GPS accuracy depends on Einstein's theory of relativity

The 24 GPS satellites orbiting Earth each carry an atomic clock, but those clocks run at slightly different rates than clocks on the ground — due to both special relativity (satellites move fast, slowing their clocks) and general relativity (weaker gravity speeds them up). The net result is that GPS clocks gain about 38 microseconds per day relative to Earth. Without corrections for these effects, GPS positions would drift by roughly 11 kilometres per day. Einstein's 1915 theory of general relativity is, in this sense, a practical engineering requirement of every smartphone.

Source: Ashby, Relativity and the Global Positioning System, Physics Today (2002)