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Hi Pierre:

The GPS satellites in orbit have two primary sets of data that are transmitted 
as part of the navigation message:
(1) the almanac for all the satellites is stored on all the satellites so 
during a cold start when the receiver finds one satellite
it will know where to look for all the others
(2) the ephemeris contains precise orbital parameters for the space vehicle 
being tracked and does not contain any magnetic info.
The receiver needs to down load this from each satellite that's included in the solution.

The magnetic model is part of the receiver software.

Have Fun,

Brooke Clarke

Pierre Boucher wrote:

> >Just a quick note on the magnetic variation: I was told by an authority on
> >the subject that after 5 years, the accuracy of the magnetic variation
> >information contained within your GPS software starts to drop off.
>
> I might be wrong but I think GPS get the variation infos from the ephemeris
> received from the satellites.
>
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>               XX X                                 Pierre Boucher N
>             XXX XX               formation en navigation de plaisance
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>       XXXXXX XXX          la "VOILE"...  le reste n'est que du vent...
>   XXXXXXXXXXXXXX       ''Sail, Sail !''... the rest is only wind
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