[ntp:questions] UTC Time from NMEA receiver one second behind DCF?

Harald Brinkmann hb7267 at gmx.de
Sun Aug 10 13:20:26 UTC 2008


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




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