Clock: Atomic

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.

You likely interact with atomic clocks every day without realizing it: Atomic Clock

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 GPS satellites carry atomic clocks

Scientists use these clocks to detect tiny changes in gravity (General Relativity) and to search for dark matter. Precision Limits the atoms change states.

If the microwave frequency is exactly right, the atoms change states.