General relativity predicts "that the relative clicking of clocks changes depending on their position in a gravitational field. Satellites located high above Earth are moving in a gravitational field that is slightly weaker than what we experience on the ground. As a result, their internal clocks tick at a different rate than those on Earth. The effect is extremely small, but it is comparable to the accuracy needed to distinguish the position of objects on the scale that is otherwise possible with needed by global positioning devices."
Thanks to Ashwani Vasishth