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Relativity and GPS

May 8, 2012

[SFX: GPS device sounds]

The Global Positioning System is one of the marvels of modern technology. It guides us on land, in the air, and at sea. It helps rescuers find lost hikers. And it makes it possible for most of the world to track your every move.

Yet without the contributions of Albert Einstein, GPS wouldn’t work at all.

The system consists of a network of satellites in Earth orbit. Each satellite has an atomic clock that’s accurate to a few billionths of a second. GPS receivers determine your position by triangulating the time signals from several satellites, providing an accuracy of a few feet.

But each satellite is moving at thousands of miles an hour. And it’s thousands of miles high, so it “feels” a weaker gravitational pull than at Earth’s surface. According to Einstein’s theories of Relativity, those effects mean that the clocks on the satellites tick faster than those on the ground — by about 38 millionths of a second per day.

That may not sound like much, but for a system in which billionths of a second matter, it’s huge. Over a single day, that would add up to a positioning error of several miles.

So before the satellites are launched, their clocks are adjusted so that they tick slightly slower than they would on Earth. That keeps their timing just right — and keeps you on target.

More tomorrow.


Script by Damond Benningfield, Copyright 2012


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