The GPS satellite’s clock

38.5 microseconds a day

GPS satellites circle Earth about 20,000 km up. Because they move fast, their clocks fall behind by about 7.2 microseconds (millionths of a second) a day. But because they are far from Earth, where gravity is weaker, their clocks gain about 45.7 microseconds a day. Together, a satellite clock runs about 38.5 microseconds a day faster than a clock on the ground. If nobody corrected this, your position would drift by about 12 km every day.

Why?GPS works out distance from how long a signal from each satellite takes to arrive. In one microsecond, light travels 300 meters. So even a tiny clock error turns into a big position error. Satellite clocks are set to run slightly slow before launch to make up for it.

Learn moreHeight changes time even on the ground. In 2010, scientists in the United States put two very precise clocks 33 cm apart in height and found that the higher one ran a tiny bit faster. This effect, gravity bending time, is at the heart of general relativity.

Clock we are watching

A GPS satellite’s clock (compared with the ground)

How much slower

38.5 μs faster per day

Clock rate

0.99999999975

seconds this clock ticks while a clock far away in empty space ticks one second · calculated with the formulas of relativity