"Three high-accuracy space clocks aboard
 
Like its predecessor, GIOVE-B carries two redundant small-size rubidium atomic clocks, each with a stability of 10 nanoseconds per day. But it also features an even more accurate payload: the Passive Hydrogen Maser (PHM), with stability better than 1 nanosecond per day. The first of its kind ever to be launched into space, this is now the most stable clock operating in earth orbit. Two PHMs will be used as primary clocks onboard operational Galileo satellites, with two rubidium clocks serving as backup. "