[ntp:questions] custom NMEA messages

Hal Murray hmurray at megapathdsl.net
Sun Dec 27 22:57:31 UTC 2015

> If neither of those can be seen, IMHO the most likely reasons are:
>  1. You do not see enough sky for the receiver to make a good fix.
>  2. Somehow the device got misconfigured.

Normally, the GPRMC gets sent every second even when the puck doesn't have a 
fix.  There is a field that says OK vs no-fix.  ntpd checks that field.

> Normal timing setup is to have the device send only one NMEA GPRMC message
> at 1s interval, and tweak driver fudge time1 until offset measured against
> other sources averages around zero. 

Beware.  The SiRF chips are well known not co cooperate with that approach, 
depending upon what you expect.  They have lots of wander that you can't 
filter out.  If you tweak the fudge so that it is working correctly, it will 
be off if you wait an hour or two.  You can correct most of the offset, but 
you will be left with roughly 100 ms peak-peak.


These are my opinions.  I hate spam.

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