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:
- Hydrogen discharge: a small storage bottle of molecular hydrogen leaks a controlled amount of gas into a discharge bulb. The molecules are disassociated in the discharge bulb into individual hydrogen atoms by an arc.
- State selector: this atomic hydrogen passes through a collimator and a magnetic state selector.
- Storage bulb: The atoms are selected for the desired state and passed on to a storage bulb. The storage bulb is 20 cm high and 10 cm in diameter and made of quartz. Its inside is coated with Teflon, allowing many collisions of the atoms with the wall without perturbation of the atomic state, and slows the recombination of the hydrogen atoms into hydrogen molecules.
- Microwave cavity: the storage bulb is located inside a microwave cavity made of copper or silver-plated ceramic cylinder. This cavity is fed from an external 1420 MHz frequency (resonance frequency of the hydrogen atoms) producing a maximum output in the cavity, that allows the use of lower hydrogen atom density.
if an hydrogen atom jump from one particular energy state to another is forced (step 3), it will radiate an associated microwave signal at an extremely stable frequency that is used as reference.