[ntp:questions] Re: Question to NTP developers

Hal Murray hmurray at suespammers.org
Thu Aug 24 05:16:56 UTC 2006

>There is only one GPS receiver and, of course, one PPS hardware signal.
>How could the PPS signal be present on one server and absent on the
>other in the same time ? This is my problem and I suspect 50% the
>hardware on the motherboards and 50% the software (OS + NTP) !

What type of GPS receiver are you using?  Have you turned on
logging and looked in (both) clockstats?

Some GPS receivers turn off the PPS signal when they know they
don't have a good signal.  Some leave it running.

Most GPS receiver give you some sort of status indication to tell you if
the signal they are receiving is reasonable.  For example, here are
a few lines from a receiver that uses NMEA:
53963 17392.501 $GPRMC,044953,A,3726.0639,N,12212.1673,W,000.3,105.4,160806,015.3,E*6B
53963 17456.337 $GPRMC,045057,V,3726.0630,N,12212.1664,W,,,160806,015.3,E*7C
53963 17519.324 $GPRMC,045200,V,3726.0630,N,12212.1664,W,,,160806,015.3,E*7C
53963 17584.547 $GPRMC,045305,A,3726.0804,N,12212.1886,W,000.1,050.9,160806,015.3,E*69
53963 17647.534 $GPRMC,045408,A,3726.0806,N,12212.1892,W,000.0,064
53963 17712.586 $GPRMC,045513,A,3726.0805,N,12212.1929,W,000.1,096

The V means bad, and the A means good.  So the signal dropped out for
somewhere between a bit over a minute and three minutes.

One possible reason why your two boxes are not doing the same thing is
that they aren't actually looking at the same data.  The info above
is only part of data that goes over the serial line.  Normally NMEA
receivers send a line of info every second (or every N seconds).
ntpd only looks for info every 64 seconds.  (handwave, that's probably
not right but it's close enough)  If you have two boxes looking
at the same signal they may be out of phase.

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