[time-nuts] TimeKeeper?

[Hal Murray]

> 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.