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China-Japan-Koreas
How Did N.Korea Sink the Cheonan?
2010-05-22
Investigators say a North Korean midget submarine and a support vessel left a naval base on the west coast two to three days before the attack on the South Korean corvette Cheonan on March 26 and returned to port two or three days later.

They said the submarine and support vessel left the base on Cape Bipagot, around 80 km from Baeknyeong Island on March 23 and maneuvered out of the sight of South Korean and U.S. intelligence. The ship apparently accompanied the submarine to provide support and offer aid in case the sub encountered difficulties. The submarine took a detour out into open seas and arrived in waters to the west of Baeknyeong Island on March 25. There it is believed to have awaited its prey 10 m under the surface for about a day.

The military believes that the submarine found the Cheonan on the evening of March 26 and fired a CHT-02D torpedo at the vessel from 3 km away. At the time of the attack, the Cheonan was in waters that are 30 to 40 m deep, while the North Korean submarine was further out at sea where the water is between 40 to 50 m deep, posing no problems to launching a torpedo.

"When the sub attacked the Cheonan at 9:22 p.m. on March 26, the tides in the West Sea were slow, and it looks like the North strategically planned the attack," said a military source. The sub apparently returned to Cape Bipagot on March 28.

The submarine class was unknown until now. The 130-ton sub ranks between the Shark (325 tons) and a Yugo class (85 tons). Air Force Lt. Gen. Hwang Won-dong, the chief of the intelligence analysis team, said, the sub "is similar to the shark-class submarine and was built recently for export, equipped with night-vision equipment and other high-tech gadgets, as well as a unique structure to enhance its stealth capabilities." Intelligence experts say the sub is the same as the three "Ghadir" class midget submarines the North exported to Iran.

At first, investigators suspected a shark-class sub of the attack because they were unaware of the movements of this class, dubbed "Yono" or salmon. But by backtracking information, they apparently discovered the new class. "Two North Korean subs left the naval base two or three days before the attack, but we were unable to detect this," said Sohn Ki-hwa of the intelligence team said. "This will not lead to major shifts in our intelligence assessment capabilities, but we will improve what needs to be improved."

But questions linger why the Cheonan, a corvette battleship equipped with sonar equipment, was unable to detect the movements of the sub or the launch of the torpedo. "The Cheonan's sonar is an old model with a limited range, so there's a strong possibility that it failed to detect the torpedo which was launched from far away," said a military source.
3 km doesn't seem that far, but readers with better insight are encouraged to explain this in the comments.
Experts say North Korean subs and other ships probably conducted several infiltration and surveillance operations in waters near Baeknyeong Island. "We have no information whether North Korea conducted prior surveillance of the waters where the attack was carried out, but we believe it carried out training missions in waters off North Korea's coast with similar underwater conditions," Hwang said.
Posted by:Steve White

#8  I think the more important question is why they sank the Cheonan.
Posted by: Deacon Blues   2010-05-22 20:47  

#7  It sounds like the kind of semi-submersible boat the drug runners use

It's a true mini-sub. Photo.
Posted by: ed   2010-05-22 10:54  

#6  ;-) Frank.
Posted by: lotp   2010-05-22 10:21  

#5  I understand that there are certain conditions where sonar actually bounces off of layers of water like a mirror. Perhaps this is relevant here, but I doubt it.

thermoclines and prolly not relevant at these depths.

/not a bubblehead, but watch a lot of movies and read a lot.... and currently staying at a Holiday Inn. No, really. I am
Posted by: Frank G   2010-05-22 09:02  

#4  If that sub is 130 tons, I think that means it displaces less than about 120 cubic meters of water, so it's maybe 3x3x12 meters or so? That doesn't sound like much of a home for two or three days.

It sounds like the kind of semi-submersible boat the drug runners use in the US that the USN cannot detect without an humint hint. CBGs are at risk to a swarm of such vessels and so not have sufficient escorts to defend against them.

The boat doesn't have to stay on station, it only has to show up when prey is in the neighborhood. Or sortie for a day or two when the weather is nice and maybe get lucky. That's all they have to do, get lucky once.

They aren't conventional warriors. They're terrorists. That's why they so quickly hitch up with drug runners. We're playing a suckers game by sending warriors to fight with them. And we're going to lose economically if it's as long a war as I suspect it will be.
Posted by: Nimble Spemble   2010-05-22 06:53  

#3  How bout you bubbleheads chime in?

They always do. Loudly.
Posted by: Shipman   2010-05-22 06:06  

#2  I understand that there are certain conditions where sonar actually bounces off of layers of water like a mirror. Perhaps this is relevant here, but I doubt it.

Was the torpedo going fast or slow? Was the Cheonan going fast or slow? Was the Cheonan heading straight for the torpedo or at an angle?
Do the Norks make this model of torpedo? Did they get an assist in designing it? What kind of technology does this torpedo have? I understand that the Norks made the sub, but did they develop the tech or did they get an assist, probably from the Chinese again? Iran will have this, no doubt. Also, a couple years back, I remember that Russia helped some drug folks, I think in Columbia, manufacture primitive drug-running subs.

If that sub is 130 tons, I think that means it displaces less than about 120 cubic meters of water, so it's maybe 3x3x12 meters or so? That doesn't sound like much of a home for two or three days. Probably a crew of two or three? Probably has a diesel motor that it used to bug out right away after the Cheonan was hit by the torpedo. I wouldn't think batteries would let it move fast for more than what, three hours or so? (WAG!)

It'd be a shame if one of these were found caught in a net somewhere. I hear that giant squid can be a real problem for them, too. Or maybe one of their propellers could get bent and cavitate a bit or mysteriously fall off in the middle of a mission. How embarrassing.
Posted by: gorb   2010-05-22 01:56  

#1  Someone with better tech knowledge can correct this, but my old Cold War memory says: Close to shore (islands), (relatively) shallow waters, high currents = tough sonar conditions likely, be it in the coastal areas of the Barents sea or the Yellow sea. Good place to hide a sub if you can keep station.

I wonder if its more difficult to detect if the Korean patrol boat were just using passive against an electric boat, no active pinging (And even then I'm not sure that active would gain much in shallows other than a lot of multipath noise).

How bout you bubbleheads chime in?
Posted by: OldSpook   2010-05-22 01:04  

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