[ntp:questions] custom NMEA messages

Hal Murray hmurray at megapathdsl.net
Mon Jan 4 08:58:07 UTC 2016

>>    http://users.megapathdsl.net/~hmurray/ntp/GPS18LVCx-off.gif

> You must have had one of the bad firmware releases: David Taylor's site
> shows the improvement he got after upgrading to better firmware releases, in
> which I believe you were involved with "Kiwi" Geoff on analysis.
> http://www.satsignal.eu/ntp/Garmin-GSP18x-LVC-firmware-issue.htm 

If you look about halfway down that page, there is a table showing the timing 
variations of firmware versions 3.70 and 3.90.  3.90 has a peak-peak of 181 

That's roughly as bad as whatever firmware I'm running.  So I haven't played 
the new-firmware game.  Anybody know if I can update the firmware from Linux? 
 (or *BSD)

> If you are looking at NMEA message timing - that's all over the board on
> every device, as those serial interface messages get to use only the time
> left after SV tracking and GPS message processing is handled, which may not
> be a lot on the microcontrollers used. Nav GPS receivers with PPS are good
> enough with NTP for most of us who want consistent, correct system time but
> do not need lab quality timing. 

No, some devices do it right.  Look at my graph (way above).  You can do a 
whole lot better than most devices do.

The issue is not the delay.  That can be fudged.  The problem is stability.

If the problem is CPU cycles to compute the answer, a solution is to wait 
until the worst case and then send the message.

These are my opinions.  I hate spam.

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