Physics
Gravity is curved spacetime
GPS satellites orbit 20,000 km up, in weaker gravity than clocks on the ground, and under general relativity that makes their onboard clocks tick faster than identical clocks on Earth. Left uncorrected, that mismatch would push GPS position errors past 11 kilometres in a single day. Engineers build the correction directly into the system.
Sources & further reading
NASA Science (Physics of the Cosmos Program)General Relativity and the Nature of SpacetimeHow curved spacetime explains bent, redshifted light near black holes.NASAEinstein's Theory of Relativity, Critical For GPS, Seen In Distant StarsWhy GPS satellites need relativity-corrected clocks to work.American Institute of Physics / Physics TodayRelativity and the Global Positioning SystemThe concrete number: GPS would drift over 11 km a day without relativity corrections.
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