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By 1977, scientists knew that relativity would cause orbiting and ground-based clocks to beat out of sync.

Two competing effects are at work. The satellite moves rapidly in its orbit, so time dilation makes it tick more slowly compared to a ground-based clock. But clocks higher up in Earth's gravitational field should tick more rapidly than Earth-bound clocks.

Special relativity is trying to slow the satellite clock down relative to the ground, general relativity says it should tick faster.