GPS satellite and satnav receiver clock synchronisation

Click For Summary

Discussion Overview

The discussion revolves around the synchronization of GPS satellite clocks with those of the GPS receiver. It explores the implications of clock accuracy on distance measurement and the methods used to achieve synchronization, particularly in the context of navigation and positioning accuracy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the process by which a GPS receiver calculates its position using signals from satellites, emphasizing the importance of clock synchronization for accurate distance measurement.
  • Another participant suggests that the receiver's clock error is assumed to be consistent during the time interval of signal reception, allowing for a method to fit the time error so that the intersection of spheres can be determined.
  • A third participant shares a link to a paper that provides a thorough explanation of GPS principles, indicating a search for deeper understanding.
  • A participant with practical experience in GPS technology notes the significant advancements in accuracy, contrasting with earlier claims of accuracy, and expresses a personal acknowledgment of gaps in fundamental knowledge despite working with precision GPS equipment.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and assumptions regarding GPS clock synchronization, with no clear consensus on the methods or implications discussed. The conversation reflects both technical explanations and personal experiences, indicating a mix of agreement on the importance of synchronization but disagreement on the specifics of the process.

Contextual Notes

There are unresolved questions regarding the exact methods used for clock synchronization and the assumptions made about clock errors. The discussion also highlights the dependence on the accuracy of the signals received from multiple satellites.

quitequick
Messages
7
Reaction score
0
As far as I understand it...

Each satellite transmits a signal that the receiver receives. This signal consists of (at least) a satellite identification together with a time-stamp of when the signal was sent and its position at that time. The receiver calculates the distance from the satellite by comparison of the time-stamp to its own internal clock (distance = speed-of-light x time-difference). With three different satellite signals the intersection of three spheres can be found and therefore the position of the receiver.

So, the accuracy of the receiver's clock against the satellite's clocks would seem to be critical - in fact fundamental to the accuracy of the distance measurement. But if the receiver does not know exactly how far away the satellite is, how could it accurately compensate for the delay in receiving a time synchronisation signal? Assuming that the receiver does not have an on-board atomic clock, any clock it does have will have to be sync'ed probably very regularly.

So the question is, how is the receiver's clock sync'ed to the GPS satellite's clocks, given that the receiver does not know far away the satellite is, so it can correct for the time-delay?

Sounds like a chicken and egg problem to me!
 
Physics news on Phys.org
http://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equations

It seems that it is just assumed the clock is wrong by some amount, but wrong by the same amount during the time interval in which the signals from the different satellites arrive, which is probably a good estimate considering the small time interval. Then they just fit the time error of the clock such that the four spheres intersect in a single spot, or try to minimize the variation of the intersection point considering all the spheres or some other method.
 
quitequick said:
Thanks, chingel. The Wikipedia page lead me to...

http://web.archive.org/web/20110719232148/http://www.macalester.edu/~halverson/math36/GPS.pdf

which explains it thoroughly.

interesting paper, thanks for the link

I'm the tech working for a company that's sells GPS gear into the survey and heavy machine industry ... road construction, mining etc
I have to admit that, even working with precision GPS equip all day every day, there's still lots I don't know about the GPS basics. ( mainly because it isn't essential to repairing the equip)

I had to giggle with that 80metre accuracy comment in the opening lines.
we are now easily down to less than 10mm ( 1cm) 5mm is quite achieveable using 2 linked receivers

Dave
 

Similar threads

Graduate 1-Way Speed of Light
  • · Replies 42 ·
2
Replies
42
Views
3K
Undergrad How does GPS receiver calculate time delay?
  • · Replies 10 ·
Replies
10
Views
13K
Undergrad GPS relativity error - really?
  • · Replies 31 ·
2
Replies
31
Views
8K
Undergrad Satellite Orbit synchronization
  • · Replies 126 ·
5
Replies
126
Views
7K
Engineering Raw data from GPS receiver
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Explore Clock Synchronisation in a Moving Train Frame
  • · Replies 32 ·
2
Replies
32
Views
4K
Undergrad Question about GPS clock synchronization
  • · Replies 22 ·
Replies
22
Views
3K
Graduate What relativistic effects to GPS receivers compensate for?
  • · Replies 10 ·
Replies
10
Views
4K
Graduate How Critical Are Relativistic Effects in GNSS Accuracy?
  • · Replies 8 ·
Replies
8
Views
7K
Undergrad Understanding Relativity of Simultaneity in Resnick, Part II
  • · Replies 20 ·
Replies
20
Views
3K