Re: Old Submarines (Why?)
Define littoral waters. The PLAN sees its scope not just the 12 mile territorial sea zone but also sees the need to exert itself throughout the claimed EEZ. That's like 200 nautical miles off the coast and includes the continental shelf. Remember the 2003 incident when a Ming suddenly surfaced near JSMDF exercises. Its been sitting there for a while and presumably that's toward the east side of the East China sea. And again the Song incident with the Kitty Hawk. Though the exact location is not disclosed, one can presume these exercises are taking place towards the eastern side of the Ryukus. I have a theory that both subs were gathering signatures and taking sampling data of the water densities and temperatures.
You don't really need subs to gather oceanic water data and acoustic signatures. You can do it covertly with disguised ships, like fishing boats or 'oceanographic research" vessels.. There are also special vessels for that task. Lately there are pictures of a ship that is built like a Swath and is intended to function as a SURTASS. Also, there is a report that the PRC is putting a network of SOSUS on its own.
One thing of note. Working with sonar signatures is a lot easier in open water than in littoral. Why? Take this analogy. You get a car. You take the car roaring in an open country road without cars. The car will be the only noise around for miles. That's your open ocean. Take the same car. Put it in the middle of the busiest intersection of New York or Hong Kong or Bangkok. Rush hour. Now try to hear and isolate the sound of your car's engine in that traffic. That's littoral environment for you.
Now if you're dealing with hundreds, if not thousands of acoustic signatures at the same time, then ask to identify the correct one within 30 seconds, that will of course be difficult. That's where computers come in with acoustic sound databases to make all these comparisons. It gets worst the more the sonar is sensitive because the more sensitive it is, the more sound sources it picks up. These computers would also factor the water conditions (temperature, density, etc,.) in analyzing the signatures, since sound signatures can be altered with water conditions.
One thing I must note that in one of the videos where CCTV showed the insides of the Song, there are crewman sitting in workstations and in the background there appears to be rack style server cabinets. So whatever they are using, it has removable and upgradable server racks, similar to commercial PC servers, and no doubt uses COTS computing technologies (e.g. commercial processor chips).
For open water, what is important is acoustic or sound management. Compared to littoral waters, cold open water not only has less extraneous sound sources (e.g. biologics, ship traffic, etc,.) but the coldness means higher water densities and more sensitivity to sound. Open water is a lot less forgiving for noisy subs than littoral waters. If the 093/094 goes to the open water, acoustic management is what they should practice.