GIOVE-B (Galileo In-Orbit Validation Element) was launched by a Soyuz rocket from Baiknour on 27 April 2008, carrying the most accurate atomic clock ever flown into space at that moment.

The 500 kg satellite was left into a circular orbit at an altitude of 23,173 km, inclined at 56 degrees to the Equator, 3 hours and 45 minutes after the launch. It makes a complete journey around the Earth in 14 hours and 3 minutes.

GIOVE-B carries the most stable clock operating in earth orbit, the Passive Hydrogen Maser (PHM), with stability better than 1 nanosecond per day. At that time it was the first of its kind to be launched into space.

These two PHMs will be used as master clocks, but this satellite also carries two rubidium atomic clocks, serving as back-up, each with a stability of 10 nanoseconds per day.