[time-nuts] TimeKeeper?
Hal Murray
hmurray at megapathdsl.net
Mon Feb 22 22:03:19 UTC 2010
> That said NTP is very conservative in validating the stability of
> clock sources. I have not delved into the code, but it is obvious
> that even a refclock like a GPS receiver doesn't get any favours. Why
> should it? Who knows whether the clock is dodgy or not?
The NMEA strings from low cost GPS units have a lot of noise/jitter.
In particular, the SiRF units are horrible. (They are also low cost and
widely available.) The time offset has a sawtooth pattern with a long time
constant that would be nasty to filter out. Think of hanging bridges.
http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
> However the times he was reporting for the offsets to drop to less
> than 1ms did look excessive.
I've seen lots of comments about ntpd being slow to converge. I haven't
investigated carefully, but they seem credible.
One way to get in trouble is to have a bad drift file. You can get that if
you have a warm system, shut it down, wait for it to cool off, then restart
it.
--
These are my opinions, not necessarily my employer's. I hate spam.
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