[ntp:questions] NTP with GPS and RTC

unruh unruh at invalid.ca
Fri Apr 26 15:53:33 UTC 2013

On 2013-04-26, Biebaut Sven <sven.biebaut at be.thalesgroup.com> wrote:
>>[ Long lines repaired. ]
> Thanks, I will take better care next time
>>Biebaut Sven wrote:
>>> If I drop the idea of the RTC as a reference clock, am I correct in
>>+ stating that, when there is no external synchronisation:
>>> - my local clock and my RTC will drift away from each other, but at
>>+ least my RTC will be closer to the mark (the DS3231 is chosen for its
>>+ precision)
>>Probably not.  ntpd will continue to apply first order frequency 
>>correction to the local clock.  
> Ah, I did not realise that. So a system with ntpd but without an external 
> reference clock would still be more accurate than a system without ntpd 
> at all ?

Yes. By far.
If you look at www.theory.physics.ubc.ca/chrony/chrony.html you can see
the rate variations on a bunch of commercial grade systems disciplined
by chrony, either with a local GPS or disciplined from that system.
The rate varioations are on the order of .2 to 1 PPM over a day, with
drifts of a few PPM per week. the daily variations are alost certainly
due to temp variations due to workloads of the machines. The weekly
drifts due to crystal ageing etc. 
Note the raw corrections are of the order of up to 50PPM from the rate
the clock would run at if it had not been disciplined. ( 50PPM is a few
seconds per day)

>>Assuming the machine temperature was 
>>reasonably stable, you would need to to trim hte DS3231 several times a 
>>year to be sure of bettering a coasting local clock.  (2ppm per year, 
>>maximum of 5ppm over first 10 years.)
> Unfortunately, access to the device will be impossible after its deployement, 
> so it seems that just running ntpd with an external reference clock that only 
> occasionally is present, is the only real option.

Define occasionally. Unfortuneately ntpd requires about 5-10 hours to
rediscipline a clock to ultimate accuracy when the connection comes up
again, so if the connection time is shorter than that, chrony (assuming
you run linux or some unix) is a better choice ( faster lock and
discipline time). It also allows you to "trim by hand". Ie, if you can
phone into the device, and find it is say 1 min off, you can tell chrony
to correct that offset by hand.

>>> - the kernel stops updating the RTC
> OK I guess, as ntpd will keep disciplining the local clock.

It really makes the rtc almost useless, EXCEPT for reboots. 

>>> - there is no other existing mechanism that disciplines the local system clock
>> You have provided insufficient information about your system to answer this.
> Agreed. Actually, there is none implemented. I should have asked if there were 
> any other mechanisms possible, but your answers to the other questions now make 
> this unnecessary :) 
> Thank you,
> Sven
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