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

girino66 at gmail.com girino66 at gmail.com
Wed Jun 14 19:32:19 UTC 2017

Le mercredi 14 juin 2017 18:58:07 UTC+2, Hans Mayer a écrit :
> Hi Fida, 
> > However, did you ever tried to achieve 1-microsecond accuracy in your used refid (in your case with driver 28 which NTP uses)? My primary goal is to attain 1-microsecond accuracy bond where I can see 3 milliseconds offset with .GPS refid.
> Definitely not. 
> With driver 22  I am in the +- 10 usec range. With driver 28 in millisecond range. 
> See my blog.
> http://blog.mayer.tv/2017/06/14/ntp-driver-22-versus-28.html
> I assume driver 20 is very similar to 28. Both are reading serial data. 
> I don't know if 1-microsecond is a easy goal. I am using a very cheap hardware. It's running on a BananaPI. All together about 150 EUR. With a good hardware it's maybe possible. 
> But why do you need this ? Is it just your ambition. This I could understand. 
> // Hans

Hi Hans,

Driver 20 is very different from driver 28.

Driver 28 receives data from a shared memory space shared with another process. It does nothing else. The structure of the memory segment is described in the NTP documentation: https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
Data can comes from any kind of program that is able to fill the structure shared in memory. It is common to see driver 28 in association with GPSD program because driver 28 is enabled by default in usual distributions like 'Raspbian'.

Driver 20 receives data from a serial port and is able to decode some NMEA sequences. That driver is also able to manage directly a PPS signal as driver 22 do and because of that it is also able to be as precise as driver 20 is.
Unfortunately its use is possible after a recompilation of ntpd with correct options to enable it. It fully works only on Unix like systems, the 'Windows' implementation is not complete.

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