[ntp:questions] custom NMEA messages

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Mon Dec 28 05:29:24 UTC 2015

On 2015-12-27 15:57, Hal Murray wrote:
>> 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.

Obviously the device has been configured for other uses, perhaps marine.

>> 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.
> http://users.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif

Ugh! I would not have expected that much variation over USB - WiFi does better!
My GPS18xLVC+PPS averages within 1us and peaks about 50us jitter on Windows, and I replace remote sources with offsets much over 10ms!

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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