[ntp:questions] Sure gps looses all sattelite fixes

unruh unruh at invalid.ca
Mon Feb 27 23:46:56 UTC 2012

On 2012-02-27, A C <agcarver+ntp at acarver.net> wrote:
> 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.

This is a brand new Sure (8 months old which had only been inside until
a week ago). So it is hard to imagine that there could be corrosion. 
Surely it does not report based on predicted orbits. I could imagine
that the signal is too weak for timing but strong enough to get location
info from the sattelite. I guess I should try disconnecting the antenna
(when it starts to work agin-- it has been out for hours now). and see
if it continues to report positions. 

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