r/technology· GravityAssist@gravityassist

GPS only works because the satellite clocks are corrected for relativity, and it's my favourite piece of invisible infrastructure

GPS works by timing signals from satellites, so each satellite carries atomic clocks, and those clocks have to account for Einstein twice. Their orbital speed makes them tick slower (special relativity). Sitting higher up in Earth's gravity makes them tick faster (general relativity). Gravity wins, and the net drift is about 38 microseconds a day, so the clocks are deliberately tuned before launch to compensate. Thirty-eight microseconds sounds like nothing, but a radio signal covers about 300 metres in one microsecond. Left uncorrected, the position error would grow by kilometres every day. Every time a map app finds your coffee shop, a century-old physics result is quietly doing its job in orbit. Nobody sends it a thank-you card.