How the Galileo Atomic Clocks Work

How Satellite Navigation Systems Use Atomic Clocks  

For a satellite navigation system to function accurately, the signals transmitted by the satellites must be synchronized. To achieve this, the satellites are equipped with highly stable clocks.

Galileo satellites are equipped with two types of atomic clocks: rubidium atomic frequency standards and passive hydrogen masers. The rubidium clock is incredibly stable, losing only three seconds every million years. The passive hydrogen maser is even more precise, losing just one second every three million years. This extreme stability is essential because even a few nanoseconds of error in Galileo's measurements could result in a positioning error of several meters, which would be unacceptable.

An atomic clock operates similarly to a conventional clock, but instead of using an oscillating mass (like in a pendulum clock), the time-base is based on the behavior of atoms transitioning between different energy states.

When an atom is excited by an external energy source, it moves to a higher energy state. It then returns to a lower energy state, releasing energy at a very specific frequency. This frequency is characteristic of the atom and serves as a "signature" of the material. To create an accurate clock, all that’s needed is a method of detecting this frequency and using it to drive a counter. This is the fundamental principle behind an atomic clock.

These transitions can occur by emitting or absorbing energy at either optical or microwave frequencies. For example, one atomic second is defined as 9,192,631,700 counts of the frequency released during the transition of the Cesium-133 isotope when exposed to proper excitation.

Rubidium Clock

The Galileo rubidium clock is made of an atomic resonator and its associated control electronics. Inside the atomic resonator there is a rubidium vapour cell. The atoms are kept in a gaseous state at high temperature. In order to initiate the resonance, the atoms of the cell are excited to a higher state by the light of a rubidium discharge lamp located in one end of the atomic resonator. At one end of the resonator, a photodiode detects the amount of light that passes through the cell.  

Once the atoms are excited, they decay to a lower energy state. From there, the atoms are re-excited to an intermediate state by injecting microwave energy into the resonator at a specific frequency. Transition to this intermediate state only occurs if the microwave frequency precisely matches the one associated with that transition. When the atoms reach the intermediate state, light absorption is at its maximum.

The output from the photodiode is fed into control circuitry, which adjusts the microwave frequency. The correct frequency is maintained by tuning the microwave source until maximum light absorption is achieved. This resonance is sustained by energy from the rubidium lamp, as the atoms in the intermediate state are re-excited to the higher state and then decay back to the lower state, continuing the cycle.

Hydrogen Maser Clock  

The Galileo passive hydrogen maser clock is built around an atomic resonator and its associated control electronics. In this clock, a small storage bottle supplies molecular hydrogen to a gas discharge bulb, where the hydrogen molecules are dissociated into atomic hydrogen. Once dissociated, the atoms pass through a collimator and a magnetic state selector before entering the resonance cavity. The magnetic state selector ensures that only atoms at the desired energy level enter the cavity.

Inside the resonance cavity, the hydrogen atoms are confined in a quartz storage bulb. These atoms naturally tend to return to their fundamental energy state, emitting a microwave frequency in the process.

This emitted frequency is detected by an interrogation circuit, which locks an external signal to the natural transition frequency of the hydrogen atoms. Locking occurs when the injected microwave frequency matches the resonant frequency of the atoms, amplifying the signal.

The resonant frequency of the microwave cavity is approximately 1.420 GHz. The clock's electronics include control circuitry for the frequency, as well as a thermal control system to maintain the resonant cavity at the correct temperature.

The atomic resonator is highly sensitive to environmental factors, such as magnetic fields. Therefore, maintaining a stable external environment is crucial to ensure the clock operates at its full performance potential.