GPS satellite and satnav receiver clock synchronisation

• quitequick
In summary: GPS receiver's clock without first syncing it to a known time source. Every so often he'll notice that his positions will be a few meters off, even after a full 24 hours of GPS operation. He's been trying to figure out a way to sync his receiver without having to manually enter the time every time he wants to use it.In summary, Dave's GPS receiver is inaccurate by about 5mm, but this inaccuracy is only apparent when he tries to use it to measure distances.
quitequick
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!

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:

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

I can explain the process of GPS satellite and satnav receiver clock synchronization. The accuracy of the receiver's clock is indeed crucial for the accuracy of distance measurements in the GPS system. However, the receiver does not need to know the exact distance to the satellite in order to synchronize its clock with the satellite's clock. This is because the GPS system uses a technique called "pseudorange" to estimate the distance to the satellite.

Pseudorange is calculated by multiplying the speed of light by the time difference between when the satellite's signal was transmitted and when it was received by the receiver. This time difference is measured using the receiver's own clock. So, even though the receiver's clock may not be perfectly synchronized with the satellite's clock, it can still accurately calculate the pseudorange.

The GPS system also uses a process called "differential correction" to further improve the accuracy of the receiver's clock. This involves comparing the time signals from multiple satellites and using the differences to adjust the receiver's clock. This is why GPS receivers often require an initial period of time to acquire signals from multiple satellites before providing accurate location information.

Additionally, many modern GPS receivers do have on-board atomic clocks, which are extremely accurate and do not require frequent synchronization. However, even if the receiver does not have an atomic clock, the regular synchronization with multiple satellites allows for accurate timekeeping.

In summary, the GPS system uses pseudorange and differential correction techniques to synchronize the receiver's clock with the satellite's clock, ensuring accurate distance measurements and location information. This eliminates the chicken and egg problem and allows for precise navigation using GPS technology.

Q: What is GPS satellite and satnav receiver clock synchronisation?

GPS satellite and satnav receiver clock synchronisation is the process of ensuring that the clocks on GPS satellites and receivers are synchronized. This is necessary for accurate positioning and navigation.

Q: Why is clock synchronisation important for GPS satellites and receivers?

Clock synchronisation is important because it allows for accurate timing of signals between satellites and receivers. This is crucial for determining the precise location of a receiver on the Earth's surface.

Q: How does GPS satellite and satnav receiver clock synchronisation work?

GPS satellites have atomic clocks that are constantly synchronized with ground control stations. The receiver also has its own clock, which is less accurate than the atomic clock on the satellite. The satellite sends a signal containing its current time and the receiver uses this information to adjust its clock to match the satellite's time.

Q: What happens if the clocks on GPS satellites and receivers are not synchronized?

If the clocks are not synchronized, the receiver will not be able to accurately determine its position. This can result in errors in navigation and positioning, which can be dangerous for activities such as aviation or maritime navigation.

Q: How often do GPS satellites and receivers need to be synchronized?

GPS satellites and receivers are automatically synchronized every 12.5 minutes. This ensures that the clocks remain accurate and any discrepancies are corrected in a timely manner.

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