Le Repteux

Since you say that you are synchronized with the main stream scientists, I accept to stop arguing, but make sure you stay adjusted with them!

Yes, they end up ticking at the same frequency, but if we let them run for a long while without adjusting them, they will get out of sync just as if we had never adjusted them before launch, what would not prevent us from adjusting them again to the right frequency since we can adjust them from the ground. To me, the fact that we could do that means that we could have adjusted them progressively once in orbit instead of adjusting them in one shot prior to launch.

No, but as I tried to make clear, I was synchronizing the frequency of the signal from the orbiting clock, which is not its original actual frequency but which we can adjust from the ground, with the actual frequency of the Earth-bound clock.

I see what you mean, and I think that you saw what I meant too, but that xyzt did not, so I repeat more clearly: I was always talking of synchronizing the ground clock's frequency with the orbiting signal frequency while changing that signal frequency progressively from earth so that the observed signal frequency that we receive on earth equals the frequency of our reference clocks on earth. I did not know about these specific noises, thanks for the information. I used the term drift because I read it right here in the beginning, and I thought that its meaning fitted the relativity frequency shift quite closely. Since frequency shift fits both doppler effect and relativity effect at the reception of the signal, what specific terms could we use to differentiate them: would "doppler shift" and "reception shift" be OK? And what specific term could we use to talk about the relativity effect at the emission of the signal: would "emission shift" be OK?

I always assumed that the clocks in orbit would not stay sync with the clocks on the ground if not adjusted to what you call their signal frequency, so I think that I made the distinction correctly between the clock frequency and the signal frequency. Does it matter that much if the relativity formulas were not absolutely necessary to the GPS? It would not change the fact that the clocks are drifting and that the drift is due to the limited speed of light, would it?

We can adjust the transmitter's frequency from the ground, and we can do it step by step until it is the same as our own clocks' frequency, in scientific research, it is called the heuristic method, and it is often used.

If it was not, then it could be taken as a periodic effect an be accounted for as it is actually. By eliminating the doppler effect, I meant that we could realize it is due to direct motion and retrieve it from the adjustment of the clock's frequency. As far as the gravitation frequency shift is concerned, all we have to know for the GPS to work properly is that the observed frequency must be the same as our own clock's frequency, and all we have to do to obtain that is change the orbiting clocks' frequency step by step until it hits the right frequency. If we could not adjust the orbiting clocks from the ground, the question would be irrelevant, but we can, and we need to also because the clocks' rates are not exactly the same and they need to stay the same.

Swansont, In my example with non relativistic doppler effect, we can easily sync the other ship's clock with ours because it is only a mechanical adjustment, because doppler effect is only a matter of frequency shift when it is not relativistic. But the GPS clocks are also adjusted mechanically, their frequency is slowed to account for gravity effects, so that they seem to be sync with our clocks when in orbit. If we can adjust the frequency of the ship's clock with ours without knowing why it is out of sync, why could not we do the same for a satellite clock? After all, if the orbit is circular and if we eliminate variables like doppler effect, it is only a matter of changing progressively the frequency of the orbiting clock until we can observe that it is sync with ours, no?

You are right Swansont, if my mind experience would concern different clocks, it could not work, but if the clocks would be the same, if both would be cesium clocks for instance, should it work? Are you talking about the GPS clocks? Because I always thought that they had to be put out of sync so as to appear sync with the ground reference clocks once in orbit. I do push some fringe ideas here, but I am now participating to a scientific subject where I know that I can't. Swansont was talking about their internal mechanism, saying that my mind experience was hypothetically feasible.

You are right, the clocks would not be sync this way, but it is the same for GPS clocks, except that it is the inverse process: we "un" sync them before launch, and they appear sync when in orbit. The idea is to better understand the sync process by inversing the tasks in a particular mind experience, that's all. If we really had to use relativity formulas to build the clocks, it would be another story, but I don't see where they could be useful. Do you?

All we know is the frequency that we observe, and we want to sync our clock with it. You conclude too fast. This is a better argument. Since it is local time that we want them to keep with precision, what would be the use to know about relativity to build atomic clocks? We do not use the formulas to build the clocks, do we?

You only said that it was impossible xyzt, without explaining why. Synchronizing two clocks at a distance without knowing why they are out of sync is not an important matter, and if you think it is impossible I'll take note of that, but before I quit, give me a last chance to illustrate what I mean. If an alien spaceship is going strait to yours, and you have the possibility to change the rate of your clock to synchronize it with the alien's one, could you nullify the doppler effect? And if so, couldn't you do the same with the alien ship's clock if you could adjust it from your own ship?

If you could put a clock in a perfect circular orbit, and if you could correct its rate from the ground as it seems to be for Rubidium clocks, it seems to me that, with time, you could find the correct rate so that it stays synchronized with the ground clock. Can you give me the reason why you say it is impossible? No, I don't think that we can play with clocks that way! But we can play with our minds that way though! Analogies are a way for a better understanding of phenomenon.

I'm through with the articles, and I fell on that consideration at page 15 in Imatfaal's one: "In the older satellites these terms were compensated by setting the atomic clock frequencies down by this amount before launch–the so-called “factory frequency offset.” Atomic clocks that have been recently launched are based on Rubidium atoms. These clock frequencies may be bumped during launch so they are measured after orbit insertion and the necessary frequency corrections are transmitted to the receivers in the navigation message." If we can completely correct the clocks from the ground, why couldn't we correct them completely by essay and error, I mean without using the relativity formulas? Of course it would be longer, but it should be possible, no? It is only an hypothetical question, it is just for fun. I am not trying to say that relativity is useless, I am just trying to see if we could operate the GPS system without knowing about the relativity effects. In the old days, we could predict the position of the planets without knowing that the sun was at the center of the system: isn't that comparable?

I began reading Imatfaal's link, and at page 14, I was glad to see that it confirmed my supposition. I'll go on with this article until I understand everything, and I'll have a look on yours too xyzt.

I suppose that the clocks on earth run at the same rate because the difference in frequency shift due to their rotational speed is compensated by the difference in frequency shift due to the deformation of the rotating earth, whose diameter is larger at the equator than at the poles. Is that so?

I was talking of the tangential speed at the earth surface, which vary with latitude. I suppose that one of the omega terms represents the tangential speed of the satellite, and the other the equator one. Am I right?

Yes I understand how it works, and I thus understand why it is simpler to adjust the clocks prior to launch, but since the tangential speed of the receivers vary with latitude, I suppose that it is the equator speed that is used the equations. Does it mean that the receivers are programmed to correct this factor?

The formula that you referred me to is: I understand that the "r" distances are those to the center of the earth, and that the "v" speeds are radial to that center, so I conclude that the omega term refers to the tangential speeds to the same center. Am I right?

Yes, I can follow the maths. So what you say is that we have to change the clocks to account for gravitational effect prior to their launch because it would be impossible to do so once in orbit, and my question is, can we change them for the radial motion effect too or do we have to program the receivers to account for that? Also, what about the transverse effect, does it affect the system?

Yes Swanson"t", I know that both relativities rely on the invariance of light, but I also know that their equations are different: one is about motion, the other is about gravitation. If it is easy for me to figure out how to adjust the satellite's clocks with GR, it is impossible to figure out how to adjust them with SR since, at the same time, each of them would have to run at a different rate from each of the others, which is in fact physically impossible. Yes xyzt, I read your link, but it did not help me a lot. I still think that, if we can synchronize the clocks for their mechanical drift once they are in orbit, then we can do so for their GR drift too. No?

Call me Le Repteut for a few times and I will remember that your "t" goes at the end. OK for GR, but what about Strange that seems to say that SR is part of the equations?

Do you mean that SR calculations would be necessary? If so, this is not what I understood from xyzt and Swantson's explanations.

OK You mean, because of the small differences in their orbital parameters, I suppose?

Without GR as a guide, once we put a couple of clocks on orbit and discover that they all drift the same amount, we can measure it with precision and include it in our corrections, no?