[ntp:questions] help - clock jumps up to 5000 ms startup ntpd with usb gps linux

unruh unruh at invalid.ca
Sun Feb 19 16:32:57 UTC 2012

On 2012-02-19, Rob <nomail at example.com> wrote:
> Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
>> *GPS_NMEA(5)     .GPS1.           0 l    4    8   37    0.000  563.733   
> Don't try to use NMEA for timesyncing!  It will only drive you crazy.

No. It is fine. 

> The timestamp in NMEA is a "time of last fix", not the current time.

And ntpd allows you to alter the offset timestamp to account for the
delay in the end of the nmea message coming through. That it uses the
end of the message rather than the beginning causes excess jitter, but
that is never anywhere near 5000ms.

> Some people are lucky and seem to be able to get a usable result, but
> they really are working outside the specs and can get bitten at any
> firmware change (as was recently seen with some Garmin firmware).

Yes, you may have to recalibrate the offset after a firmware change. So?

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