[ntp:questions] Garmin GPS18x LVC 5m - Centos 5.5 - Kernel 2.6.18 - gpsd

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Dec 28 09:25:33 UTC 2010

"Q" <.. at ..> wrote in message 
news:4d18cb5a$0$12172$fa0fcedb at news.zen.co.uk...
> I wonder if someone would be kind enough to help me out a little here. I 
> have done lots of googleing and found nothing that helps me out with the 
> issues I'm seeing.
> I am trying to reduce the PPS offset & delay (Which seem quite high for 
> a local device) and stop the NMEA driver from swinging and becoming a 
> false ticker.
> Any ideas please would be great. This is only a personal project, but I 
> was expecting slightly better results! Below are debug outputs, configs 
> and some other 'useful' stuff if you could take a look over them and let 
> me know what I could be doing wrong, or would otherwise benefit from 
> changing.
> Cheers!
> GPS;
> GPS18x - NMEA outputting GPRMC to serial port.
> GPS18x - PPS outputting to Pin 1 of serial port.

For what it's worth, I have been using a GPS18x LVC for quite some time, 
and seen no problems.  As it happens, this is on a Windows-based stratum-0 
clock.  Then a few months back, time keeping became erratic.  I saw there 
was a new firmware out, so I flashed the puck with that firmware and all 
was well.  For a few more weeks, at least.  Then the same happened again 
and, as it happened, there was another new firmware out, so I flashed it 
again.  This time it didn't cure the problem.  More careful observation 
showed that the NMEA output was being seen as up to a second out compared 
to the PPS output.  Like you, I have only the one sentence being used, but 
I was still at 4800 baud (which should easily allow one sentence per 
second, and had for about a year).

The situation now is:

- I don't know why this GPS18x LVC is behaving like this, and why its 
behaviour has changed.

- as this system has good links to other reliable servers, I commented out 
the NMEA driver, and left in just the ATOM driver:

server	minpoll 4
server	minpoll 4	prefer  mode 33

server	minpoll 4

and marked one of the reliable servers as "prefer".  I wonder if anyone 
else has seem similar problems with the GPS18x LVC?

As to PPS offset, I can't measure it accurately here but it appears to be 
under a millisecond, and perhaps under 100 microseconds.


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