This is a fun little program. ACTUV, Anti-submarine, Continuous Trail Unmanned Vessel. A not particularly tiny submerged craft that’s not really a submarine (its always got its snort up providing air for its diesel engines.} ACTUV is designed to folloow opposition submarines around instead of having dedicated manned plaforms like a destroyer or Anti Submarine Patrol Aircraft like a P-3 or P-8 do the job.
In certain circumstances ACTUV could probably be used as a form of cordon, spaced a few miles apart across a channel a submarine is or has to use they can wait patiently for days, weeks, months, until the spot something, whereupon they lock up and follow the target till something with more decision making authority comes along to deal with the matter.
ACTUV would be the end of the drug smuggling semi submersibles or even true submarines. They can probably greatly enhance and reduce the load on forces tracking bad actors with subs who we need to keep an eye on.
Even more, this is a way of dealing with the expanding threat of conventional submarines (SSKs) that have Air Independent Propulsion (AIP) and can operate nearly silently for many days at sea. These small vessels are devilishly hard for even modern Anti Submarine Systems to find and in shallow seas and near shore are deadly dangerous.
Once found if you can keep an asset ‘on’ them you can usually stay locked on to an SSK, but how do you do that if the asset you are using is a destroyer worth billions or an aircraft worth hundreds of millions with crews who need to go home sometime? ACTUV solves the problem, localize the target and send for the nearest ACTUV, it sprints to you and locks up on the target. Thereafter it follows the submarine around day after day after day, until the crew of the submarine, whose only real protection is hiding, give it up and go home for some sessions with the shrinks. There is even a game that was used to crowd source ideas on how to use ACTUV.