[ntp:questions] project ntp.br - discrepancy from UTC

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Oct 5 20:24:37 UTC 2007

In article <1191543815_6985 at sp12lax.superfeed.net>,
Antonio M. Moreiras <antonio at moreiras.eng.br> wrote:

> After one year, with the discrepancy at 16ms it will be probabily of the 
> same order than the half round trip time for the majority of the clients.
> Given this, do you you think it will be necessary any modification?

Speaking personally, I would say definitely yes.

> I don´t know if I correctly understood what this discrepancy can cause. 
> If a client is using other sources attached directly to GPSs, for 
> example, there is a risk that our servers being considered falsetickers. 

It can certainly declare you a falseticker.

> Is it? Or is there other problems?

At least for some configurations, it could declare all the external servers
(maybe all the servers) as falsetickers.  It could also hop between the
two versions of the time.

> A discrepancy of 16ms in a Internet NTP primary server is acceptable?

It would exceed modern expectations, even if not necessary for most
applications.  People expect this sort of accuracy on end nodes.

> The majority of the Internet NTP primary servers are GPS based. For 
> curiosity: what is the discrepancy of GPS time from UTC (without 
> considering the leap seconds)?

50 nano seconds at about the 50 percentile, is the official specification.
The constellation needs to be in synch with each other to rather better
than this for GPS to work at all, although it is not strictly necessary
that they match UTC.  Even if they don't match UTC exactly, the offset
will be available.

I seem to remember that typical NTP servers can lock to this to
microsecond accuracies, although typical network delays will degrade
this to around 1 ms.

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