Global Positioning System
In order for your car’s GPS navigation to function as accurately as it does, satellites have to take relativistic effects into account. This is because even though satellites aren’t moving at anything close to the speed of light, they are still going pretty fast. The satellites are also sending signals to ground stations on Earth. These stations (and the GPS unit in your car) are all experiencing higher accelerations due to gravity than the satellites in orbit.
To get that pinpoint accuracy, the satellites use clocks that are accurate to a few billionths of a second (nanoseconds). Since each satellite is 12,600 miles (20,300 kilometers) above Earth and moves at about 6,000 miles per hour (10,000 km/h), there’s a relativistic time dilation that tacks on about 4 microseconds each day. Add in the effects of gravity and the figure goes up to about 7 microseconds. That’s 7,000 nanoseconds.
The difference is very real: if no relativistic effects were accounted for, a GPS unit that tells you it’s a half mile (0.8 km) to the next gas station would be 5 miles (8 km) off after only one day.