GPS satellites have to account for Einstein's relativity

GPS satellites travel fast enough and sit high enough in Earth's gravity well that time passes at different rates for them than for us — general relativity speeds their clocks by 45 microseconds per day, while special relativity slows them by 7 microseconds. Engineers deliberately set the satellite clocks slightly slow before launch so they drift into perfect sync once in orbit. Without these corrections, GPS would accumulate errors of 10 kilometres per day.

Source: Ashby, N., 'Relativity in the Global Positioning System', Living Reviews in Relativity (2003)