[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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