[ntp:questions] Sure gps looses all sattelite fixes

unruh unruh at invalid.ca
Tue Feb 28 18:35:04 UTC 2012

On 2012-02-28, John Hasler <jhasler at newsguy.com> wrote:
> I wrote:
>> It doesn't get location data from the satellite.  It uses the
>> ephemeris[1] to plot satellites that should be above the horizon.
>> [1]Updated periodically, of course.
> unruh writes:
>> Updated how?  The only thing it could update them from is the sattelites
>> themselves. So the sattelites DO give their location to the system.
> Every satellite preiodically transmits the full ephemeris for all the
> satellites, not just its own, and the receiver retains it until it gets
> a new one or it expires.  Thus it can have been quite a while since your
> receiver has gotten data from any satellite but it will still know where
> they all are.
>> And why does the Sure plot 12-14 sattelites at all times while the
>> Gamin only does 9-11? both have the same sattelites above the horizon
>> at any one time. And the sure plots them right down to the horizon (
>> and gets signals from them) while the Garmin only reports them down to
>> about10 degrees above the horizon.
> Most likely because Sure decided to show you all above-horizon
> satellites while Garmin decided to show you only ones they think are
> high enough to be useful.

Thanks for the lesson. Interesting.

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