How GPS works: how your phone knows where you are
Relativity and GPS: why satellite clocks are set to run slow
Before a GPS satellite goes up, engineers set its atomic clock to tick a little too slowly. It is not a mistake. The reason comes from Einstein, and from two things that change how fast time passes.
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Before launch, while the satellite is still on the ground, engineers set its atomic clock to tick a tiny bit too slowly. Next to a normal clock, it falls a little more behind every day.
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It is one of the best clocks in the world, and the engineers make it tick wrong on purpose. Why would anyone do that?
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