Einstein Proven Wrong, Yet Again

a reply to: Greggers

The way I have always viewed it, GPS does provide empirical validation of Relativity because it is possible to know the clock speed at the satellites and compare it to atomic clocks running on earth, and the difference in the speeds of the clocks does indeed match what is predicted by Einstein's equations.

A bit assbackwards. The rate at which the clocks on the satellites run is the same as that as the rate at which clocks on Earth run because they are set (before launch) to run slow. Not doing so would further complicate some already complex calculations even further because, on orbit, they would be running faster. This makes it all somewhat more "simple."
www.aapt.org...

Coincidentally, that pre-launch adjustment uses Einstein's equations. So, maybe he was wrong but I would like to be wrong like that more often.

originally posted by: Phage

A bit assbackwards. The rate at which the clocks on the satellites run is the same as that as the rate at which clocks on Earth run because they are set (before launch) to run slow. Not doing so would further complicate some already complex calculations even further because, on orbit, they would be running faster. This makes it all somewhat more "simple."
www.aapt.org...

Coincidentally, that pre-launch adjustment uses Einstein's equations. So, maybe he was wrong but I would like to be wrong like that more often.

Assbackwards or not, I believe we're on the same page. Even if the clocks are adjusted pre-launch to run at the same speed, the fact that this adjustment is made using Einstein's equations provides empirical validation of relativity, even if an earth based clock is not used for positioning information.

Or are you saying that an earth based clock is used for positioning information? I have in fact read BOTH, but my latest understanding is what I thought you had indicated in your last post -- specifically, that the only clocks used to calculate the user's location were located in orbit.

Or are the complex calculations of which you speak merely related to the satellite availability information?

a reply to: Greggers

My point was that, contrary to what you said, the clocks on the satellites run at the same rate as those on the ground. Through forethought. The designers knew it was problematic and came up with a solution.

Without that prelaunch adjustment, positional calculations would be more nightmarish than they are with. As the paper I linked shows, time dilation is only a part of it.

originally posted by: Phage
a reply to: Greggers

My point was that, contrary to what you said, the clocks on the satellites run at the same rate as those on the ground. Through forethought. The designers knew it was problematic and came up with a solution.

Without that prelaunch adjustment, positional calculations would be more nightmarish than they are with. As the paper I linked shows, time dilation is only a part of it.

Yes, I get what you're saying. What I'm hoping you can clarify for me is whether an earth based clock is involved in determining the user's position.

I have read both that it is, and that it isn't.

a reply to: Phage

"Simple" is somewhat *ahem* relative...

That paper is going to take me a while to plow through...

Meanwhile a certain someone should really give some thought to, instead of snarky little condescending posts, actually answering questions that have repeatedly been ignored.

Certainly my patience is running thin.

The challenge here (for me, anyway) is that I can find documents on the internet that say different things about how GPS works, many of which were authored by physicists or published on university websites (or both).

This *might* be the website where I first read about this (although it was formatted differently and I can't promise it's the same one): www.astronomy.ohio-state.edu...

It seems to suggest that an earth based clock is involved in the positioning calculations, and furthermore goes on to describe how the entire GPS system would become completely unusable within a very short amount of time if the clocks weren't programmed to account for relativistic differences between earth and orbit.

Later, at some other website, I read that the positioning information was calculated entirely using clocks in orbit. So, that made me question everything else on that page, including whether the clocks in the satellites were actually coordinated to run at the same rates with the earthbound clock.

One more post before I'm out for the night.

I found this interesting discussion on a physics forum (it appears to be in basic agreement with Phage's initial post): physics.stackexchange.com...

The satellites' clocks are corrected for GR and SR but this is irrelevant for navigation purposes. Your receiver is comparing the time difference between the time sent by a number of different satellites. If this time is in 'earth' seconds or 1 part in 10^10 speeded up 'space seconds' is to first order irrelevant - so long as all the satellites experience the same effect.

So the choice is to broadcast at 10.23 MHz and let the signal be a slightly different frequency when it reaches the ground, or adjust the frequency to 10.22999999543 MHz onboard so it's 10.23MHz on the ground. I think this is where the urban legend of the 'USAF didn't believe in relativity and weren't going to correct the clocks' comes from. Of course although your position relative to the satellites is unaffected by the time dilation - the satellites' own knowledge of time and so its position in its orbit would accumulate an error.

To allow you to find your absolute position the satelite also broadcasts its own position in orbit. The satellites are in orbit at about 20,000km altitude, 26500km from the centre of the Earth so have an orbit of 165,000km which they cover every 12hours. An error of 38.6 μs/day in a path of 333,000km/day still gives a position error (of the satellite) of only a fraction of a meter - although this accumulates with time. This could be corrected by giving the satellites an adjusted figure for their orbital speed or by updating their empheris as they pass over the ground station.

Like the author of the question posed at that link, I've seen different experts saying different things. I'm going with the above for now. Maybe I'll change my mind tomorrow.

a reply to: Greggers
An Earth based clock is used in establishing the ephemerides and that is used in determing the user's position. Yes.

But the claim is that, since the satellites transmit a time synchronization signal, there is no need for adjustments due to relativity. That synchronization, as I said, may get you to your appointment on time but if your appointment involves delivering a bomb exactly where you want it, it ain't good enough.

Remember, the system was conceived and designed for military use.

It seems to suggest that an earth based clock is involved in the positioning calculations, and furthermore goes on to describe how the entire GPS system would become completely unusable within a very short amount of time if the clocks weren't programmed to account for relativistic differences between earth and orbit

That would be true. Without ground based updates. Which there are.

a reply to: Phage
In stark contrast to the initial OP and said OP's lack of information, I've actually learned a lot about this and how these timings work from your posts. Fascinating stuff.

Working my way through that paper now.

Thanks Phage.

Thread Closed.

OP check your PMs.