GPS and relativity

In order for GPS to work correctly, the software must explicitly take into account both special relativity and general relativity.

Adjustment for special relativity is needed because satellites fly around the Earth at approximately 8 km/s, which by itself results in time aboard a satellite flowing about 7 microseconds per day slower compared to time on Earth.

Adjustment for general relativity is due to the fact that GPS satellites orbit at a distance from Earth, where its gravitational pull is somewhat weaker compared to that on the Earth’s surface. This (again, by itself) leads to time aboard a satellite flowing approximately 45 microseconds per day faster compared to time at the sea level.

(Source: public lecture by Barry Barish at the Fermilab)

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