Relativity and the Global Positioning System

An ideal companion to "The Universe in a Nutshell" is this article by Neil Ashby, a Professor of Physics at University of Colorado - Boulder. Hand- held GPS devices have become standard equipment for boaters, hikers, and technology junkies. It is now taken for granted that one's position on the earth can be obtained within a few seconds, accurately, and free. But few users are aware of the degree to which the GPS system depends on general relativity. The system of 24 satellites and their atomic clocks would lose 11 km of accuracy per day, if the bending of space and time were left out of the equations. Ashby has written this article for an audience of physicists, but even those without a great deal of background can get the idea.