[ntp:questions] Single GPS/PPS time source gets marked as a falseticker

David Lord snews at lordynet.org
Fri Jul 15 18:51:31 UTC 2011


unruh wrote:
> On 2011-07-15, David Lord <snews at lordynet.org> wrote:
>> Michael Eder wrote:
>>> Again we are talking effectively what happens.  There is a good deal of
>>> logic what to do if the NMEA and/or pps does not come in and if the system
>>> clock is significantly different from either the NMEA or pps.  A whole day
>>> on the lab bench with a scope on both the pps and the NMEA these never
>>> happen.  The pps comes in and by definition it must be represent the second
>>> + 1 of the previous NMEA string.  This is exactly the same logic that gpsd
>>> uses and is why we tell NTP that the NMEA has an offset.  
>>>
>>> We scope the oscillator also and it is a very expensive oceanographic device
>>> that adjusts for temperature automatically.
>>>
>>>
>>> Sounds like a common problem and I note when we add an additional network
>>> clock the problem seems to go away.
>>>
>>> I note there are multiple SHM segments in NTP.  Anyone tried writing the pps
>>> time into two locations to fix this problem?
>> Did you try my suggestion of marking the NMEA as prefer rather
>> than the PPS?
>>
>> server 127.127.28.0 minpoll 4 maxpoll 4 prefer
>> fudge 127.127.28.0 time1 0.680 refid NMEA
>> server 127.127.28.1 minpoll 4 maxpoll 4
>> fudge 127.127.28.1 refid PPS
>>
>> instead of:
>>
>> server 127.127.28.0 minpoll 4 maxpoll 4
>> fudge 127.127.28.0 time1 0.680 refid NMEA
>> server 127.127.28.1 minpoll 4 maxpoll 4 prefer
>> fudge 127.127.28.1 refid PPS
> 
> What are you feeding the shm with? Ie, what are you putting into the
> shm How do you deterime the "seconds" for the PPS?
> 
> I would also make sure that the system did not think that nmea was a
> stratum 0 server, but jack its stratum up, so the system always thought
> that the PPS was a higher stratum than nmea. 
> I am not sure why you would make nmea a preferred source?
> 

When I tried when I was using only a single source and normal
type 20 NMEA driver it was needed otherwise ntpd doesn't sync.

Having the type 22 PPS driver as prefer gave the same bad
results as posted by the OP.

I've been told my config is wrong but my server hasn't had
any problems (although once it was setup with the new GPS
there have been other servers added).

<http://www.lordynet.org.uk/mrtg/stats/ntp/me6000_ntp.html>


David




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