" The global positioning system consists of 24 satellites orbiting about 11,000 miles above the Earth, arrayed so that any spot on the planet is visible to at least six of the satellites at any time. Each satellite carries four atomic clocks on board, synchronized within a billionth of a second of one another by the master superclock in Boulder. Any GPS receiver, like those found in expensive cars or on handheld devices, receives signals from four of these satellites (at least), and uses those four numbers to calculate its three-dimensional location and the current time. "

― Steven H. Strogatz , Sync: The Emerging Science of Spontaneous Order