Ok, so pardon my ignorance if this can found easily online but I have these few questions. I was actually reading quite a complicated article about GPS and relativity but it had too much maths in it for my abilty. Ok, first of all, apart from the complicated details regarding the workings of laser, cryogenic cooling etc, does a Cs atomic clock work by cooling Cs133 ions to a low (how low?) temperature and then inserting them inside a microwave cavity and as they get radiated by the microwaves they then fall into resonance but only if the frequency is the correct one, so by looking at the feedback they can then determine that the highest peak (electric current or what?) is where the resonance is at. Now I assume this can only work so precisely due to the fact that the resonant peak of the CS ions is a very well defined and precise one correct? But if we are speaking about precision rates into the millionth's of a second, then how can they achieve those if the rest of the atomic clock is usual electronics, I'm talking about the feedback system and the microwave generating system etc? They use ordinary components like quartz crystal frequency oscillators circuits and microwave emitters etc right? so what physical process here governs the extreme accuracy which one can never expect from an ordinary quartz crystal controlled simple electronic clock? given the fact that quart crystal degrades over time and use and so it resonant frequency changes. If possible I would love a more in depth explanation of the innards of a CS atomic clock, maybe someone here has done some work with them. As for the GPS, I read that it is very important to have precise timing which is synchronized so they use atomic clocks both here on earth for time keeping and also in satellites used for GPS, now ok I think I get the part where a GPS device like my smartphone can calculate my physical location based on the signal from atleast 3 satellites as it probably receives a signal from each of the satellites which has a "time stamp" as I read but most importantly which travels as all Em waves travel at the speed of light in vacuum, but here's my question , earth's atmosphere is not vacuum instead it has many obstacles like fog and rain etc, so the speed of EM waves is slower than c, but how do we then know precisely what that speed is, so how can the GPS then add or subtract the speed of the person here on earth using the Doppler effect on top of the signal speed itself if we cannot know the precise signal speed? Since I don't know I assume i'm then asking does the receiver device (my smartphone) calculate my speed, either based on the constantly changing timestamps from satellites with respect to the atomic time standard here on earth or by the signal that it gets from the satellite, enlighten me please. I understand they slow down a tiny bit the frequency of the atomic clock onboard the satellite to compensate for gravitational and relativistic effects due to high speeds involved in satellite motion. Oh also since satellites transmit their signal, what kind of power source they use for that as I imagine space and weight is limited and a radiosingal that is strong enough requires some energy. Thank you.