POSYDON: GPS-Like Audio Transmitters Planned For The Oceans

Defense Researchers Plan To Bring "GPS" Where It's Never Gone Before: Under The Sea

A system of audio transmitters would stand in for GPS's satellite broadcasts and let underwater drones navigate without surfacing.

Steven Melendez | June 11, 2016

We take for granted that GPS can get us where we’re going pretty much anywhere on earth, but there’s one important place satellite navigation systems are essentially guaranteed not to work: under the sea.
The satellite broadcasts that GPS systems rely on can’t penetrate very far below the ocean’s surface, and that’s a problem for unmanned underwater vehicles—essentially, drone submarines—designed to autonomously navigate below the sea.
That’s why the Defense Advanced Research Projects Agency has announced plans to build an underwater GPS-style system called Posydon—which stands for Positioning System for Deep Ocean Navigation—that will use underwater sound broadcasts to let submarines determine their own positions without coming to the surface.

"Illustration explaining undersea positioning of POSYDON system" Source: http://www.baesystems.com/en-us/article/undersea-navigation-and-positioning-system-development-to-begin-for-u-s--navy

<more at http://www.fastcompany.com/3060116/defense-researchers-plan-to-bring-gps-where-its-never-gone-before-under-the-sea; related articles and links: http://www.darpa.mil/program/positioning-system-for-deep-ocean-navigation (Positioning System for Deep Ocean Navigation (POSYDON). April 2016) and  http://www.theatlantic.com/technology/archive/2016/06/its-gps-underwater-for-robots/486656/ (GPS Doesn't Work Underwater. So the U.S. Navy is developing a new kind of system—built specifically for drone submarines. June 13, 2016)>