[ntp:questions] custom NMEA messages

Leon McCalla leonmccalla at hotmail.com
Mon Dec 28 15:16:56 UTC 2015

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

10 satelite fix indoors. GPS is operating.

> 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.

the GPS sends 3 GPRMC messages in a 3 second window but NOT 1 per second.

>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 

This is my problem. I bought a GPS based on an inferior chipset.

>My GPS18xLVC+PPS averages within 1us and peaks about 50us jitter on 

Garmin is a real GPS. please don’t mock me...

it is because of these instability that I analyzed that messages in detail 
and noticed GPGBS being sent every 3 seconds with under 10 ms jitter. This 
is the reason I am asking how difficult will it be for me to modify the NMEA 
driver to accept GPGBS messages with mode 16? Should this question be on 
another list?


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