Margen
GPS satellites carry atomic clocks, and those clocks don't tick at the same rate as ours. General relativity says that with less gravity up there, time runs slightly faster; special relativity adds that at such high speeds, it runs slightly slower. The net result: orbiting clocks gain about 38 microseconds every day.
That sounds ridiculous, but the system pinpoints your position by timing signals that travel at the speed of light — which covers about 300 metres per microsecond. Left uncorrected, the accumulated error would reach roughly ten kilometres per day: GPS would be useless within an afternoon.
That's why engineers adjust the clocks' frequency before launch. Every time your phone guides you to a café, you are quietly using the equations Einstein published more than a century ago.