Mel Beckman wrote:
To be useful, any atomic clocks you operate must be synchronized to a Stratum Zero time source, such as GPS.
Only initially.
Precise time is crucial to a variety of economic activities around the world. Communication systems, electrical power grids, and financial networks all rely on precision timing for synchronization and operational efficiency. The free availability of GPS time has enabled cost savings for companies that depend on precise time and has led to significant advances in capability.
FYI, time difference between two points is not noticeable, that is, does not affect correctness of any distributed algorithm, if the difference is below the communication delay between the points, which means rough synchronization by NTP is good enough. That is an information theoretic version of relativity of simultaneity: https://en.wikipedia.org/wiki/Relativity_of_simultaneity For information theoretic simultaneity, you can consider, instead of light cone, information cone. Masataka Ohta