DARPA program seeks to give subs and undersea drones an acoustic GPS

A conceptual illustration of how POSYDON will work. (credit: BAE Systems)

A technology being developed under a Defense Advanced Research Projects Agency (DARPA) program could soon bring GPS-like navigation below the waves. The POSYDON program seeks to create a network of acoustic underwater beacons that act like GPS satellites—broadcasting a burst of data encoded into sound waves that underwater craft can use to get a fix on their location.

GPS uses radio signals from satellites carrying time and position data, allowing a receiver to passively pick up that data and calculate its position. But while GPS works well for ships, ground vehicles, and aircraft, the radio signal from GPS satellites doesn't penetrate very far below the ocean's surface. It’s a technical problem that submarines have dealt with since long before GPS was available. It has forced subs to come close to the surface and raise an antenna mast if crews want to figure out where they are.

During the Cold War, the US developed an incredibly accurate—and expensive—technology for helping submarines navigate the seas without surfacing. The solution was based on inertial sensors: gyroscopes measured acceleration and movement relative to the Earth in a fashion similar to the guidance systems used for ballistic missiles. Gyroscopes have since gotten a lot smaller, and the fundamental parts of inertial navigation are now part of most smartphones. But precise inertial systems are still very expensive and not easy to miniaturize. Really accurate inertial navigation has continued to be a problem for undersea drones.

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