In mid-2019 the U.S. Navy established an Aggressor Squadron (AGGRON) to provide submarine and ship crews with better information about their ASW (Anti-Submarine Warfare) capabilities versus potential opponents This is mainly for submarine crews as AGGRON is part of the UWDC (Undersea Warfighting Development Center). The United States has the largest submarine force in the world with 54 SSN (nuclear attack subs) and 14 SSBNs (ballistic missile carrying subs). The rest of the world has about 450 submarines, most of them non-nuclear and all trained and equipped to try and deal with American subs. The U.S. boats have the best passive sensors and guided torpedoes, as well as anti-ship missiles for use against surface ships. Two potential foes; China and Russia are putting a lot of money and effort into developing equipment and tactics for dealing with American subs in particular and Western nuclear and non-nuclear subs in general.
AGGRON will be of use to surface ship ASW officers because of the detailed data collected about how other nations conduct ASW and what special (not available to the U.S.) equipment they use. For example, Russia is developing a sub launched EW (Electronic Warfare) jammer that would broadcast a signal interfering with the ability of airdropped sonobuoys to pinpoint the location of a submarine so the helicopter or fixed-wing ASW aircraft could drop an ASW torpedo. If this Russian concept works, changes will have to be made in American ASW tactics and equipment. Subs already have decoys they can eject to degrade the accuracy of an incoming torpedo. There are other new concepts for ASW EW devices and some of them will work and force everyone else to adapt. This is nothing new as electronics developments have shaped submarine capabilities and tactics for over a century. In the 21st century there are more electronic and mechanical devices being developed to protect or hunt submarines.
AGGRON may also use one or more SSNs for realistic training exercises, the same way warplanes have been using aggressor squadrons with aircraft and pilots capable to operating like potential enemies. The navy has ship and submarine simulators that can help out with this OPFOR (opposing force) training but nothing does it as well as using actual subs and ships.
The U.S. Navy has another unique problem. For over fifty years all American subs have been nuclear while most of the subs worldwide are non-nuclear. The U.S. Navy continues to work on the problem of just how effective non-nuclear submarines would be in wartime. This occasionally raises the question of whether the U.S. should buy some of these non-nuclear boats itself. This radical proposal is based on two compelling factors. First, the U.S. Navy may not get enough money to maintain a force of 40-50 SSNs (attack subs). Second, the quietness of modern diesel-electric boats puts nuclear subs at a serious disadvantage, especially in coastal waters. With modern passive sensors, a submerged diesel-electric sub is often the best type of sub for finding and destroying other diesel-electric boats. While the nuclear sub is the most effective high seas vessel, especially if you have worldwide responsibilities and these nukes have to quickly move long distances to get to the troubled waters, the diesel electric boat, operating on batteries in coastal waters, is quieter and harder to find.
There are 39 nations operating nearly 400 diesel electric subs. Only three of these nations (China, Iran, North Korea) are likely to use their subs against the U.S. or its allies. China has fifty of these boats, Iran has three (plus 25 much smaller mini-subs) and North Korea has 20 (plus 50 much smaller mini-subs). The U.S. has to worry about 73 diesel electric subs and 75 mini-subs. But about half the full-size subs are elderly, obsolete, and noisy. The same can be said for at least half the mini-subs. That leaves about 36 full size subs and 40 mini-subs that are a clear threat. At the same time the older stuff can be a threat if you get sloppy. That’s a lot of subs, and they make the East Asian coast and the Persian Gulf dangerous places for American warships.
Since the 1990s the U.S. Navy has been trying to get an idea of just how bad the threat is. Some interesting solutions have been tried. For example, from 2005 to 2007 the United States leased a Swedish sub (Sweden only had five subs in service) and its crew to help American anti-submarine forces get a better idea of what they were up against. This Swedish boat was a "worst case" scenario, an approach that is preferred for training. The Gotland class Swedish subs involved were small (1,500 tons, 64.5 meters/200 feet long) and have a crew of only 25. The Gotland was based in San Diego, along with three dozen civilian technicians to help with maintenance.
For many years before the Gotland arrived, the U.S. Navy had trained against larger Australian diesel-electric subs and often came out second. The Gotland has one advantage over the Australian boats because of its AIP system. This allows Gotland to stay under water, silently, for several weeks at a time. The Gotland was something of a worst case in terms of what American surface ships and submarines might have to face in a future naval war. America's most likely naval opponents (China, North Korea, or Iran) now have some AIP boats as well as more and more diesel-electric subs which, in the hands of skilled crews, can be pretty deadly.
Based on the experience with Australian and Swedish subs, the U.S. Navy has been developing new anti-submarine tactics and equipment. All this is done in secret, obviously, except when new organizations like AGGRON appear and confirm that the Gotland experiments yielded much useful information. Apparently the modern, quiet diesel electric boats continue to be a major threat to U.S. surface warships and subs. Meanwhile, potential enemies build more of their cheaper and higher quality diesel-electric boats and train their crews by having them stalk actual warships, including U.S. ones. The subs are getting more numerous, while U.S. defenses are limping along because of the sheer technical problems of finding quiet diesel-electric boats in coastal waters.
One reason China wants to keep American naval forces out of their economic zone (which does not bar foreign warships) is so that Chinese diesel electric subs can train without being stalked by American subs, surface ships, and aircraft looking for realistic practice tracking Chinese boats. At the same time the U.S. Navy has lost the full use of its most effective underwater anti-submarine training area (a well mapped and instrumented area off southern California) because environmentalist activists have convinced judges that the use of active sonar in this training area is harmful to some species of aquatic animals.
Moreover, the North Korean and Iranian fleets (and governments) are in decline, while China is pouring more cash into their armed forces. If there’s any diesel-electric boats the U.S. Navy has to be extremely concerned about, it’s the Chinese. While China continues to try and develop world class nuclear subs, they are also moving ahead in creating world class diesel electric boats.