[ntp:questions] Shall I use a fudge on PPS?

William Unruh unruh at invalid.ca
Wed Jul 17 06:15:35 UTC 2019

On 2019-07-16, Andreas Mattheiss <please.post at publicly.invalid> wrote:
> Hello,
> I have a cheap GPS hooked onto my PC via a MAX232 level converter - the
> PPS goes directly into the DCD pin of a native serial port. 
> Everything is fine; after some time ntpd locks onto the PPS signal, and
> any other servers from the net show a small offset in ntpq -p. 
> What is the correct way of dealing with this? We are talking about 3 ms
> here. Is it appropriate to use a fudge on the PPS to bring it in line with
> the mean of a handfull of other servers from the net, or is is more
> appropriate to use the PPS's pings face value, not using any fudge?

A couple of possibiliites-- your connection to the net is highly
assymetric, so that it send path is 6 ms longer than the receive
path.That will make the ntp software think that everything outside is later than
it is.
Or, you are triggering pps on the falling rather than the rising edge of
the pulse. 
Oh, a third possibility-- your level converter is really really really
slow. GPS PPS  (even cheap ones) should be good to 1 usec, not 3 ms. 

> Thanks, regards
> Andreas

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