[ntp:questions] UTC Time from NMEA receiver one second behind DCF?
Richard B. Gilbert
rgilbert88 at comcast.net
Sun Aug 10 16:36:34 UTC 2008
Harald Brinkmann wrote:
> This is my setup:
>
> I am using a Navilock NL-320U connected to a small Linux box running ntp
> 4.2.4p4-44.1 that came with the openSuSE 11.0 distribution. This is
> supposed to supply a time service to the local network without the use of
> external network ntp servers purely from the received GPS signal.
>
> I know that using a USB connection is not optimal, but the achieved accuracy
> is fine for my needs.
>
> Last Saturday (2008-08-02) I noticed for the first time that the time off
> the GPS unit was one second behind the DCF time, which I monitor on a
> separate radio clock. A reboot of the system did not help. On Sunday
> (2008-08-03) everything was back to normal. I noticed the same effect again
> on Friday evening (2008-08-08) and through yesterday (2008-08-09), but
> everything seems fine today (2008-08-10).
>
> I added a network server to my ntp configuration to double check the effect.
> This is how the output "ntpq -p" looks like when everything is fine:
>
> remote refid st t when poll reach delay offset
> jitter
> ==============================================================================
> LOCAL(0) .LOCL. 10 l 19 64 377 0.000 0.000
> 3.906
> *GPS_NMEA(0) .GPS. 3 l 22 64 377 0.000 14.902
> 3.906
> xptbtime2.ptb.de .PTB. 1 u 422 1024 377 66.375 -8.106
> 5.216
>
> And this is the output when I observe the one second lag:
>
> remote refid st t when poll reach delay offset
> jitter
> ==============================================================================
> LOCAL(0) .LOCL. 10 l 33 64 17 0.000 0.000
> 3.906
> *GPS_NMEA(0) .GPS. 3 l 30 64 17 0.000 -978.27
> 11.515
> xptbtime2.ptb.de .PTB. 1 u 31 64 17 67.160 1.002
> 3.906
>
> Looking at the raw NMEA output, the UTC info in there also seems to be one
> second slow.
>
> In all of this I presume the PTB time to be correct.
>
> My question is, has anyone else observed this and how can I fix this?
>
> Thanks
>
> Harald
>
It sounds as if the GPS receiver you have was designed for navigation
rather than timing!
Timing receivers typically have a Pulse Per Second (PPS) output and use
a binary protocol rather than NMEA to transmit the time to a serial port.
Using the proper tool for the job should solve many of your problems.
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