The United States Air Force, which runs the G.P.S. Master Control Station, in Colorado, calls G.P.S. “the world’s only global utility.” Wholly owned by the U.S. government, the system is available free to everyone, everywhere; an ISIS terrorist glancing at his phone for a position fix benefits from the Pentagon’s largesse as much as a commuter on I-95. Since the first G.P.S. satellite was launched, in 1978, the system has steadily become the most powerful of its kind. (Other countries have navigation satellite networks, but none are as dependable or as widely available.) There are now around seven G.P.S. receivers on this planet for every ten people. Estimates of the system’s economic value often run into the trillions of dollars. ... The Pentagon’s Defense Advanced Research Projects Agency recently determined that, within thirty seconds of a catastrophic G.P.S. shutdown, a position reading would have a margin of error the size of Washington, D.C. After an hour, it would be Montana-sized. Drivers might miss their freeway exits, but planes would also be grounded, ships would drift off course, commuter-rail systems would be tied up, and millions of freight-train cars with G.P.S. beacons would disappear from the map. ... Fortunately, a worldwide G.P.S. failure is unlikely. A hacker or terrorist would require either a weapon powerful enough to destroy the satellites or a way to infiltrate the heavily fortified Master Control Station. The bigger worry is spoofing, the transmission of a bogus G.P.S. signal that nearby receivers mistake for the real thing.