Friday’s 9.0-magnitude earthquake was so powerful that it actually widened Japan. While parts of the country barely moved, other regions are now 13 feet closer to the United States. The earthquake may also have tilted the earth’s axis and shortened the day by 1.8 millionths of a second. Does this mean that GPS navigation systems in Japan are broken?
Only slightly. Modern GPS receivers have a 10-foot margin of error. A 13-foot displacement means the navigators are currently miscalibrated in the most-shifted portions of Japan. They should still get the job done, however. Only a few years ago, the margin of error was around 50 feet, and GPS devices still managed to direct people to the grocery store with little trouble.
Responsibility for keeping a commercial GPS working properly is split between the U.S. government and the company that manufactured the unit. The Air Force takes care of the space stuff. They make sure the 32 or so satellites that run the global navigation system remain in orbit. * However, the mapping software that enables a GPS receiver to determine street location and give directions is the responsibility of the unit’s manufacturer. When streets move, either because of municipal construction or massive earthquakes, it’s up to the company to update its software.
Soon, representatives of GPS manufacturers will walk and drive through the affected areas, armed with GPS locators, to change the official coordinates of streets and addresses. The process will probably take a few months.
While GPS receivers may be a little confused right now, our clocks are just fine. There are two official, globally recognized systems of time. The atomic clock defines a second as 9,192,631,770 radiation cycles of the cesium-133 atom, and a day as 86,400 of those seconds. The universal clock, by contrast, defines a day as one rotation of the earth. Because the earth’s rotation is irregular, and almost never takes exactly 86,400 atomic seconds, the two clocks can drift apart. When the difference between the two exceeds 0.9 seconds, an international consortium of astronomers called the International Earth Rotation and Reference Systems Service adjusts universal time. But a change of 1.8 millionths of a second isn’t going to affect the universal clock anytime soon. In fact, astronomers at different observatories sometimes differ in their estimate of a day’s length by more than that amount.
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Explainer thanks Carly Baltes of Garmin International.
Correction, March 16, 2011: This article incorrectly stated that the GPS satellites are in geosynchronous orbit. (Return to the corrected sentence.)