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

David Lord snews at lordynet.org
Wed Jun 7 20:13:16 UTC 2017

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:)


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


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