How do you keep track of increasingly stealthy Russian, Chinese and Iranian submarines? If you’re the U.S. military, you build a robotic ghost ship to follow them around the high seas.

In 2010, the Defense Advanced Research Projects Agency, or DARPA, announced that they were building a 132-foot autonomous boat to track quiet, diesel-powered submarines. The program was dubbed Anti-submarine Warfare Continuous Trail Unmanned Vessel, or ACTUV.

To little notice, the system earlier this year passed a critical test, moving much closer to actual deployment and potentially changing not just naval warfare but also the way humans, ships, and robotic systems interact across the world’s waters.

In six weeks of tests along a 35-nautical mile stretch of water off of Mississippi, testers at engineering company Leidos and DARPA put the ACTUV’s systems through 100 different scenarios. The test boat, equipped with nothing more than off-the-shelf radar components, a digital area chart and some proprietary software, was able to complete an autonomous trip without crashing into rocks, shoals, or erratically behaving surface vessels. In future tests, the ship will tail a target boat at 1 kilometer’s distance.

Most importantly, the tests showed that the robot boat could execute a difficult military mission without violating the maritime laws outlined in the Convention on the International Regulations for Preventing Collisions at Sea. They also provided a critical proof-of-concept for machine-learning systems at sea, showing that big robots can, indeed, navigate the open seas along with cruise ships and shrimp boats. The next big challenge for the ACTUV will be the same kind of tests, but with “enemy ships” trying to block or interfere with it.  

The world’s waters could soon be crowded with robot ships that almost never hit land. 

Speaking at a National Defense Association Event in Virginia, DARPA program manager Ellison Urban outlined why the Navy needs sub-hunting boat bots. Diesel-electric submarines, with their nearly-noiseless engines, are incredibly difficult to track from afar. They’re also cheap at $200 million to $300 million apiece, making them affordable to the likes of Iran, which claims to have a fleet of 17. “Instead of chasing down these submarines and trying to keep track of them with expensive nuclear powered-submarines, which is the way we do it now, we want to try and build this at significantly reduced cost. It will be able to transit by itself across thousands of kilometers of ocean and it can deploy for months at a time. It can go out, find a diesel-electric submarine and just ping on it,” said Urban.

Leidos conducted the tests on a 42-foot surrogate boat while they finish construction of the ACTUV prototype vessel, the Sea Hunter, which is expected to launch this fall.

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