Re: Galileo

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
.

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