GPS positioning systems.

[Handy andy]

Global Positioning systems are extremely accurate typically <10m, assuming there is good satellite coverage. Satellites have both high and low orbit, Inmarsat coverage is particularly poor.

 

I would like to start a discussion ref the reliability and accuracy of the various GPS networks.

 

I am particularly interested in how the accuracy, or time, of the clocks may be affected if they are in different orbits around the planet, or at different altitudes or orbiting in different directions. For example if in orbit around the equator, at higher latitudes or perhaps orbiting over the poles.

 

The reason the question occurs to me is that I use GPS in navigation off shore, and have had times when the GPS signal vanishes completely, in clear skys. Also 2 years ago leaving the Carribean a general notice came over navtex system that the GPS system may be down for an hour due to a reset. Obviously if a GPS reset is done over open ocean, it is safer than doing it in congested waters as there is less traffic etc.

 

[Bender]

In a couple of years, you can use Galileo, which will be more accurate and more reliable for civilians.

 

In a couple of months, you can already buy galileo-ready smartphones.

 

I did a paper on the accuracy of gps at university. You can increase the accuracy by measuring the atmosphere with a laser and compensating for it (IIRC, thickness and water content are important).

If used for relative rather than absolute measurements, an accuracy of 2-3 mm was possible. Perhaps someone did better by now.

[Klaynos]

Immersat is not a positioning network.

 

The clocks on the satellites need to have corrections applied. These include special and general relativistic corrections and will vary with orbit.

 

There's a few GNSS networks, glonass, Galileo, GPS and beidou all exist to a certain extent. They're all in similar altitude orbits and not geostationary.

 

Your local antenna is where I'd start fault finding. If this is really safety critical you should use more than one network.

[Strange]

This is a big topic. I suggest reading something like this: https://en.wikipedia.org/wiki/Global_Positioning_System

And then asking more specific questions.

 

 

 

The reason the question occurs to me is that I use GPS in navigation off shore, and have had times when the GPS signal vanishes completely, in clear skys. Also 2 years ago leaving the Carribean a general notice came over navtex system that the GPS system may be down for an hour due to a reset.

 

I would guess these were issues with the receiver rather than the GPS system itself.

[swansont]

There is only one system labeled GPS.

 

In a couple of years, you can use Galileo, which will be more accurate and more reliable for civilians.

 

... if it works as advertised. They've had problems.

[Klaynos]

There is only one system labeled GPS.

 

 

 

... if it works as advertised. They've had problems.

The last thing I read suggested that there will be significant interoperability with GPS and Galileo.

[swansont]

Also 2 years ago leaving the Carribean a general notice came over navtex system that the GPS system may be down for an hour due to a reset. Obviously if a GPS reset is done over open ocean, it is safer than doing it in congested waters as there is less traffic etc.

That's somewhat vague. What exactly did the message say? (or what date was it, so that one could check it?)

The last thing I read suggested that there will be significant interoperability with GPS and Galileo.

Which is a separate issue from them having clock failures and schedule slips, and the number of satellites being scaled-back.

 

Or that while GPS and Galileo are both GNSSs (Global Navigation Satellite Systems), Galileo is not GPS.

[Klaynos]

 

Which is a separate issue from them having clock failures and schedule slips, and the number of satellites being scaled-back.

 

Or that while GPS and Galileo are both GNSSs (Global Navigation Satellite Systems), Galileo is not GPS.

I completely agree. I shan't be moving my systems from GPS to another GNSS any time soon.

[Handy andy]

The loss of GPS was not an antennae problem, I carry 3 independent GPS on my boat, in the Pacific when the GPS failed I went around all the other boats in the anchorage 6 boats approx. All their GPS had lost position also, this lasted for about an hour, when the signal returned.

 

I did not store the Navtex message received in the carribean, and I do not have the exact wording time or date. Sorry for the vagueness

 

My Inmarsat phone system does have a GPS fix which it must have before being able to talk. It is completely independent of my other systems.

 

Thank you for the links, again I will read them all.

 

Would any one like to comment on differences in atomic clocks on GPS satellites in different orbits at different altitudes, orbiting equatorially and or over the poles or in different directions.?

 

The earth is an oblate spheroid, would a satellite orbiting over the poles find a stable orbit.?

 

@ SWANSONT I understand you are the undisputed expert on GPS on this forum, since you have worked on it for a number of years in Washington USA, not Washington County Durham UK. Perhaps you would like to lead the thread, and educate us all ref GPS systems and accuracy of atomic clocks etc. The subject is huge, and strangely it might take an expert to cast light on the nuances off the GPS operating system.

Edited May 5, 2017 by Handy andy

[swansont]

The loss of GPS was not an antennae problem, I carry 3 independent GPS on my boat, in the Pacific when the GPS failed I went around all the other boats in the anchorage 6 boats approx. All their GPS had lost position also, this lasted for about an hour, when the signal returned.

 

I did not store the Navtex message received in the carribean, and I do not have the exact wording time or date. Sorry for the vagueness

 

My Inmarsat phone system does have a GPS fix which it must have before being able to talk. It is completely independent of my other systems.

 

Thank you for the links, again I will read them all.

 

Would any one like to comment on differences in atomic clocks on GPS satellites in different orbits at different altitudes, orbiting equatorially and or over the poles or in different directions.?

 

The earth is an oblate spheroid, would a satellite orbiting over the poles find a stable orbit.?

 

@ SWANSONT I understand you are the undisputed expert on GPS on this forum, since you have worked on it for a number of years in Washington USA, not Washington County Durham UK. Perhaps you would like to lead the thread, and educate us all ref GPS systems and accuracy of atomic clocks etc. The subject is huge, and strangely it might take an expert to cast light on the nuances off the GPS operating system.

I am not an expert in GPS, though I have some knowledge of it.

 

Navtex has an archive of their messages. There are announced local outages for a variety of reasons. A "reset" just doesn't sound right.

[Strange]

There is a neat graph of the effects of orbital velocity vs gravityational effects here

https://en.wikipedia.org/wiki/Gravitational_time_dilation#Experimental_confirmation

[Handy andy]

I am not an expert in GPS, though I have some knowledge of it.

 

Navtex has an archive of their messages. There are announced local outages for a variety of reasons. A "reset" just doesn't sound right.

 

Sorry I must have misread a thread where someone had worked on GPS for 15 years in Washington for the military.

 

Why would there be a local GPS outage?

There is a neat graph of the effects of orbital velocity vs gravityational effects here

https://en.wikipedia.org/wiki/Gravitational_time_dilation#Experimental_confirmation

 

That is an excellent link and answers most of my questions and mostly answers my next questions also.

 

My next question was going to be does the moons gravitational field affect the clocks on satellites when they pass between the moon and earth. From the link you posted it will. I was also going to ask another question ref time dilation with a little thought experiment along the same lines, if a space ship was to travel from earth at constant speed towards the sun and then return to earth at the same speed, would the elapsed time towards the sun be different to the elapsed time on the return journey. Perhaps not going all the way to the sun to avoid being cooked. Again I think your link confirms this.

 

Gravitational time dilation is different from time dilation under special relativity, could you explain this further?

 

Are all these effects taken into account in GPS systems.?

 

A Photons track can be bent by gravity, a change in a gravitational field causes time dilation, an atom goes to a higher energy level if it absorbs a photon, and to a lower energy level when it emits a photon as in a radioactive clock, could gravity be affecting the clock.

[Handy andy]

In the pacific ocean when all the boats in the same anchorage as me lost there GPS signals, it is not impossible one of them was using a Single Side Band Radio. I have heard of led switch panels lighting up when the boat next door uses an SSB

 

Does any one have an opinion, ref how overhead electrical storms or other interference may affect the GPS signal. How do the military block the signal, other than switch it off.

 

Does anyone have any sentient comments or questions on the link https://en.wikipedia.org/wiki/Global_Positioning_System posted by strange.

Edited May 7, 2017 by Handy andy

[Handy andy]

With reference to atomic clocks used in space as in gps systems.

 

If two atomic clocks are in free fall, will they both read the same time.

 

If an atomic clock is in free fall around this planet or the sun, would it read the same as an atomic clock at the other side of the universe regressing away from us in space at 3c.

[Strange]

With reference to atomic clocks used in space as in gps systems.

 

If two atomic clocks are in free fall, will they both read the same time.

 

 

No. Satellites in orbit are in free fall and their clocks run at different rates depending on altitude.

 

 

 

If an atomic clock is in free fall around this planet or the sun, would it read the same as an atomic clock at the other side of the universe regressing away from us in space at 3c.

 

Definitely not. That is why distant galaxies are red-shifted (and therefore time dilated).

[Handy andy]

 

 

No. Satellites in orbit are in free fall and their clocks run at different rates depending on altitude.

 

 

Definitely not. That is why distant galaxies are red-shifted (and therefore time dilated).

 

I mean free fall in open space or being accelerated directly downwards towards the centre of a stretched area of space, (a gravitational body), I do not mean moving at an angle to the gravitational force, like satellites do.

 

The speed of light in another galaxy is the same as the speed of light here, why would the clocks not run at the same speed? If neither of them were moving in their own reference frame, (galaxy) or are being accelerated towards a more stretched area of space, caused by masses of equal size.

Are you not just talking about the time it takes to get the information back here, rather than the rate the two clocks would tick?. The red shift is caused by movement through space is it not. If the clock is stationery in space in its reference frame, or galaxy how can they tick slower?

 

I understand Time dilation to be measured by atomic clocks, that must tick at the same rate if they are not undergoing motion through space. If space is moving with them, they will tick at the same speed wont they? Have missed something important, and what is it please?

[Bender]

I mean free fall in open space or being accelerated directly downwards towards the centre of a stretched area of space, (a gravitational body), I do not mean moving at an angle to the gravitational force, like satellites do.

It depends. For two random objects in a random position in space, the clocks won't be ticking at the same speed, although if you carefully choose those two points, it is possible.

 

The speed of light in another galaxy is the same as the speed of light here, why would the clocks not run at the same speed? If neither of them were moving in their own reference frame, (galaxy) or are being accelerated towards a more stretched area of space, caused by masses of equal size.

Are you not just talking about the time it takes to get the information back here, rather than the rate the two clocks would tick?. The red shift is caused by movement through space is it not. If the clock is stationery in space in its reference frame, or galaxy how can they tick slower?

 

I understand Time dilation to be measured by atomic clocks, that must tick at the same rate if they are not undergoing motion through space. If space is moving with them, they will tick at the same speed wont they? Have missed something important, and what is it please?

If you could see the clocks in those distant galaxies, the clocks would be ticking slower. In the time between one tick reaching us and the next tick reaching us, the space between us increased, and thus the second tick had a larger distance to travel.

[Strange]

 

I mean free fall in open space or being accelerated directly downwards towards the centre of a stretched area of space, (a gravitational body), I do not mean moving at an angle to the gravitational force, like satellites do.

 

 

The time dilation will change as they approach the area of increasing gravitational potential.

 

 

 

I understand Time dilation to be measured by atomic clocks, that must tick at the same rate if they are not undergoing motion through space. If space is moving with them, they will tick at the same speed wont they? Have missed something important, and what is it please?

 

There are multiple causes of time dilation. One is cosmological, due to the changing scale factor of the universe. So distant sources are time dilated, which is why we see red shift (they are the same thing).

[Handy andy]

It depends. For two random objects in a random position in space, the clocks won't be ticking at the same speed, although if you carefully choose those two points, it is possible.

 

If you could see the clocks in those distant galaxies, the clocks would be ticking slower. In the time between one tick reaching us and the next tick reaching us, the space between us increased, and thus the second tick had a larger distance to travel.

 

Basing my understanding on an assumption the speed of light is the same in distant galaxies as it is here, those galaxies are moving away from us and they appear to be ticking slower, from their perspective our clocks would be ticking slower. However if one could adopt some science fiction worm holes, or incorporate an idea from quantum entanglement, and entangle the clocks so that information was transmitted instantly across space, is it not the case there would be no time dilation, between two stationery clocks operating under the same gravitational forces at the opposite sides of the universe.

 

Are you possibly saying that the time dilation is just a function of what we are able to observe with our instruments. Whereas it doesn't really happen in galaxies at opposite sides of the universe.?

 

 

The time dilation will change as they approach the area of increasing gravitational potential.

 

 

There are multiple causes of time dilation. One is cosmological, due to the changing scale factor of the universe. So distant sources are time dilated, which is why we see red shift (they are the same thing).

 

Agreed the time dilation will change as the gravitational potential increases, but if the gravitational changes are equal are the time dilations measured in the atomic clocks the same, in different galaxies.

 

Agreed for information to get to us it is restricted by the speed of light, and is red shifted as the space between galaxies is increased.

 

If a star super novaes a million light years away, it will take a million years before we know what has happened, this isn't time dilation is just a delay in the information being transmitted, even if it is red shifted.

 

What I am trying to get at, is how does the distortion of space affects the clocks on satellites.

 

You have stated above satellites in orbits at different altitudes have different time dilation. Assuming all the satellites have the same mass(which I know they wont have), I assume these are not orbiting around the planet with the same angular velocity, and those at higher altitude are going slower, otherwise they would fly off into space.

 

Do the satellites at higher altitude have more or less time dilation than those at lower altitude. Those at higher altitude are going slower but space is less stretched less further away from the planet.

[Strange]

If a star super novaes a million light years away, it will take a million years before we know what has happened, this isn't time dilation is just a delay in the information being transmitted, even if it is red shifted.

 

 

But red shift is just one "symptom" of time dilation. So, for example, the light from a supernova falls off at a very characteristic rate (the "light curve"). For more distant galaxies, the light curve is slowed by exactly the same amount that the light is red shifted.

 

 

What I am trying to get at, is how does the distortion of space affects the clocks on satellites.

 

"How" is a tricky one. In the case of special relativity, where we are just concerned about relative velocity, then we can view this as a rotation of the coordinate system. The faster an object is moving through space, the less it moves through time - it swaps some of its time dimension for some of the spatial dimension.

 

Similarly, for gravitational time dilation, although it is not a simple rotation, you can consider that the time dimension is shrunk while the spatial dimensions are stretched (although the details of what happens are complex). So much so that, at the event horizon of a black hole, the space and time dimensions get completely switched: the singularity ceases to be ahead of you, and is now in your future (and what used to be your future is now a position some distance ahead of you). And they say you wouldn't even notice!

 

 

 

Do the satellites at higher altitude have more or less time dilation than those at lower altitude. Those at higher altitude are going slower but space is less stretched less further away from the planet.

 

I would refer you back to that graph earlier which showed the relative effects of velocity and gravity.

[swansont]

 

Basing my understanding on an assumption the speed of light is the same in distant galaxies as it is here, those galaxies are moving away from us and they appear to be ticking slower, from their perspective our clocks would be ticking slower. However if one could adopt some science fiction worm holes, or incorporate an idea from quantum entanglement, and entangle the clocks so that information was transmitted instantly across space, is it not the case there would be no time dilation, between two stationery clocks operating under the same gravitational forces at the opposite sides of the universe.

 

 

 

Science fiction is fiction.

[Bender]

 

Basing my understanding on an assumption the speed of light is the same in distant galaxies as it is here, those galaxies are moving away from us and they appear to be ticking slower, from their perspective our clocks would be ticking slower. However if one could adopt some science fiction worm holes, or incorporate an idea from quantum entanglement, and entangle the clocks so that information was transmitted instantly across space, is it not the case there would be no time dilation, between two stationery clocks operating under the same gravitational forces at the opposite sides of the universe.

That's not how entanglement works. It does not allow for information to move faster than light.

[Handy andy]

 

 

But red shift is just one "symptom" of time dilation. So, for example, the light from a supernova falls off at a very characteristic rate (the "light curve"). For more distant galaxies, the light curve is slowed by exactly the same amount that the light is red shifted.

 

 

"How" is a tricky one. In the case of special relativity, where we are just concerned about relative velocity, then we can view this as a rotation of the coordinate system. The faster an object is moving through space, the less it moves through time - it swaps some of its time dimension for some of the spatial dimension.

 

Similarly, for gravitational time dilation, although it is not a simple rotation, you can consider that the time dimension is shrunk while the spatial dimensions are stretched (although the details of what happens are complex). So much so that, at the event horizon of a black hole, the space and time dimensions get completely switched: the singularity ceases to be ahead of you, and is now in your future (and what used to be your future is now a position some distance ahead of you). And they say you wouldn't even notice!

 

 

I would refer you back to that graph earlier which showed the relative effects of velocity and gravity.

 

Thanks for that, I had studied the written text in the link, but the graph was out of focus so had not paid too much attention. Here is the link again to save any one going back https://en.wikipedia.org/wiki/Gravitational_time_dilation#Experimental_confirmation

 

Satellite clocks are slowed by their orbital speed but speed up by their distance out of the Earth's gravitational well. Just as I thought.

 

How do geostationary clocks vary with reference to one on the surface of the planet. Do they speed up??

 

Would a clock in free fall moving towards a stationery clock on the surface of the planet experience faster or slower time than the one fixed on the surface of the planet. Does it go faster ???

That's not how entanglement works. It does not allow for information to move faster than light.

 

My understanding is entanglement allows information to move instantaneously. Do you have a link explaining otherwise. I know quantum entanglement is not clearly understood, I was using it as analogy to try and explain what I was getting at and that is also why included worm holes, to transmit information instantly

 

 

Science fiction is fiction.

 

I was using it to try and explain what I was getting at, not anything else

[swansont]

 

I was using it to try and explain what I was getting at, not anything else

 

 

As they allegedly say in the backwaters of New England, "You can't get there from here."

 

Entanglement does not permit instant communication. Pop-sci is sometimes worse than science fiction.

[Handy andy]

 

 

As they allegedly say in the backwaters of New England, "You can't get there from here."

 

Entanglement does not permit instant communication. Pop-sci is sometimes worse than science fiction.

 

Those new Englanders knew a thing when they saw it.

 

Trying to use the analogy of quantum entanglement and pop scifi was a blunder on my part. I am sure there is a thread somewhere discussing quantum entanglement in the speculation bin.

 

I asked strange a question ref time dilation above, with reference to geostationary satellites and ground based clocks, also ground based clocks compared to those falling directly down in free fall towards a ground based clock. Do you have any thoughts or links that could cast light on this. I suspect that time dilation might speed up under these circumstances compared to the ground based clock.

Edited May 9, 2017 by Handy andy