GPS

The GPS constellation.

Rubidium. Cesium atomic clocks are more expensive.

Galileo sats use these

https://www.esa.int/Applications/Navigation/Galileo/Galileo_...

so, basically magic.

You think that's magic, check out optical lattice clocks. Blows my mind every time I re-read about them and accept "yes, this actually works".

Some Pulsars have extremely stable frequencies https://www.nytimes.com/1983/02/09/us/pulsar-termed-most-acc...