Galileo - Principles of the atomic clock

For a satellite navigation system to work correctly, it is necessary that the signals broadcast by the satellites are transmitted synchronously. In order to achieve this, the satellites carry very stable clocks.

The Galileo satellites will carry two types of clocks: rubidium atomic frequency standards and passive hydrogen masers. The stability of the rubidium clock is so good that it would lose only three seconds in one million years, while the passive hydrogen maser is even more stable and it would lose only one second in three million years. However this kind of stability is really needed, since an error of only a few nanoseconds (billionths of a second) on the Galileo measurements would produce a positioning error of metres which would not be acceptable.

An atomic clock works like a conventional clock but the time-base of the clock, instead of being an oscillating mass as in a pendulum clock, is based on the properties of atoms when transitioning between different energy states.

An atom, when excited by an external energy source, goes to a higher energy state. Then, from this state, it goes to a lower energy state. In this transition, the atom releases energy at a very precise frequency which is characteristic of the type of atom. This is like a signature for the type of material used. All that is needed for making a good clock is a way of detecting this frequency and using it as an input to a counter. This is the principle behind an atomic clock.