How Do Satellite Atomic Clocks Work?

Atomic Clocks

An atomic clock is an extremely precise tool used to set the standard of time. The best atomic clocks are accurate to 1 second in 300 million years. Even older atomic clocks can go many thousands of years without gaining or losing a second. Satellite atomic clocks are usually cesium clocks, which means they use the resonance of cesium molecules to calculate the precise length of a second.

Resonance

All clocks work on resonance. Old grandfather clocks use a pendulum, which swings at a particular rate. More accurate quartz clocks use a tiny crystal tuning fork to make a more precise clock. Cesium clocks work by finding cesium's resonant frequency. Since that frequency, 9,192,631,770 cycles per second, is known, it can be used to derive the exact length of any other unit of time.

How it Works

The cesium is heated, energized and trapped in a magnetic field. Then, it is bombarded with microwaves in a very narrow frequency range. The circuit that controls the microwaves uses a crystal to get very near the resonant frequency of the cesium. It moves up and down slightly until it finds the exact frequency. When the oscillator hits the right frequency, it excites the molecules, sending some toward a detector at the end of the cesium chamber. When the detector registers a bombardment of cesium molecules, the clock has reached the resonant frequency of cesium. A computer can then use that frequency to calculate the length of a second.