[ntp:questions] GPS-PPS, standalone server. NTP

girino66 at gmail.com girino66 at gmail.com
Thu Jun 8 02:52:18 UTC 2017

Le mercredi 7 juin 2017 22:37:04 UTC+2, David Lord a écrit :
> girino66 at gmail.com wrote:
> > Le mardi 6 juin 2017 15:28:27 UTC+2, David Taylor a écrit :
> >> On 06/06/2017 13:11, girino66 at gmail.com wrote:
> >> []
> >>> Some GPS will continue to deliver a PPS signal even if the lock is lost. I'm thinking particularly about the Garmin 18xLVC where it is clearly indicated in the documentation (4.4.1): 'After the initial position fix has been calculated, the PPS signal is generated and continues until the unit is powered down.'
> >>>
> >>> With the use of that 'kind of' GPS, ntpd will continue to provide time service.
> >> As I understand it, NTP will only continue to provide a service if it 
> >> has other "time-of-day" sources available.  Should the NMEA output (as 
> >> the only time-of-day source) become invalid, NTP would reject it, and 
> >> gradually ramp itself up to stratum-16 so as to become invalid as a 
> >> server to its clients.
> >>
> >> [1 - I'm unsure off the top of my head what NTP checks to know whether 
> >> NMEA is valid or not.
> >> 2 - I wonder what the drift in the GPS 18x LVC is when unlocked?]
> >>
> >> -- 
> >> Cheers,
> >> David
> >> Web: http://www.satsignal.eu
> > 
> > At least on my 'Garmin', when a fix is not valid the position is not given but the time message remain available. The GPS internal clock continue to work.
> > 
> > One question is to know how stable and precise can be the internal clock of a 18xLVC GPS model? I don't have yet the answer but if it's comparable with the one in a Raspberry or Odroid chip then I'm an happy man for some hours:)
> Hi
> NMEA from my 18xLVC was +/- 300ms so I used fudge stratum so that
> it didn't affect time accuracy if PPS wasn't available. Sometimes
> there was an inversion layer preventing good GPS reception. The
> LVC was swapped out to be replaced by a SURE which was still
> reliable when the PC went down in March this year an has not yet
> been replaced.
> from my ntp.conf:
> server mode 18 prefer
> fudge stratum 7 time2 0.407 flag1 0 refid GPSb
> server minpoll 4 maxpoll 4
> fudge flag2 0 flag3 1 refid PPSb
> David


My 18x is linked with an Odroid C1+ and I use only driver 20 with 0x10000 mode enabled to have a better look on NMEA sentences. It's by looking 'ntpq -c clockvar' results that I discovered the quality of my 18x comparing with some others 'Adafruit' or basic GPS chips.

from my ntp.conf:
server mode 65553 minpoll 5 maxpoll 5 
fudge time2 0.550 flag1 1 flag3 1 flag4 1  

Knowing that by default flags are set to 0 I don't explicitly feel the need to reset them again to 0. 
Having only one physical device (GPS) connected I also don't feel the need to rename the driver with 'refid' option. 


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