GPS satellites carry atomic clocks. Because radio signals travel at the speed of light, even a billionth of a second of error could result in your GPS being off by several miles.
The best modern atomic clocks are so stable that they wouldn't lose or gain a single second in —a span of time longer than the age of the universe. Atomic Clock
If the microwave frequency is exactly right, the atoms change states. GPS satellites carry atomic clocks
Since 1967, the International System of Units (SI) has defined one second as exactly 9,192,631,770 oscillations of the radiation produced by this transition in a cesium-133 atom. Why It Matters If the microwave frequency is exactly right, the
Scientists use these clocks to detect tiny changes in gravity (General Relativity) and to search for dark matter. Precision Limits
You likely interact with atomic clocks every day without realizing it:
Cesium atoms are sprayed into a vacuum chamber and hit with microwaves.