[ntp:questions] Time server question

Jakob Bohm jb-usenet at wisemo.com.invalid
Wed Jul 24 05:49:36 UTC 2019

On 21/07/2019 16:02, Terje Mathisen wrote:
> William Unruh wrote:
>> On 2019-07-19, Chris <xxx.syseng.yyy at gfsys.co.uk> wrote:
>>> On 07/18/19 11:13, William Unruh wrote:
>>>> Sure, but I do not have faith in the "averaging" If one is always 30us
>>>> after the other, then the average will always be out by 15us.
>>> One would expect a difference, but how can you tell which one is right
>>> using just 2 pps ?. With three, you could choose the closest to average
>>> and discard the outlier, or if it was outside a defined window. Ok,
>>> it's a bit nitpicking, but would still be interesting to try it.
>> No. The mechanism is clear. While one is answering its interrupt the
>> other gets to wait. So, it is the earliest one that is closest to
>> "right" Ie, do not try to use more than one interrupt on the same
>> computer. It does not work
> A good timing-optimized gps unit, like the original Oncore, have a sw 
> mechanism to offset the PPS event away from the actual top of the 
> second, as well as a way for the sw protocol that numbers the PPS 
> signals to also specify how far away this particular pulse is from the 
> actual event.
> I.e. with an internal 10 MHz clock, PPS signals will be synced to one of 
> those 100 ns-wide periods, so it can/will be at least up to +/-50 ns 
> away from the proper moment, but when the driver knows about this, it 
> can adjust perfectly for that effect.
> Terje

I happen to have a GPS unit (not yet connected) that is documented to do
this too: The PPS pulse occurs at an edge of the internal crystal clock,
but a special NMEA statement states (based on the 4D GPS solution) how
many ns it is off for each pulse.  I have yet to find the point to pass
this offset to ntpd after capturing the PPS arrival time.


Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
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