The theory of relativity provides the basis for GPS

　　The timing signals of the GPS satellites of the Global Positioning System provide information on latitude, longitude and altitude, and precise distance measurement requires an accurate clock. Therefore, accurate GPS receivers use relativistic effects. A GPS receiver that is accurate to within 30 meters means that it has exploited relativistic effects. "If you don't take into account relativistic effects, the clocks on satellites are out of sync with the Earth's," explains Clifford M. Will, a physicist at the University of Washington. Relativity says that fast-moving objects move more slowly over time than stationary ones. Will calculates that each GPS satellite travels about 14,000 kilometers per hour, which means its onboard atomic clock is 7 microseconds slower per day than the clock on Earth. And gravity exerts a greater relativistic effect on time. At an altitude of about 20,000 kilometers, the gravitational pull experienced by GPS satellites is about a quarter of that on the ground. The result is that the onboard clock is 45 microseconds faster per day, and the GPS accounts for a total of 38 microseconds of deviation. "Without frequency compensation on the satellite, that would add to the error of 11 kilometers per day," Ashby explained. (The effect is actually more complicated because the satellite follows an eccentric orbit, sometimes closer to Earth and sometimes closer to Earth. farther away.

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