Timekeeping to a few billionths of a second: Twin ‘passive hydrogen maser’ atomic clocks are the master clocks onboard, using the ultra-stable oscillations of a hydrogen atom to measure time to an accuracy of one second in three billion years. Two smaller rubidium clocks provide an independent time source, accurate to three seconds in one billion years. In practice an ensemble of atomic clocks on the ground keep these orbital clocks synched to ‘Galileo System Time’, via timing corrections included in the navigation messages sent up to satellites by Galileo’s worldwide ground segment.
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