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Your phone finds you in seconds, anywhere

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on Earth. Yet it carries no map

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that knows where you are. So how

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does it know?

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High above the Earth, more than thirty

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satellites circle the planet twice a day.

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Each one carries an atomic clock, and

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calls out the same two things, again

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and again: where it is, and exactly

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what time it is.

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Those calls travel at the speed of

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light, and your phone times how long

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each one took to arrive. That timing

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is the measuring tape. A timing mistake

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of one microsecond, a millionth of a

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second, means roughly three hundred metres of

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error.

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One satellite only tells your phone it

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is somewhere on a giant sphere. A

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second satellite narrows that down to a

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circle. A third brings it down to

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two points, and one of those points

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is you.

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But your phone has no atomic clock

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of its own, so a fourth satellite

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does one more job. It lets the

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phone correct its own clock, and solve

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for latitude, longitude, height, and time, all

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at once.

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Even Einstein is on duty. Satellite clocks

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gain about thirty eight microseconds a day

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because of relativity. Left uncorrected, that would

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drift positions by about ten kilometres a

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day. Your phone knows where you are

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because it knows the time, to a

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billionth of a second.