Technology creeps up on us until it runs everything. From the New Yorker:
The radio signal that is the lifeblood of the Global Positioning System originates from a constellation of twenty-four satellites, orbiting more than twelve thousand miles above Earth. When it reaches the ground, after about sixty-seven milliseconds, it is so weak as to be almost imperceptible. (G.P.S. experts often compare processing the signal to trying to read by the light of a single bulb in a city thousands of miles away.) The signal tells the receiver the precise moment at which it left the satellite. Given four of these cues, processed simultaneously, the receiver can extrapolate its position in three dimensions. A timing error of as little as a millisecond can throw its calculation off by nearly two hundred miles. […]
That’s a lot of responsibility for such a weak signal. 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.