3 ms·

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 preci

by 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.
- Shiva2012 11y ago
  Oh one other thought. It's been a while since I've been on a deep-dive with this material so I'm considering the possibility that perhaps the updates to the clocks that the engineer spoke of are some ground storage of corrected time. Let's presume that we're both right but I've misinterpreted where the corrections are kept. Then I could totally see how the clocks could have drifted many minutes over their lifetimes but the actual "time" number sent is an abstraction and some central corrective facility keeps up with the correction for each clock. So then the daily drift updates are on the ground and done to the central system and it's corrective notation. It's effectively the same but technically different. I'll have to look into this possibility. I'm not sure how individual devices would then access this central repository of corrections.
- Shiva2012 11y ago
  Reference for the half effect by Newtonian: (bending of light and gravitational time dilation are effectively a single phenomena) https://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun https://en.wikipedia.org/wiki/Tests_of_general_relativity#De...

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.
Shiva2012 11y ago
Oh one other thought. It's been a while since I've been on a deep-dive with this material so I'm considering the possibility that perhaps the updates to the clocks that the engineer spoke of are some ground storage of corrected time. Let's presume that we're both right but I've misinterpreted where the corrections are kept. Then I could totally see how the clocks could have drifted many minutes over their lifetimes but the actual "time" number sent is an abstraction and some central corrective facility keeps up with the correction for each clock. So then the daily drift updates are on the ground and done to the central system and it's corrective notation. It's effectively the same but technically different. I'll have to look into this possibility. I'm not sure how individual devices would then access this central repository of corrections.
Shiva2012 11y ago
Reference for the half effect by Newtonian: (bending of light and gravitational time dilation are effectively a single phenomena) https://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun https://en.wikipedia.org/wiki/Tests_of_general_relativity#De...