It is the most inexpensive, compact, and widely used type of atomic clock, used not only in Galileo and GPS but also to control the frequency of television stations, cell phone base stations, etc.

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.

When an atom is excited by an external energy source, it 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. Read more…

Video explanation on how PHM works:

The Passive Hydrogen Maser (PHM) is the master clock on the Galileo satellite’s payload. Its extremely good performance makes it the most stable of all clocks currently in space, better than 1 nanosecond per day. Some other features: 18 Kg of mass, 28 liters of volume and 20 years lifetime.

The hydrogen maser uses the properties of the hydrogen atom to serve as a precision frequency reference. But, how does it work? Let’s see what is the process: Read more…