Re: OpenNTPD behaviour with sensors

Thanks for your reply, also to Kevin. This makes sense and explains the
behaviour I'm seeing.
I now know that I have to make some other choices in my setup, but that's
OK.

Best regards and all the best for 2026 (for everyone),
Maurice


On Wed, Dec 31, 2025 at 09:27:51PM +0300, Kihaguru Gathura wrote:
>Hi Maurice
>
>
>This is expected behavior with OpenNTPd and not a problem with your
>hardware.
>
>With only a local sensor, ntpd can control the clock very tightly so that
>microsecond offsets are normal but once you add remote servers, their
>network delay and asymmetry start to influence the clock control as well.
>
>Even if the sensor remains selected in ntpctl, the other peers will still
>affect the filtering and frequency correction, which typically results in
>millisecond-level variation.
>
>Weights don't fully isolate the sensor, and seeing it selected does not
>mean the other sources have zero impact.
>
>If you want to keep microsecond accuracy, the usual approach is to run the
>machine as a pure stratum-1 server with only the sensor configured and get
>redundancy by running multiple such boxes.
>
>However, If you need fallback to network time in the same daemon, then
>OpenNTPd is simply not designed for that use case.
>
>Regards,
>
>Kihaguru Njenga Gathura
>
>
>On Wed, 31 Dec 2025, 17:28 Maurice Janssen, <maurice@z74.net> wrote:
>
>> Hi,
>>
>> I have a couple of small machines (Soekris 6501 and 5501) that I use as
>> NTP server in the NTP pool.
>> When I use a sensor as the only source of time (Garmin 18lvc on the serial
>> port or Meinberg PZF180PEX DCF77 receiver card), the offset is quite good:
>> nearly allways below 20 us, most of the time below 5 us and sometimes even
>> less than 1 us.
>> But when I add another server (mainly as backup in case the sensor doen't
>> work), the offset drifts away to 1 or 2 ms. I tried with 1 sensor and
>> 1 server, with 1 sensor and multiple servers, I tried adding weight 5 or 10
>> to the sensor, but this doesn't seem to have any effect. In all cases ntpd
>> stays synchronised to the sensor (with poll=15s), but after an hour or so
>> after starting ntpd the offset starts to increase. And from that moment,
>> the
>> offset wanders between roughly -2 and +2 ms.
>> It almost seems as if ntpd is synchronised with one of the servers (with
>> much higher polling interval), however ntpctl still shows that it is
>> synchronised to the sensor (albeit with the wandering offset as described
>> above).
>>
>> I would prefer to have the microsecond offset _and_ some servers as backup,
>> but can't get it working like that.
>>
>> Am I missing something? Or is this a bug in ntpd?
>>
>> Thanks for any help.
>>
>>
>>