Answer Geek: How GPS Works

The Global Positioning System does have its limits. Satellites transmit very low-power signals which don’t pass through walls, rocks, city buildings, or even thickly wooded forests. So it helps if you are out in the open. GPS isn’t 100 percent accurate either. Everything from atmospheric conditions to the position of the satellites relative to one another to small errors in the atomic clocks can affect the accuracy of the system. And in the GPS business, small errors add up: a single nanosecond difference in those atomic clocks up in space translates to about a 12 inches of error down here on the ground. It doesn’t take much before your car’s GPS navigation device is telling you that you are on Fourth Avenue when you are really on First.

But for the average user, the most important reason for GPS inaccuracy went away last December when Bill Clinton ordered the U.S. military to stop intentionally scrambling the signal. Before that date, the Department of Defense, which maintains the system, introduced artificial clock errors into satellite signals in the name of national security. That limited the accuracy to about 100 yards for most users. Eliminating the artificial error resulted in an instant 10-fold improvement in the accuracy of the system.

So far, at least, the national security still seems to be intact, and those of us with GPS units are all a quite a bit more location-savvy with our appointments and on our travels.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.

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