Relativity

On February 22, 1978, the first Navstar GPS satellite lifted off from Vandenberg Air Force Base. The engineers who built it had solved a problem that seemed impossible: determining a position anywhere on Earth to within meters, using signals from satellites orbiting 20,000 kilometers away. The solution required not just advances in electronics and rocketry, but a practical application of Einstein’s theory of relativity that affects every GPS receiver in existence today. ...