One day in January 2016, the U.S. Air Force decommissioned a single satellite and accidentally uploaded an incorrect time to the clocks aboard 15 others in the process. The time difference was tiny -- only 13 millionths of a second -- but the results were significant. Police, fire, and EMS radio equipment in some parts of North America stopped functioning for more than 12 hours, and BBC digital radio went out for two days in several areas. Luckily, short-term backup systems helped ensure that the general public wasn't affected by many of the problems, but the scare posed an important question: what do we do if GPS fails completely? We might be able to go back to paper maps, but navigation is a very small part of what makes the system important. What's most essential about GPS is time: each satellite contains multiple atomic clocks that are synchronized to nanosecond precision and continually broadcast signals to Earth. For navigation, GPS units use the tiny differences in the arrival time of each of these signals to know exactly where they are in the world. But some things, like cell towers, power grids, and ATMs, rely on the time alone to stay synchronized. This means that a GPS failure could disrupt an untold number of systems essential to daily life -- and there's no backup plan.
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