[ntp:questions] Garmin LVC 18x jitter problem

David Woolley david at ex.djwhome.demon.invalid
Fri Jul 12 23:12:02 UTC 2019

On 12/07/2019 11:32, William Unruh wrote:
> Do not use nmea as a time source. It it has a long delay ( hundreds of
> ms) which depnds on the speed of connection, on length of the nmea
> sentance, etc. EAch character takes from 100 of usec to ms to be
> delivered, and the lengthof sentences is variable, and the gps sends out
> a sentence only when it is not terribly busy ( which depends on how many
> sattelites it is trying to handle, etc) It is good for setting the
> seconds (sometimes-- if you ask it to send too many sentences it can be
> out by more than a second totally messing up your timing).
> One sentence (RMA) and a high baud rate make it useful for setting the
> second.

I believe the OP is using it for seconds.  The problem is, I think, that 
ntpd will only use it, and only accept the PPS, if it is a true chimer.

Moreover, I think the OP's problem is that the NMEA timing is too 
repeatable, so its root dispersion is too small to cover the static 
error, although, if they had a lot of jitter, they would then advertise 
a root dispersion that doesn't give credit to the PPS accuracy.

This is a degenerate case of root dispersion, as it is measured at the root.

Rather than setting as separate error bound for the NMEA, is there any 
option to set a static offset separately for the NMEA.

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