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Author Topic: Do satellites need time synchronization among one another?  (Read 455 times)

Offline hamdani yusuf

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According to theory of relativity, moving satellites are experiencing time dilation hence require time compensation when measured from "stationary" earth based station.
Following the premis, according to one satellite, another satellite located on the other side of the earth is moving twice as fast as it is measured from the earth, assume they have the same height and speed, and direction as going around the earth. Do they need periodic time adjustment to synchronize their time keeping?
What if the other satellite is moving in the opposite direction?


 

Online Colin2B

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The GPS satellites don't use timing information from the other satellites. Each sallelite's speed and height are calculated relative to earth centre and the atomic clock adjusted to allow for both speed and position in the gravitational field. The earth control station has the ability to make additional clock changes if necessary.
 

Offline hamdani yusuf

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That would mean relative motion between two satellites doesn't produce time dilation as supposedly measured by the other satellite as predicted by theory of relativity.
 

Online Colin2B

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That would mean relative motion between two satellites doesn't produce time dilation as supposedly measured by the other satellite as predicted by theory of relativity.
The time dilation takes place but the satellites don't need to know the time on the other satellites so they don't measure any relative difference.
As I said "Each sallelite's (sic) speed and height are calculated relative to earth centre and the atomic clock adjusted to allow for both speed and position in the gravitational field". This is because the receivers are on earth, so earth is used as the 'stationary' reference frame. The receiver works with each visible satellite to work out its distance from the receiver based on the transmission time delay. Once it has the distance from at least 3 satellites and using earth as a 4th sphere the receiver knows that it is at a specific position* on the intersect of the spheres.

Edit- *within the level of uncertainty of course.
« Last Edit: 26/10/2016 09:03:30 by Colin2B »
 

Offline Fruityloop

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It would seem that if you have two satellites, each on the opposite side of the Earth, moving in opposite directions from each other, you would need some kind of clock adjustment due to time dilation.  What actually happens, however,  is that they set the clock rate for the satellites prior to launch and then the clocks don't need adjustment after that.  Is this correct?
 

Online Colin2B

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It would seem that if you have two satellites, each on the opposite side of the Earth, moving in opposite directions from each other, you would need some kind of clock adjustment due to time dilation.  What actually happens, however,  is that they set the clock rate for the satellites prior to launch and then the clocks don't need adjustment after that.  Is this correct?
Yes, they are set with an adjustment to correct for the situation relative to an earth centred, earth fixed frame, but not for satellite relative to satellite.
There is also capability for the earth control station to make further adjustments if necessary. Apparently earth control can switch the adjustment off, story goes that the customer - US military - didn't believe the engineers who said it was necessary!
 

Offline yor_on

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Who is important in this story yusuf?
You, right?

Now, where do you live, and why would a gps be more important than accounting for numerous satellites?
 

Offline hamdani yusuf

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Yes, they are set with an adjustment to correct for the situation relative to an earth centred, earth fixed frame, but not for satellite relative to satellite.
There is also capability for the earth control station to make further adjustments if necessary. Apparently earth control can switch the adjustment off, story goes that the customer - US military - didn't believe the engineers who said it was necessary!
Do you know what kind of adjustment they made?
If satellite A and B are in orbit on the opposite side of the earth, while C is the earth control, which clock goes fastest, which clock goes slowest?
 

Offline Bored chemist

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Yes, they are set with an adjustment to correct for the situation relative to an earth centred, earth fixed frame, but not for satellite relative to satellite.
There is also capability for the earth control station to make further adjustments if necessary. Apparently earth control can switch the adjustment off, story goes that the customer - US military - didn't believe the engineers who said it was necessary!
Do you know what kind of adjustment they made?
If satellite A and B are in orbit on the opposite side of the earth, while C is the earth control, which clock goes fastest, which clock goes slowest?

From the point of view of me and my GPS, they are all running at the right speed.
 

Offline hamdani yusuf

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Yes, they are set with an adjustment to correct for the situation relative to an earth centred, earth fixed frame, but not for satellite relative to satellite.
There is also capability for the earth control station to make further adjustments if necessary. Apparently earth control can switch the adjustment off, story goes that the customer - US military - didn't believe the engineers who said it was necessary!
Do you know what kind of adjustment they made?
If satellite A and B are in orbit on the opposite side of the earth, while C is the earth control, which clock goes fastest, which clock goes slowest?

From the point of view of me and my GPS, they are all running at the right speed.
Do you mean they don't need any adjustment?
 

Online Colin2B

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Re: Do satellites need time synchronization among one another?
« Reply #10 on: 31/10/2016 17:26:13 »
Do you mean they don't need any adjustment?
The preset adjustment is about 38μs, but moon and sun can cause minor deviations to the orbits so ground control monitors and make any necessary adjustments via the uplink.
 

Offline Bored chemist

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Re: Do satellites need time synchronization among one another?
« Reply #11 on: 31/10/2016 21:38:36 »

Yes, they are set with an adjustment to correct for the situation relative to an earth centred, earth fixed frame, but not for satellite relative to satellite.
There is also capability for the earth control station to make further adjustments if necessary. Apparently earth control can switch the adjustment off, story goes that the customer - US military - didn't believe the engineers who said it was necessary!
Do you know what kind of adjustment they made?
If satellite A and B are in orbit on the opposite side of the earth, while C is the earth control, which clock goes fastest, which clock goes slowest?

From the point of view of me and my GPS, they are all running at the right speed.
Do you mean they don't need any adjustment?
In principle, no- not from my point of view. (Other external factors may influence them and they will need to correct for that)

The point is that there isn't someone sat on one of the satellites looking at the time as registered on anther one.
If there was, then they would see changes.
I only see the ones on my side of the Earth and they are all moving in much the same way, so I don't see them as "wrong"- they all agree.
 

Offline hamdani yusuf

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Re: Do satellites need time synchronization among one another?
« Reply #12 on: 01/11/2016 12:00:55 »
Do you mean they don't need any adjustment?
The preset adjustment is about 38μs, but moon and sun can cause minor deviations to the orbits so ground control monitors and make any necessary adjustments via the uplink.
Do you mean that if I send unadjusted clock to the satelite, then its reading would be slower by 38μs for every second of earth based identical clock?
 

Offline Bored chemist

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Re: Do satellites need time synchronization among one another?
« Reply #13 on: 01/11/2016 20:15:19 »
38Ás per day, not per second
 

Offline hamdani yusuf

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Re: Do satellites need time synchronization among one another?
« Reply #14 on: 01/11/2016 22:13:58 »
38Ás per day, not per second
Thanks for the clarification.
Now that I think again, there would be relative velocity between a clock on the surface of the earth and another clock on the other side of the earth. Do they need adjustment too to be synchronized?
 

Online Colin2B

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Re: Do satellites need time synchronization among one another?
« Reply #15 on: 01/11/2016 23:04:52 »
Sorry, answered in a rush and forgot to say what the adjustment related to, thanks BC.
Now that I think again, there would be relative velocity between a clock on the surface of the earth and another clock on the other side of the earth. Do they need adjustment too to be synchronized?
Everything is calculated on an earth centred (ie centre of earth) earth fixed (ie not rotating) frame so the surface of the earth is not moving relative to earth centre in that frame, nor are 2 points on the surface moving relative to each other. The algorithm for calculating position contains an adjustment for difference between earth centre and surface.
 

Offline hamdani yusuf

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Re: Do satellites need time synchronization among one another?
« Reply #16 on: 07/11/2016 17:25:30 »
Sorry, answered in a rush and forgot to say what the adjustment related to, thanks BC.
Now that I think again, there would be relative velocity between a clock on the surface of the earth and another clock on the other side of the earth. Do they need adjustment too to be synchronized?
Everything is calculated on an earth centred (ie centre of earth) earth fixed (ie not rotating) frame so the surface of the earth is not moving relative to earth centre in that frame, nor are 2 points on the surface moving relative to each other. The algorithm for calculating position contains an adjustment for difference between earth centre and surface.
So geostationary satellites don't need time adjustment? Clocks on the earth poles don't need adjustment either compared to those on the equator?
 

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Re: Do satellites need time synchronization among one another?
« Reply #16 on: 07/11/2016 17:25:30 »

 

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