Galileo satellite navigation and GPS satellites carry two types of clocks.
One will keep time with an accuracy of around one nanosecond (one one-thousand-millionth of a second) in 24 hours – equivalent to losing or gaining a second in 2.7 million years.
The other onboard clocks are accurate to 10 nanoseconds per day. In comparison, an ordinary digital wristwatch has an accuracy of about one second per day.
The 10 year Galileo program is a constellation of thirty satellites. It will be the first civilian positioning system to offer global coverage. Their accuracy will provide meter-wide positioning by measuring radio waves. Travelling at about 300 million meters per second, the radio wave covers a distance of around one-third meter in one nanosecond. [ScienceDaily]
An upcoming mercury clock is accurate to one second in about 400 million years – give or take a systemic perturbation or two.