4 ms·

Sigh... sorry but Einstein had nothing to do with putting satellites in space. If anything you're talking about the working of the GPS system and even in that

by Shiva2012 11y ago

Sigh... sorry but Einstein had nothing to do with putting satellites in space. If anything you're talking about the working of the GPS system and even in that case, the effects of gravity are below the error of the device. (clock drifts dealt with via regular ground synch) The only appreciable effect is the ephemeris calculation which uses relativistic doppler calculations. Unfortunately the difference between normal doppler and relativistic doppler is minuscule at these speeds and are also within error ranges... So ultimately even the GPS argument doesn't hold water.. So... like, go with Jonas Salk who gave away the fortune that was the Polio vaccine. That's a real hero.

dnautics 11y ago
Except Jonas Salk didn't take government money. The money to research the Polio vaccines (salk and sabin) was raised through a private nonprofit called the March of Dimes.
- Shiva2012 11y ago
  Hmm... good point, but I think it's about valuing the advancement of knowledge over monetary issues that is the heart of the matter right?
burkaman 11y ago
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.
LeoPanthera 11y ago
"The effect of gravitational frequency shift on the GPS due to general relativity is that a clock closer to a massive object will be slower than a clock farther away. Applied to the GPS, the receivers are much closer to Earth than the satellites, causing the GPS clocks to be faster by a factor of 5×10^(−10), or about 45.9 μs/day. This gravitational frequency shift is noticeable. When combining the time dilation and gravitational frequency shift, the discrepancy is about 38 microseconds per day, a difference of 4.465 parts in 10^10. Without correction, errors in the initial pseudorange of roughly 10 km/day would accumulate. To compensate for the discrepancy, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz. Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was proposed by Friedwardt Winterberg in 1955." 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_...
- jessriedel 11y ago
  Doesn't shiva2012 specifically address this in his point? You are describing a slow drift that would accumulate over many days and eventually lead to an error, but that is avoided by regularly re-syncing the clock on the satellite and the ground ("clock drifts dealt with via regular ground synch"). In a hypothetical world where Einstein didn't figure out GR, nor the thousands of physicists who followed him in the intervening decades, the implication for GPS would be an unexplained drift that would be easily correctable though still mysterious.
  - LeoPanthera 11y ago
    To my knowledge there is no "ground sync" - as described in the previous quote, the clocks are intentionally set to run slightly slow in advance to offset the effects of relativity.

dnautics 11y ago

Except Jonas Salk didn't take government money. The money to research the Polio vaccines (salk and sabin) was raised through a private nonprofit called the March of Dimes.

Shiva2012 11y ago
Hmm... good point, but I think it's about valuing the advancement of knowledge over monetary issues that is the heart of the matter right?

burkaman 11y ago

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.

LeoPanthera 11y ago

"The effect of gravitational frequency shift on the GPS due to general relativity is that a clock closer to a massive object will be slower than a clock farther away. Applied to the GPS, the receivers are much closer to Earth than the satellites, causing the GPS clocks to be faster by a factor of 5×10^(−10), or about 45.9 μs/day. This gravitational frequency shift is noticeable. When combining the time dilation and gravitational frequency shift, the discrepancy is about 38 microseconds per day, a difference of 4.465 parts in 10^10. Without correction, errors in the initial pseudorange of roughly 10 km/day would accumulate. To compensate for the discrepancy, the frequency standard on board each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz. Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. Placing atomic clocks on artificial satellites to test Einstein's general theory was proposed by Friedwardt Winterberg in 1955." 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_...

jessriedel 11y ago
Doesn't shiva2012 specifically address this in his point? You are describing a slow drift that would accumulate over many days and eventually lead to an error, but that is avoided by regularly re-syncing the clock on the satellite and the ground ("clock drifts dealt with via regular ground synch"). In a hypothetical world where Einstein didn't figure out GR, nor the thousands of physicists who followed him in the intervening decades, the implication for GPS would be an unexplained drift that would be easily correctable though still mysterious.
- LeoPanthera 11y ago
  To my knowledge there is no "ground sync" - as described in the previous quote, the clocks are intentionally set to run slightly slow in advance to offset the effects of relativity.