[ntp:questions] GPS_NEMA, but high offset and jitter?

David J Taylor david-taylor at blueyonder.co.not-this-part.nor-this.uk
Wed Feb 24 20:07:43 UTC 2010


"Kelsey Cummings" <kgc at sonic.net> wrote in message 
news:4b8580e5$0$1616$742ec2ed at news.sonic.net...
[]
> I'm not sure why it lost sync but I went ahead and upgraded to 4.2.7p5 
> from the ntp-dev port.
>
> I'm still seeing high offset and variable jitter.
>
> ==============================================================================
> xGPS_NMEA(1)  .GPS.      0 l   45   64  377    0.000  -311.36  75.859
> -time.sr    207.200.81.. 2 u  348  512  377    0.643    0.977   2.984
> *clock.sjc.he .CDMA.     1 u  362  512  377    6.348   -2.584   0.289
>
> Or: http://kgc.users.sonic.net/ntp_127_127_20_1-day.png
> The initial spike was the last restart.
>
> # Garmin GPS 18 LVC
> server  127.127.20.1    mode 0  prefer
> fudge   127.127.20.1    flag1 1
>
> Does the GPS_NEMA driver actually make use of the PPS signal (currently 
> wired to DCD on serial port) at all or does it just use the GPS 
> sentences?
>
> I'm wired up following http://www.satsignal.eu/ntp/GPS-interface-2.png 
> except that I used a NPN transistor to drive the PPS led so it wouldn't 
> load the PPS signal.
>
> -K

Kelsey, at least you now have a reach of 377 for the GPS!

The offset of -300ms suggests to me that you could be syncing on the wrong 
edge of the PPS signal.

Cheers,
David 




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