We would be lost without GPS. The Global Positioning System constellation of satellites started life as a military project, but now enables turn-by-turn directions in the palm of your hand, and confidence that it's almost impossible to get lost as long as you can see the sky.

Underwater, it's a different story. GPS doesn't penetrate the briny deep, so Darpa, the Pentagon’s research arm, wants a system that will keep the robots plumbing the oceans on the map, and it's asking for proposals from industry. The proposed solution also wins this week’s award for best military acronym: "Posydon," for Positioning System for Deep Ocean Navigation.

“What they are trying to do here is revolutionize underwater navigation in a way that is similar for what GPS did for above water,” says Neil Adams, director of defense systems at Draper, the non-profit R&D lab that's working on an answer.

GPS satellites use very high frequency radio waves (in the L band, between one and two GHz), which can't penetrate more than a few inches of sea water. To navigate, today's unmanned underwater vehicles (UUVs), like some submarines, usually rely on dead-reckoning: They start out with an accurate fix on their location, then keep track of where they are, based on how far they’ve travelled, how fast, and in which direction.

That works okay for short trips, but long voyages demand the occasional GPS lock. That means popping to the surface, where military vehicles are vulnerable to detection. And dead reckoning requires expensive, power-hungry inertial measurement units. So, yeah, it's not an ideal system.

A far better technology for underwater communication is acoustic signals—sound waves—at very low frequencies that travel well over long distances. Instead of satellites transmitting radio frequencies, an underwater GPS system could use a constellation of acoustic transmitters—powered underwater beacons, tethered to the seafloor, sending pings that propagate through the water.

A network of transmitters will allow a UUV to hear signals from several beacons at once and triangulate its position—the way our phones listen for signals from multiple GPS satellites. Now Darpa wants someone to find an appropriate type of acoustic technology, and figure out how many beacons you'd need to give good coverage across an entire ocean basin.

The ocean isn't the easiest place to work. Posydon's designers will have to cope with noise pollution, avoiding interference from and with all the other sources of sound in the oceans we make through shipping, drilling, and military exercises. And they need a decent enough data transfer rate to be able to communicate meaningful information to drone-mounted nav systems. Even subtle changes in water temperature can alter the speed of sound. If the signals sent out from the beacons are going to be reliable enough for navigation, whoever's running things will need a detailed understanding of how the waves propagate through seawater. Draper plans to start with high-fidelity computer "virtual-ocean" models to figure that out, and back it up with real-world tests.

Adams says any proposal will be cognizant of wildlife concerns. Animals like whales, which sound to communicate over long distances, may suffer from the noises humanity already makes. "We had to submit environmental assessment information, and it's all well within the acceptable parameters," he says. The proposal states "Darpa intends that execution of the Posydon program obey all applicable laws and regulations protecting marine life."

Other researchers looking at this question are interested in bathymetric navigation—the comparison of scans of the ocean floor with known maps. Sonar or LIDaR can be used to do that, but both require a line of sight, and for the vehicle to be actively transmitting. With an acoustic system, the vehicles themselves would be passive listeners, waiting for pings. That level of stealth could be vital in military applications.

Civilians, too, could benefit from a reliable way to navigate underwater. Scientific and commercial research missions using UUVs could be easier and quicker, if the vessel didn't have to regularly surface. That could happen soon: Darpa wants to see at-sea demonstrations by 2018. If it has its way, there soon won’t be anywhere on the planet where you can still get lost.