[ntp:questions] Sure gps looses all sattelite fixes

A C agcarver+ntp at acarver.net
Mon Feb 27 22:48:11 UTC 2012


On 2/27/2012 08:49, unruh wrote:

> Ssure tracks with a S/N of between about 18 and 54. (higher is stonger)
> The Garmin trackes with S/N between about 35 and 54.
>
> What happens here is that the sattelites will be strong (S/N of 45-54)
> then suddenly over the time frame of seconds, all loose tracking ( but
> the gps still sees the sattelites and still reports their location in
> the sky), and when they come back again they are at the 45-54 level.
> I have now seen one such dropout from the Garmin 18 that is located
> about 5 m away from the sure antenna. (The sure at that time was offline
> for about 15 hrs).
>
> There seems to be some problem, wither with the antenna or with the
> board. This kind of dropout behaviour makes the Sure much less useful
> for timing purposes.
>
>   have written to Sure, but have had no response.
>
The GPS doesn't see the satellites if your signals hit zero.  It does 
happen to know the ephemeris data for all the satellites and simply 
plots their position based on its internal clock.  If it didn't have 
this capability, any reboot would take many minutes until the next 
ephemeris transmission.  With the tracking capability you get teh 
ability to have 10-12 second warm boots.

That said, I would check your antenna cables for broken or corroded 
connections.  I had an external antenna on a Garmin 12XL receiver that 
worked great for years until one day I got exactly the behavior you 
describe.  Eventually I discovered that the shield crimp on the 
connector to the receiver (an MCX connector) had become loose and the 
shield was making intermittent contact.  Sometimes, the on-board 
amplifier in the antenna was no longer receiving power if the shield 
disconnected.  The loose crimp also allowed the wire to corrode slightly 
which creates a rectifying junction.  The connection would be mostly ok 
in that power would be flowing to the amplifier but the rectifying 
junction makes the long coax into an antenna capable of receiving and AM 
demodulating (via the rectification of the junction) any signal nearby. 
  A signal strong enough would flood the RF front-end and all my signals 
would go away.


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