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

David Lord snews at lordynet.org
Tue Jun 13 18:52:33 UTC 2017


Fida Hasan wrote:
> On Thursday, June 8, 2017 at 6:37:04 AM UTC+10, David Lord wrote:
>> :
>>> Le mardi 6 juin 2017 15:28:27 UTC+2, David Taylor a écrit :
>>>> On 06/06/2017 13:11,  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 127.127.20.2 mode 18 prefer
>> fudge 127.127.20.2 stratum 7 time2 0.407 flag1 0 refid GPSb
>> server 127.127.22.2 minpoll 4 maxpoll 4
>> fudge 127.127.22.2 flag2 0 flag3 1 refid PPSb
>>
>>
>> David
> 
> Hi David,
> I was just wondering to know the accuracy you have achieved through driver 20? Did it turned down to 1-miro or around it?
> 
> Regards,
> Fida

Hi

with the two gps I've tried, driver 20 is only accurate
enough for numbering the second, say +/- 400ms for my LVC
and maybe +/- 100ms for the Sure.

Combined with driver 22 I had accuracy down to few us,
mostly better than 1us.

If you have other internet sources use of driver 20 isn't
essential.


David







More information about the questions mailing list