From NextGov:

To prepare for the possibility that it will one day deploy swarms of uncrewed drone submarines, the U.S. Navy is developing a system that will allow the global positioning system to function deep below the ocean’s surface. If successful, the technology could start to appear as soon as the 2020s.

The global positioning system is a marvel of accurate clocks and simple physics. A GPS receiver, like the kind in smartphones or car navigation systems, is little more than a radio antenna tuned to satellites. It listens for signals from the three GPS satellites that happen to be closest (there are more than 50 satellites in total, all constantly broadcasting their time), then it triangulates its own location from where it knows those satellites to be.

Though composed only of silent listeners and a constellation of passive beacons, GPS can tell someone where they are on or above the planet’s surface. 

But take heed of the prepositions—on or above. Right now, GPS signals can barely go below.

“Radio signals do not propagate very far underwater,” says Joshua Niedzwiecki, the director of sensor processing at BAE Systems. BAE is researching and developing the new technology for the government. “If you put your cellphone in a waterproof case and went into the ocean, as you start getting deeper and deeper, you’d very soon lose connectivity.”

And because GPS radio signals don’t reach very far underwater, they’re basically useless for any submarine below the surface. The Navy solves this problem now by outfitting each sub with a “very large and very expensive inertial measurement unit,” says Niedzwiecki. (Imagine a tricked-out, military-grade version of the accelerometer in your phone.) 

By remembering the submarine’s last known surface position, as measured by GPS, then keeping close track of its forward movement and…

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