8 ms·

Do you have some source for the claim that relativity doesn't really matter for GPS? If I google "GPS relativity", I get results from several universities, Wiki

Do you have some source for the claim that relativity doesn't really matter for GPS? If I google "GPS relativity", I get results from several universities, Wikipedia, Stack Exchange, Wired, etc. that all disagree with you. I don't know the science well enough to figure it out myself, but I can't find any trustworthy source that contradicts this idea.

jessriedel 11y ago
See my reply to LeoPanthera. Shiva2012 doesn't claim that GR effects can't be detected using GPS satellites, he's saying they are already corrected in a way that doesn't require understanding GR at all: by regularly re-syncing the clock with ground. In a hypothetical world without GR, this would be a mysterious drift rather than a non-mysterious one, but it wouldn't make GPS any less possible. Do you have a link from any of those prestigious places that says GR is actually necessary for making GPS work?
- dogma1138 11y ago
  Shiva2012 wasn't exactly correct, there is no "clock-sync" from ground stations, each satellite has a specifically fined tune atomic clock that is designed to adjust the drift and each adjustment is unique for each satellite based on it's position in the constellation. Some additional GR related shifts are corrected in the receivers however GR isn't the only thing that can affect the accuracy of GPS and since it's constant (although cumulative) it could be potentially corrected by other means if GR was not known at the time. And there are of course other issues that cause errors in GPS many of them were discovered post launch and the system was fine tuned to correct for them mainly on the receiver side. https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...
  - Shiva2012 11y ago
    Incorrect. I've spoken directly to the engineers and read through a lot of the material. The Rubidium clocks have a lot of error and drift for such a high precision application and have to regularly be updated. Daily or sometimes more frequently.
    - dogma1138 11y ago
      You have any sources for that? "GPS satellites revolve around the earth with a velocity of 3.874 km/s at an altitude of 20,184 km. Thus on account of the its velocity, a satellite clock appears to run slow by 7 microseconds per day when compared to a clock on the earth’s surface. But on account of the difference in gravitational potential, the satellite clock appears to run fast by 45 microseconds per day. The net effect is that the clock appears to run fast by 38 microseconds per day. This is an enormous rate difference for an atomic clock with a precision of a few nanoseconds. Thus to compensate for this large secular rate, the clocks are given a rate offset prior to satellite launch of - 4.465 parts in 1010 from their nominal frequency of 10.23 MHz so that on average they appear to run at the same rate as a clock on the ground. The actual frequency of the satellite clocks before launch is thus 10.22999999543 MHz. Although the GPS satellite orbits are nominally circular, there is always some residual eccentricity. The eccentricity causes the orbit to be slightly elliptical, and the velocity and altitude vary over one revolution. Thus, although the principal velocity and gravitational effects have been compensated by a rate offset, there remains a slight residual variation that is proportional to the eccentricity. For example, with an orbital eccentricity of 0.02 there is a relativistic sinusoidal variation in the apparent clock time having an amplitude of 46 nanoseconds. This correction must be calculated and taken into account in the GPS receiver." I've had a chance to speak with a few engineers from Motorola that worked on GPS and a very interesting fellow that worked on GLONASS all of them said that they had a tuned atomic clock and everything was done on the receiving end and for the most part the clocks on the satellites could not be even corrected after they were initialized.
      - Shiva2012 11y ago
        For references, no I don't have anything other than email conversations with FAA engineers, but I'll see if I can find it somewhere else too. Both of us will have to do better than he said she said, but since you're with consensus the burden is on me. One thing I'm not disputing is that the clocks run at a different rate. What I guess I haven't made clear is that any rational extension of classical physics would presume the clock rates would be different. Bear in mind that Lorentz's Aether theory was ultimately classical in nature because even though he had the Lorentz transform and all the time effects, light speed constancy was modeled as an illusion within a preferred frame. IE from a purely newtonian perspective, there is a "catching up" that light would have to play in a moving apparatus such as an atomic clock. This naturally lends itself to time dilation from motion. Furthermore Newtonian mechanics also predicted deviation of starlight and therefore gravitational time dilation of exactly one half that predicted by Relativity. Einstein mentions it in his book "Relativity". So what I'm talking about is a little more nuanced than it seems. I'm simply talking about the actual difference between Einstein's theories and what classical theory would have come up with is insignificantly small. I'm not saying that the whole time effect is insignificantly small.
- burkaman 11y ago
  Well I guess that depends on your definition of prestigious, but: Ohio State - http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.... University of Colorado (hosted by LSU for some reason) - http://www.phys.lsu.edu/mog/mog9/node9.html http://www.phys.lsu.edu/mog/mog9/node9.html US Naval Observatory - http://tycho.usno.navy.mil/ptti/1996papers/Vol%2028_16.pdf http://tycho.usno.navy.mil/ptti/1996papers/Vol%2028_16.pdf That's a paper from 1996, which is actually discussing relativistic effects in GPS receivers, which you might encounter in high speed aircraft, or satellites communicating with each other. 20 years ago, the author writes: "There is a very good reason for the omission: the effects of relativity, where they are different from the effects predicted by classical mechanics and electromagnetic theory, are too small to matter - less than one centimeter, for users on or near the earth. However, a new class of users, who employ satellites that obtain time and position in space from GPS, cannot be satisfied with the approximations in the current OCS." That paper is interesting mostly because it's the basis of the Conservapedia article on why the theory of relativity is wrong: http://www.conservapedia.com/Theory_of_relativity#Lack_of_evidence_for_Relativity http://www.conservapedia.com/Theory_of_relativity#Lack_of_ev... And there are non-academic sources that I tend to trust: https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Special_and_General_Relativity https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_... http://physics.stackexchange.com/questions/1061/why-does-gps-depend-on-relativity http://physics.stackexchange.com/questions/1061/why-does-gps... http://www.wired.com/2011/06/st_equation_gps/ http://www.wired.com/2011/06/st_equation_gps/ https://www.quora.com/How-is-the-theory-of-relativity-used-in-GPS https://www.quora.com/How-is-the-theory-of-relativity-used-i...
  - Shiva2012 11y ago
    There are tens of thousands of regurgitations of the basic idea of including relativistic effects but very few sources that have looked directly at the necessity. It was certainly included. The question of necessity, however, is another matter. Once again, I must repeat that there are components of relativistic theory that are nearly indistinguishable from classical at such low proportions of C. For instance, the entire relativistic doppler effect is often cited as a relatvistic effect. However, the portion of that whole large effect that is purely and uniquely relativity is miniscule because the difference between classical and relativistic are negligible at such low speeds (Super-sonic jets are dead snails covered in cold molasses compared to C) Bear in mind that all the hyper-precise navigation we do within our solar system, from landing on mars to landing on a comet is PURELY NEWTONIAN. (You should gasp with surprise here... I know I did when I learned that)

jessriedel 11y ago

See my reply to LeoPanthera. Shiva2012 doesn't claim that GR effects can't be detected using GPS satellites, he's saying they are already corrected in a way that doesn't require understanding GR at all: by regularly re-syncing the clock with ground. In a hypothetical world without GR, this would be a mysterious drift rather than a non-mysterious one, but it wouldn't make GPS any less possible. Do you have a link from any of those prestigious places that says GR is actually necessary for making GPS work?

dogma1138 11y ago
Shiva2012 wasn't exactly correct, there is no "clock-sync" from ground stations, each satellite has a specifically fined tune atomic clock that is designed to adjust the drift and each adjustment is unique for each satellite based on it's position in the constellation. Some additional GR related shifts are corrected in the receivers however GR isn't the only thing that can affect the accuracy of GPS and since it's constant (although cumulative) it could be potentially corrected by other means if GR was not known at the time. And there are of course other issues that cause errors in GPS many of them were discovered post launch and the system was fine tuned to correct for them mainly on the receiver side. https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...
- Shiva2012 11y ago
  Incorrect. I've spoken directly to the engineers and read through a lot of the material. The Rubidium clocks have a lot of error and drift for such a high precision application and have to regularly be updated. Daily or sometimes more frequently.
  - dogma1138 11y ago
    You have any sources for that? "GPS satellites revolve around the earth with a velocity of 3.874 km/s at an altitude of 20,184 km. Thus on account of the its velocity, a satellite clock appears to run slow by 7 microseconds per day when compared to a clock on the earth’s surface. But on account of the difference in gravitational potential, the satellite clock appears to run fast by 45 microseconds per day. The net effect is that the clock appears to run fast by 38 microseconds per day. This is an enormous rate difference for an atomic clock with a precision of a few nanoseconds. Thus to compensate for this large secular rate, the clocks are given a rate offset prior to satellite launch of - 4.465 parts in 1010 from their nominal frequency of 10.23 MHz so that on average they appear to run at the same rate as a clock on the ground. The actual frequency of the satellite clocks before launch is thus 10.22999999543 MHz. Although the GPS satellite orbits are nominally circular, there is always some residual eccentricity. The eccentricity causes the orbit to be slightly elliptical, and the velocity and altitude vary over one revolution. Thus, although the principal velocity and gravitational effects have been compensated by a rate offset, there remains a slight residual variation that is proportional to the eccentricity. For example, with an orbital eccentricity of 0.02 there is a relativistic sinusoidal variation in the apparent clock time having an amplitude of 46 nanoseconds. This correction must be calculated and taken into account in the GPS receiver." I've had a chance to speak with a few engineers from Motorola that worked on GPS and a very interesting fellow that worked on GLONASS all of them said that they had a tuned atomic clock and everything was done on the receiving end and for the most part the clocks on the satellites could not be even corrected after they were initialized.
    - Shiva2012 11y ago
      For references, no I don't have anything other than email conversations with FAA engineers, but I'll see if I can find it somewhere else too. Both of us will have to do better than he said she said, but since you're with consensus the burden is on me. One thing I'm not disputing is that the clocks run at a different rate. What I guess I haven't made clear is that any rational extension of classical physics would presume the clock rates would be different. Bear in mind that Lorentz's Aether theory was ultimately classical in nature because even though he had the Lorentz transform and all the time effects, light speed constancy was modeled as an illusion within a preferred frame. IE from a purely newtonian perspective, there is a "catching up" that light would have to play in a moving apparatus such as an atomic clock. This naturally lends itself to time dilation from motion. Furthermore Newtonian mechanics also predicted deviation of starlight and therefore gravitational time dilation of exactly one half that predicted by Relativity. Einstein mentions it in his book "Relativity". So what I'm talking about is a little more nuanced than it seems. I'm simply talking about the actual difference between Einstein's theories and what classical theory would have come up with is insignificantly small. I'm not saying that the whole time effect is insignificantly small.
burkaman 11y ago
Well I guess that depends on your definition of prestigious, but: Ohio State - http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.... University of Colorado (hosted by LSU for some reason) - http://www.phys.lsu.edu/mog/mog9/node9.html http://www.phys.lsu.edu/mog/mog9/node9.html US Naval Observatory - http://tycho.usno.navy.mil/ptti/1996papers/Vol%2028_16.pdf http://tycho.usno.navy.mil/ptti/1996papers/Vol%2028_16.pdf That's a paper from 1996, which is actually discussing relativistic effects in GPS receivers, which you might encounter in high speed aircraft, or satellites communicating with each other. 20 years ago, the author writes: "There is a very good reason for the omission: the effects of relativity, where they are different from the effects predicted by classical mechanics and electromagnetic theory, are too small to matter - less than one centimeter, for users on or near the earth. However, a new class of users, who employ satellites that obtain time and position in space from GPS, cannot be satisfied with the approximations in the current OCS." That paper is interesting mostly because it's the basis of the Conservapedia article on why the theory of relativity is wrong: http://www.conservapedia.com/Theory_of_relativity#Lack_of_evidence_for_Relativity http://www.conservapedia.com/Theory_of_relativity#Lack_of_ev... And there are non-academic sources that I tend to trust: https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Special_and_General_Relativity https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_... http://physics.stackexchange.com/questions/1061/why-does-gps-depend-on-relativity http://physics.stackexchange.com/questions/1061/why-does-gps... http://www.wired.com/2011/06/st_equation_gps/ http://www.wired.com/2011/06/st_equation_gps/ https://www.quora.com/How-is-the-theory-of-relativity-used-in-GPS https://www.quora.com/How-is-the-theory-of-relativity-used-i...
- Shiva2012 11y ago
  There are tens of thousands of regurgitations of the basic idea of including relativistic effects but very few sources that have looked directly at the necessity. It was certainly included. The question of necessity, however, is another matter. Once again, I must repeat that there are components of relativistic theory that are nearly indistinguishable from classical at such low proportions of C. For instance, the entire relativistic doppler effect is often cited as a relatvistic effect. However, the portion of that whole large effect that is purely and uniquely relativity is miniscule because the difference between classical and relativistic are negligible at such low speeds (Super-sonic jets are dead snails covered in cold molasses compared to C) Bear in mind that all the hyper-precise navigation we do within our solar system, from landing on mars to landing on a comet is PURELY NEWTONIAN. (You should gasp with surprise here... I know I did when I learned that)