[ntp:questions] like a kid with a new toy (PPS jitter)

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Thu Feb 5 21:33:51 UTC 2009

Dave Hart wrote:
> After close to a week of calibration, I fudged then flipped off
> noselect on my GPS+PPS refclock in the early minutes of the UTC day.
> It looks to me (from my limited perspective of no other local
> refclocks) that I've gone from single-digit milliseconds to double-
> digit microseconds accuracy.
> Looking at the last 800 loopstats lines for the refclock, representing
> a bit more than 3 hours of 16s polls, the offsets in microseconds look
> like
> -79.497	min
> -26.961	mean
> 20.565	max
> 17.381	stddev
> The jitter (us)
> 1.907	min
> 7.974	mean
> 35.64	max
> 4.651	stddev
> How does that look to more jaded PPS eyes?

Pretty bad actually: After a day or so you should see 0-5 us offset and 

> The refclock is a Garmin GPS 18x LVC connected to a dual PII 400 via

I have one of those! (A Dell?)

It might work even better in single-cpu mode if it is a dedicated ntp 

> serial with PPS wired to the carrier detect pin.
> Cheers,
> Dave Hart

- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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