Re: Galileo


"Paul Cooper" <a.paul.r.cooper@xxxxxxxxxxxxxxxxxx> wrote in message
news:pbvlr197qu8cemvqr2dr37qm1l9s9j2rsl@xxxxxxxxxx
> On Tue, 03 Jan 2006 21:25:25 GMT, Ian Sandell <ian@xxxxxxxxxxxxx>
> wrote:
>
> >On Mon, 02 Jan 2006 15:19:50 +0000, Nick <do.not@xxxxxxxx> wrote:
> >
> >
> >>>
> >>> Paul
> >>It does depend on the receiver's clock. The satellites transmit a very
> >>accurate time signal and the receiver has to measure the time
> >>differences between the satellite signals as they arrive. If that time
> >>measurement is not very accurate, the fix will be degraded.
> >
> >My universal computer modelling programme, Bak-***-Paq, suggests that
> >receiver accuracy needs to be at least 10 e-9. Is that right, do you
> >know?
> >
> >Ian
> The whole point of the system is that the accuracy is located on the
> satellites, and is not required at the receiver. Each satellite
> carries an atomic clock; the signals from the satellites are
> synchronized. It is the difference in arrival times at the receiver
> that gives the position, not the absolute time. The signal also
> contains a time signal synched to the satellite's atomic clock,
> allowing the receiver to iteratively set the local time precisely. You
> use an approximate location to determine the delays, then use the
> delays to determine a more precise location and repeat until you are
> wihin the system errors. That's why it takes so long to determine
> position when switched on for the first time a long way from the last
> location. The receiver unit does not need an accurate clock for thsi
> purpose, only a reasonably stable frequency source.
>
> Paul

Thank you Paul - that is the clearest explanation I've heard so far.

Floatything

ps. does this thread set a record or have the 'thread dividers' messed it
up?


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