As the U.S. Navy continues to investigate how to deal with diesel submarines (or SSKs, in naval shorthand), especially by using diesel subs from friendly nations for anti-submarine training exercises. Despite the large number of submarine detection devices available to the U.S. Navy, hunting down diesel subs in coastal waters is proving to be quite difficult. Typically, diesel subs lurk near coasts, taking cover among rocky outcroppings, or even shipwrecks, underwater. The SSKs remain stationary, or move slowly (100-150 meters a minute), and are nearly impossible to pick up with sonar. By staying near the coast, the SSKs are within range of friendly anti-ship and anti-aircraft missiles. This means that the SSKs can only be safely gone after with larger, and noisier, nuclear subs.
And because coastal waters probably would be within range of shore-based anti-shipping missiles or land-based aircraft, any sea-based ASW operation other than by submarine, could be vulnerable.
If anti-aircraft fire is not a problem, the dipping sonar from us ASW (anti-submarine warfare) helicopters has proven effective against SSKs. But that's only if you have a general idea where the SSK is. You need detection systems that cover a wider area to sweep a larger area for the possible presence of SSKs. But most of these systems (magnetic field or heat detectors) are much less effective in coastal waters, where they pick up a lot of false positives.
Synthetic aperture radar, introduced during the 1990s, was seen as particularly lethal to SSKs, as this new radar, used on ASW aircraft, could spot the periscope of an SSK. But the latest SSKs can launch torpedo attacks without the periscope, and if they have one of the new engines that does not need a large supply of fresh air (like diesel engines do), they will rarely show anything on the surface. Fortunately, the hundred or so SSKs belonging to likely foes (North Korea, China, Iran) don't have the new engines, or many of the "periscopeless" fire control systems. But even these SSKs can stay underwater for 12 hours or more.
The high-frequency dipping sonar operated from ASW helicopters is said to give submariners their biggest headaches and may prove to be the most effective solution. There is a range of alternative non-acoustic ASW methods, although these have limitations. Magnetic anomaly detection (MAD) picks up variations in the magnetic field surrounding a submarine, but in shallow waters it is very prone to picking up false alarms. In muddy waters, forward-looking infra-red detectors (FLIR) can identify temperature differences between a submarines wake and the surrounding sea, but it cannot differentiate between those and irrelevant heat sources, such as industrial effluent, commercial ship wakes and river outflows.
More modern, and effective, detections systems (blue-green laser and light or chemical detectors), that were so promising at the end of the Cold War, have not panned out. So, for the moment, 20 year old submarines are being sought using 20 year old technologies. And there is considerable fear that the SSKs will come out on top.