Once the yacht was off the coast of Italy, the team — with the knowledge and permission of the vessel's owner, a wealthy individual who doesn't want to be named — used a briefcase-sized device to slowly drown out legitimate GPS signals and take control of the ship's guidance. It worked. Using the false signals, the team gradually turned the ship while its chart display continued to show it traveling in a straight line.

Since the U.S. developed the Global Positioning System, or GPS, in the 1970s, the nation has become increasingly reliant on this satellite-based method of determining position, time and velocity. Today GPS is used to do everything from calling an Uber and navigating about town to time-stamping financial transactions and dropping guided bombs.

That growing reliance could be a problem, especially if the 30 GPS satellites or their ground stations ever went down, experts say. And that has led a number of government organizations and companies to start thinking about how to fortify the system and whether other signals could be used to supplement or back up the ever-ubiquitous GPS.

In December, Air Force Secretary Heather Wilson told The Times in an interview that "there's not a military mission that doesn't depend on space," and that the Air Force practices as if it didn't have GPS.

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