The Rubidium gas cell atomic clock is another source with 10 times less the stability of the Cesium clock but 100 times the stability of TCXO quartz oscillator. At any given time and place on earth at least 4 satellites can be seen, although up to 12 satellites could be in view under special circumstances. The earth is NOT a inertial frame because it is rotating.
Gravitational field affects time.
I have attempted to give a little insight and summarize some key problems on the science of timekeeping. All comments and questions are welcome.