Last January, as the U.S. Air Force was taking one satellite in the country’s constellation of GPS satellites offline, an incorrect time was accidentally uploaded to several others, making them out of sync by less time than it takes for the sound of a gunshot to leave the chamber. The minute error disrupted GPS-dependent timing equipment around the world for more than 12 hours. BBC digital radio was out for two days in many areas, and the anomaly was even detected in electrical power grids.
Despite its name, the Global Positioning System is not about maps; it’s about time. Each satellite in the constellation (24 are needed, plus the U.S. has several spares) has multiple atomic clocks on board, synchronized with each other and to Coordinated Universal Time (UTC)—the time standard used across the world—down to the nanosecond. The satellites continually broadcast their time and position information down to Earth, where GPS receivers in equipment from iPhones to automated tractors acquire signals and use the minuscule differences in their arrival time to determine an exact position.