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…