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.
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 Atomic Clock
High-speed data transfers and cellular networks require perfectly synchronized "stamps" to ensure packets of data arrive in the correct order. GPS satellites carry atomic clocks
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