Trilateration from 20,000 kilometers
The Global Positioning System (GPS) uses atomic clocks on orbital satellites. To pinpoint your phone, your receiver computes the time difference of arrival from at least four satellites.
Why Relativity is essential
Because the satellites move at high speeds and experience weaker gravity than on Earth's surface, General Relativity causes satellite clocks to tick ~38 microseconds faster per day. Without relativistic correction, navigation errors would accumulate by 10+ kilometers every single day.
