[ntp:questions] I'm back to using GPGGA sentence on USB GPS

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Fri Mar 9 16:41:44 UTC 2012

Hi all,

I have, in several threads, recommended using the GPZDA NMEA sentence 
for timekeeping.  The basic reason was that it does not report 
position.  It is therefore mostly fixed length and not as variable.  The 
manual for the Trimble Resolution T GPS receiver also recommends this.  
This produces less jitter in the NMEA data string.  By using a high baud 
rate, and GPZDA only, and polling the GPS every 8 - 64 seconds, I was 
getting offsets almost always within the 4 ms range with spikes to 7 
ms.  I don't have PPS at the moment.

However, an email conversation with David Taylor has convinced me to go 
back to using GPGGA instead.  The basic reason is that the GPGGA 
sentence has a data validity field, whereas GPZDA does not.  If you look 
in the C code for the refclock, you will find that they check this 
validity field and do something with it.  I'm not sure exactly what they 
do, but presumably, invalid data will cause the selection of another 
clock.  I don't know what will happen if that's the only clock 
available, as in my case, since I have all internet clocks noselected 
for testing.

Using the GPGGA sentence does increase jitter, as I suspected.  Early 
data on this sentence indicates I'm getting offsets mostly within the 7 
ms range with spikes to about 11 ms.  Most of the time, though, I'm 
under 10 ms, which is the psychological target I set for this 
equipment.  Overall, I guess it's worth trading a bit of jitter to allow 
the system to do validity checks on the clock.

Just thought I'd pass it along.




(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such.  I don't always see new messages very quickly.  If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)

Ron Frazier
timekeepingdude AT c3energy.com

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