# Does the clock aboard a GPS ...

by vector22
Tags: aboard, clock
 P: 56 sattelite tick slower than an earth bound clock. Lets say an atomic clock. From what Ive gathered so far, the clock aboard the GPS sattelite ticks slower The sattelite is in free fall so the clock aboard the sattelite is in zero G. Is the zero G condition the only condition that cause the sattelite clock to tick slower?
 Emeritus Sci Advisor PF Gold P: 5,500 http://www.lightandmatter.com/html_b...ch02/ch02.html Use your browser's search function to find the text "Let's determine the directions and relative strengths of the two effects in the case of a GPS satellite."
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## Does the clock aboard a GPS ...

 Quote by vector22 sattelite tick slower than an earth bound clock. Lets say an atomic clock. From what Ive gathered so far, the clock aboard the GPS sattelite ticks slower The sattelite is in free fall so the clock aboard the sattelite is in zero G. Is the zero G condition the only condition that cause the sattelite clock to tick slower?
In one case you have the clocks speeding up since they are experiencing less gravity than clocks on earth are. On the other hand they are at a high velocity which will slow them down relative to ours here on earth. The two equal out to being a bit slower than earth clocks i believe.
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 Quote by Drakkith In one case you have the clocks speeding up since they are experiencing less gravity than clocks on earth are. On the other hand they are at a high velocity which will slow them down relative to ours here on earth. The two equal out to being a bit slower than earth clocks i believe.
The gravitational affect that detemines the clock rate due to potential (GR) is more significant than the velocity difference due to special relativity - GPS clocks are preset before launch to run faster so they are approximately synchronized with earth clocks
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 Quote by yogi The gravitational affect that detemines the clock rate due to potential (GR) is more significant than the velocity difference due to special relativity - GPS clocks are preset before launch to run faster so they are approximately synchronized with earth clocks
Of course. I meant that the clocks WOULD run slower, but as you pointed out they are corrected to run with the correct time.
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 Quote by yogi The gravitational affect that detemines the clock rate due to potential (GR) is more significant than the velocity difference due to special relativity - GPS clocks are preset before launch to run faster so they are approximately synchronized with earth clocks
Sorry that is half wrong, the effect of gravitation is just the other way round. For clarity, please allow me to correct your statement:

The effect due to gravitational potential is more significant than the velocity effect - GPS clocks are preset before launch to run slower so that in orbit they run approximately synchronous with earth clocks.

Without that adjustment they would tick very slightly faster than earth clocks.
See the already provided references for details.
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 Quote by harrylin Without that adjustment they would tick very slightly faster than earth clocks.
Yes.
 P: 56 So i gather the GPS clocks tick faster than earth bound clocks - any links to reference materials on that. So evidently, zero gravity (free fall) is not the only condition. The current science suggests that there is more than one condition that can effect a clock's timing . If there is more than one condition, are the conditions dependant on each other? Oh i found the above link - time dialation looks like pretty exotic stuff
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 Quote by harrylin Sorry that is half wrong, the effect of gravitation is just the other way round. For clarity, please allow me to correct your statement: The effect due to gravitational potential is more significant than the velocity effect - GPS clocks are preset before launch to run slower so that in orbit they run approximately synchronous with earth clocks. Without that adjustment they would tick very slightly faster than earth clocks. See the already provided references for details.
Yes -quite right
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 Quote by vector22 So i gather the GPS clocks tick faster than earth bound clocks - any links to reference materials on that. So evidently, zero gravity (free fall) is not the only condition. The current science suggests that there is more than one condition that can effect a clock's timing . If there is more than one condition, are the conditions dependant on each other? Oh i found the above link - time dialation looks like pretty exotic stuff
Free fall has nothing to do with time dilation. The gravitational pull at whatever distance the satellites are at, and their velocity relative to ours here on earth are the only factors. The satellites further up or lower in orbit have different rates of time dilation due to different gravitational pull and velocities compared to the GPS satellites.

Imagine a spaceship hovering stationary at a height equal to the GPS satellites. It has the same gravitational effects on its time that the GPS satellites have. However, since it is not moving at a high speed, it does not have the same effects from velocity.
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 Quote by Drakkith Free fall has nothing to do with time dilation. The gravitational pull at whatever distance the satellites are at, and their velocity relative to ours here on earth are the only factors. The satellites further up or lower in orbit have different rates of time dilation due to different gravitational pull and velocities compared to the GPS satellites.
Actually it's the gravitational potential that matters, not the gravitational force.

 Quote by Drakkith Imagine a spaceship hovering stationary at a height equal to the GPS satellites. It has the same gravitational effects on its time that the GPS satellites have. However, since it is not moving at a high speed, it does not have the same effects from velocity.
Experiments of this type have actually been done with atomic clocks, one in a valley and one at the top of a nearby mountain.
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 Quote by bcrowell Actually it's the gravitational potential that matters, not the gravitational force.
Oh. What's the difference?

 Experiments of this type have actually been done with atomic clocks, one in a valley and one at the top of a nearby mountain.
Sure. I was just relating it to the current topic of GPS.
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Quote by Drakkith
 Quote by bcrowell Actually it's the gravitational potential that matters, not the gravitational force.
Oh. What's the difference?
Gravitational potential is potential energy per unit mass.
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 Quote by bcrowell Gravitational potential is potential energy per unit mass.
Exactly what does that mean in regards to time dilation?
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 Quote by Drakkith Exactly what does that mean in regards to time dilation?
The ratio of the rate of flow of time between two points is equal to $e^{\Delta\phi}$, where $\Delta\phi$ is the gravitational potential difference.
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 Quote by bcrowell The ratio of the rate of flow of time between two points is equal to $e^{\Delta\phi}$, where $\Delta\phi$ is the gravitational potential difference.
So is using "Force" instead of "Potential" just a bad choice of words for me, or is it just pretty much wrong on all levels?
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