Submariners like to say there are two kinds of ships: subs and targets. The Pentagon's futurists want to turn that aphorism on its head, and develop a new kind of surface ship that can turn a sub into a target. Naturally, the sub-hunter won't have a human on board. Here's how it's going to work.

The video above is a new promotional piece of machinima (do people still say that?) released by the defense contractor Science Applications International Corporation, which has a $58 million contractwith Darpa to build its unmanned sub-hunter of the future. That maritime robot, called the Anti-Submarine Warfare Continuous Trail Unmanned Vehicle, or ACTUV, doesn't exist yet and won't for years. But here SAIC at least sketches out how the long, thin and "radically different" ACTUV can keep surface ships from becoming targets.

The really interesting thing here is how different the surface-gliding ACTUV is from the now-familiar drones that litter the skies. Even the longest-flying drones can only stay in the air for 30 hours or so. SAIC intends for this thing to stay on a hunt for 60 to 90 days.

What's more, SAIC is designing the ACTUV to be way more autonomous than contemporary drone aircraft: After a sailor powers it up and helps guide it out of port, she can go on a long vacation while the ACTUV speeds out to the open water to use its long-range acquisition sonar and other advanced sensors to scan for submarines, while automatically steering clear of any nearby surface ships.

Assuming SAIC isn't over-promising (much), the sonar pods underneath the belly of the ACTUV create an acoustic image of a submarine and pursue it at high speed – although that's something that can only happen when the ACTUV gets fairly close to its quarry. (More on that in a second.) Once the ACTUV thinks it's got something, it pings nearby Navy ships through a satellite link. If a sailor thinks the ACTUV has made a mistake, he can convey that back to the unmanned ship and it'll move on.

If not, the ACTUV operates alongside the fleet, with coordination not often seen with aerial drone tactics. SAIC apparently wants the ACTUV in constant communication with a mothership and Naval aircraft that would fly overhead and drop sonar charges to hunt the mystery sub, with the ACTUV speeding along to keep pace with the swift submarine. SAIC seems to intend for the ACTUV to follow the sub back to its home port (!) if necessary, or until a human in the fleet commands it to break contact. The ACTUV, in case you were wondering, isn't armed.

How all this will happen isn't yet clear. The subs that really give the U.S. Navy pause are cheap diesel-electric models, which are technologically puny compared to the Navy's nuclear-powered subs but can be much quieter and harder to track. Russia sells them; Iran claims to have them. SAIC's video suggests that the ACTUV can't actually find the diesel-electric sub on its own: The scenario here depends on a Navy commander suspecting there's an enemy sub in the area and deploying a P8 Poseidon surveillance aircraft (successor to the P-3C Orion) to drop sonar buoys to find it. The ACTUV sprints out in a certain pattern while "predict[ing] that long-range sensors will be able to completely envelop" the area where the sub might be "and prevent successful evasion." So not an exact science, but its sonars are said to get more precise the closer the ACTUV gets to the suspected sub target.

The on-board hardware described generically in the video relies on "collected data and sophisticated logics" to "infer the intent" of watercraft. So that should at least make the ACTUV cognizant of any sizable metal thing that seems to be tracking a Navy ship. And if SAIC is right that the ACTUV can really hear the diesel-electric subs, then that enemy sub really may become the ocean's newest target.