh water rivers dumping into the colder salt water ocean. This creates layers at different depths which the sonar waves can bounce off and a good sub skipper will use that to his advantage, hiding under those layers.

Passive sonar is like a big underwater microphone - it listens for noise. Passive sonars in the open ocean can achieve incredible ranges on noisy targets - hundreds or even thousands of miles. In shallow water they can be nearly useless however with all the varying currents and noise sources from nearby land.

The Cheonan was sunk near land in 80 meters of water.

To give you an idea of what is involved: In 1982 the British Royal Navy fought a war against Argentina for control of the Falkland Islands. This was the biggest naval war the world had seen since World War 2 and the Royal Navy was a specialized anti-submarine force whose primary Cold War task was to keep the shipping lanes from the U.S. to Europe safe from Soviet subs. Argentina had just 2 operational submarines - a World War 2 vintage relic which was knocked out of action while on the surface early in the conflict and a relatively new but not ultra-sophisticated German-built Type 209 export submarine the San Luis. The Royal Navy while hunting this one sub expended around 250 depth charges, Limbo bombs and anti-submarine torpedo's and were in fact in danger of running out of ASW ordnance. This came as a surprise to the commander of the San Luis who in interviews after the war stated his boat never came under direct attack. The RN - the best ASW force in the world - expended millions of Pounds worth of ordnance on whales and phantom noises.

So, an inherently quiet diesel-electric sub, laying silent in ambush in shallow water could certainly put a fish in a small, unsophisticated corvette that has little ASW capability to begin with.

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