[ntp:questions] Sure gps looses all sattelite fixes

unruh unruh at invalid.ca
Tue Feb 28 06:13:30 UTC 2012


On 2012-02-28, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Mon, Feb 27, 2012 at 4:29 PM, John Hasler <jhasler at newsguy.com> wrote:
>
>>> I could imagine that the signal is too weak for timing but strong
>>> enough to get location info from the sattelite.
>
>
> It works the other way around.  You need four satellites to get
> location but once you know your location only one is needed for time.
> (This assumes the sure GPS has a timing mode where you set a fixed
> location.  All timing GPSes have this.)   The other thing is that GPS

Unless this is supposed to be a tautology (Ie, yo udefine a timing GPS
as one that has that feature) then it is wrong. AFAIK the Sure, and teh
Garmin 18 do not have the feature where you can input their location.
They solve for the location and the time all at once. 
Since the location is good to something like 5m, that would give a
maximum time error of about 15ns, obtained by solving for latt, long,
height and time from the data from 4 sattelites. . 

> satellites only send and time their orbital elements.  They leave it
> to the receiver to use that to compute the satellite's location.   So
> as the signal got worse and worse the last thing you'd loose is the
> ability to keep time.

No. Many of the cheaper GPS solve for time as part of the equation as
above. 
I assume that the receiver is still obtaining the orbital elements from
the sattelites and that is how the receiver figures out where in the sky
they are. Some say it remembers the orbital elements forever, until the
sattelite changes them. Maybe, I do not know. I was assuming that it was
receiving the obits from the sattelites even though it could not send it
reliable timing data-- since noise could well mess up the time far more
easily than it does the digital data. 

>
>
> Chris Albertson
> Redondo Beach, California



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