[ntp:questions] Sure gps looses all sattelite fixes

unruh unruh at invalid.ca
Tue Feb 28 06:15:48 UTC 2012


On 2012-02-28, A C <agcarver+ntp at acarver.net> wrote:
> On 2/27/2012 15:46, unruh wrote:
>> 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.
>
> Every GPS I've ever had knows where the satellites are supposed to be 
> given the last known ephemeris data.  That's why they all have backup 
> batteries and how they manage to calculate the position and also 
> reacquire the position if the receiver loses signal for a while.  It's 
> able to make a very reasonable guess at where the satellites are, scan 
> only those visible PRNs to lock onto the signal and then compute a 
> solution.  My very old 12XL will remember the satellites and if I shut 
> it off for an hour then turn it back on, the sky map is different but 
> very close to that of a GPS that is still running with an active map. 
> The map even updates when I have the GPS indoors and not receiving any 
> signal at all (it isn't using a SiRF chipset so it has very poor indoor 
> performance).  Reporting the sky map is trivial for the GPS whether it 
> has signal or not, they all do it.
>
>
> As for age, it doesn't really matter.  Copper corrodes easily, even 
> tinned copper.  It doesn't take much to create a rectifying junction 

They actually look like gold plated connectors, not copper or tinned
copper. 

> that will swamp your RF input on the receiver and render it useless. 
> The receiver front end is very wide bandwidth, strong signals will 
> desense it (just look at the LightSquared fiasco).  Even if there were 
> no corrosion it's entirely possible to have a bad crimp connection.

Could be. Does not look like it, but then again, it probably would not
look like it anyway. 



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