Your GPS Needs Einstein

The GPS system depends on 31 satellites orbiting at 20,200 km altitude, each carrying atomic clocks accurate to billionths of a second.

Special relativity: satellites move at 14,000 km/h, so their clocks tick slower by about 7 microseconds per day. General relativity: satellites sit in weaker gravity, so their clocks tick faster by about 45 microseconds per day. Net result: clocks gain 38 microseconds daily.