[ntp:questions] Sure gps looses all sattelite fixes

A C agcarver+ntp at acarver.net
Tue Feb 28 00:25:16 UTC 2012


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 
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.


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