Europe's Galileo is the world's most precise satellite navigation system, providing meter-level accuracy and very precise timing to its four billion users. An essential ingredient to ensure this stays the case are the atomic clocks aboard each satellite, delivering pinpoint timekeeping that is maintained to a few billionths of a second. These clocks are called atomic because their "ticks" come from ultra-rapid, ultra-stable oscillation of atoms between different energy states. Sustaining this performance demands, in turn, even more accurate clocks down on the ground to keep the satellites synchronized and ensure stability of time and positioning for users.
