[ntp:questions] NTP with GPS and RTC

Biebaut Sven sven.biebaut at be.thalesgroup.com
Fri Apr 26 14:09:18 UTC 2013

>[ 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 ?

>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.

>> - the kernel stops updating the RTC


OK I guess, as ntpd will keep disciplining the local clock.

>> - 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,


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