Global Positioning System (GPS).This is my understanding... Each satellite carries an atomic clock, these clocks have been 'adjusted' so as to allow for SR and GR. SR for the satellite motion and GR for distance from earth's surface. My question. Just say, we wanted to build the GPS using only Newtonian physics (NP). But NP assumes an absolute or universal time, so how do we get around this? I'm thinking something to do with measuring or allowing for redshift/blueshift of signal. Am I wrong or can it be done only with SR and GR?
What do you mean by "using only Newtonian physics?" Newtonian Physics, General Relativity, Quantum Mechanics, etc. are all models that predict the behaviors of physical systems. We use these models to design real devices, but the devices themselves don't really use any models; they just behave the way they behave, and if we used the right models to design them, then they behave the way we want them to. If we designed a GPS satellite using only NP, it would not behave the way we wanted it to, because the clock would keep getting desynchronized. Adjusting the clock to account for SR and GR is the most straightforward way to fix this. There may be other ways to fix the problem, like clever measurement tricks or red/blueshifting the signal, but none of these solutions would be "using only NP" because NP doesn't predict the problem in the first place.
My bold above. The same as you... a Newtonian physics model. Where I said ''build The GPS'', I now see I should have used the word design. So, we have...using Newtonian physics (model) to design (build) a GPS. From my OP. Your saying both systems would work , but the Newtonian design would be just too inconvenient or rather impractical for task. (I think). Thanks Fendis.Please Register or Log in to view the hidden image! I can see your point.
I think such a system could be designed. There will need to be an adjustment between the known physics, GR, the speed of light, the distances between positioning satellites and the ground, etc., and some absolute measure of time passing, like assigning a specific clock in a specific location as the official absolute time.
I think that's actually the opposite of what I'm saying. Please Register or Log in to view the hidden image! If we design/build a satellite using only NP, our calculations will tell us that we don't need to do anything special with the clocks. When we send up the satellite, the clocks will keep getting desynchronized, so the GPS won't work. It's not a matter of practicality; NP simply does not predict the important phenomenon of clock desyncronization, so using only NP we can never properly account for that phenomenon in our design. If we're making an adjustment to account for the differences with GR, then we're not using only NP any more.
That is true, but NP fails with Mecury precession, so to be accurate in global positioning, that factor, though tiny, needs to be accommodated.
Ok, I see where your coming from... my suggesting (OP) measuring or allowing for red/blue shifts would not account for the GR part ( surface clocks slower than higher clocks). So, this really answers my question... you can't operate a GPS using just a NP model. thanks Fendis.Please Register or Log in to view the hidden image!
It wouldn't account for the SR part either. Time dilation in SR is a separate effect that must be "added in" to the Newtonian Doppler shift to correct for relativity.
I think I can see where your coming from... that SR time dilation due to motion is very real and has to be allow for, which Newtonian physics (NP), due to its absolute time, would not account for. Would I be right in saying...It's something like the slower ageing of the space shuttle astronauts in orbit, which NP would not predict? Aside: I understand that with the shuttle astronauts you also have them ageing faster due to GR (faster clocks up there). But in the case of the shuttle SR wins out and the astronauts don't age as fast as those at mission control.
It might take a lot of experimentation using classical physics. However, using classical physics only, it seems to me that we could detect the errors in our measurement of the signals & make suitable corrections.
sweatpea: Yes. For GPS satellites, it turns out that GR predicts that satellite clocks should run 45 nanoseconds per day faster than ground clocks due to gravitational time dilation, and 7 nanoseconds per day slower than ground clocks due to their relative speed (which is the SR relative-motion time dilation effect). The net effect is that clocks on GPS satellites run 45-7=38 microseconds per day faster than ground clocks. So, in this case gravity wins out over relative velocity, but both effects need to be taken into account. In practice, the satellite clocks are manufactured to run slow by 38 microseconds per day before launch, so that when in orbit the clocks are synchronised with the ground clocks.
James R, Actually, the "satellite clocks" are not "manufactured to run slow" - the multiple Atomic Clocks on board each of the GPS Satellites have an adjustable Tick-Rate. The ticking frequency can be adjusted as referenced to a Rubidium Vapor-Cell. There is also a microcomputer on board each GPS Satellite that constantly computes and makes further relativistic timing adjustments as necessary. BTW...James, was your "Everybody has acted in good faith..." etc... remarks, merely senseless attempts to exhibit acerbic wit?
Hey, sweetpea, That is basically what I was trying to relate to James R., in my Post #29... I actually worked with TECOM/DARCOM in Yuma, Az. in late 70's - early 80's in the development of the Global Positioning Satellite system for the DOD... Nice that you took it upon yourself to do your own research, the more you learn - the more you know there is to learn... The following links may be decent "stepping off points" for furthering your research : https://timeandnavigation.si.edu/multimedia-asset/rubidium-frequency-standard http://www.excelitas.com/Pages/Product/Space-Qualified-Rubidium-Frequency-Standards.aspx http://www.afcea.org/content/?q=small-atomic-clocks-chart-new-horizons-0 http://science.nasa.gov/science-news/science-at-nasa/2002/08apr_atomicclock/
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