[ntp:questions] Performance of the shared-memory reference clock

unruh unruh at wormhole.physics.ubc.ca
Wed Feb 24 18:57:41 UTC 2010

On 2010-02-23, Loretta Goldberg <loretta_1958 at yahoo.com> wrote:
> The?shared-memory reference clock was modified (ntp-4.2.5p138) to collect data each second, rather than once per polling interval.? What would be the impact of this modification on how accurately I can estimate the current time from?a given GPS?? 

What are your requirements. The main problem is that ntp has a very slow
response to temp changes and that is the limiting feature on the
accuracy you will get. Otherwise, you should get a few usec accuracy. Is
that OK for you?

> ?
> If my system requirements specify that I have to use a version of NTP that predates this modification, can I attain the same accuracy from my GPS?
> ?
> Is the limitation that it will take me longer to synchronize to my GPS?? Or does the increased polling interval prevent me from attaining the same degree of accuracy in my estimation of the current time?

ntp takes a long time to synchronize.

> ?
> I do have both a pulse-per-second signal and a time-of-day message coming from my gps.? The pulse-per-second will be designated as the "prefer" instance of the shared-memory reference clock.? The time stamp for the pulse-per-second will not be passed to NTP until NTP has "locked on" to the time stamp coming from the time-of-day message.

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