Einstein shows the way

Einstein shows the way

So GPS satellite clocks are deliberately set a little slow at the factory before they go up. A signal that should beat 10,230,000 times a second is set to beat 0.0046 times a second fewer. Once in orbit, it speeds up by exactly that much, so from the ground it’s spot on. Control stations on the ground also watch the satellite clocks nonstop and send up fixes for whatever error is left. A theory scribbled on a chalkboard more than 110 years ago is guiding your trip home today.

Why?A clock keeps time by counting something that swings or shakes at a steady pace. In a pendulum clock it’s the pendulum; in a wristwatch it’s a tiny quartz crystal. A GPS satellite uses its atomic clock to make a signal that beats 10,230,000 times a second as its reference. Make that count a little smaller, and the clock runs a little slow.

Learn moreThe main control station is in Colorado, USA, and monitoring stations around the world watch the satellite signals too. The equipment at the station on screen is an example, drawn huge so you can see it. By this site’s calculation the factory setting is 10.22999999544 MHz; the value actually used is 10.22999999543 MHz.

The beat a satellite clock is set to at the factory

10.22999999544 MHz

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