[ntp:questions] ntpd - gpsd communication

juergen perlinger juergen.perlinger at t-online.de
Sat Jun 6 06:03:20 UTC 2020

On 6/4/20 9:15 PM, ah at avtodoria.ru wrote:
> Hello!! 
> I have time jumps made by ntpd because gpsd sometimes puts wrong data to SHM because of wrong data from gps receiver(very bad chips)
> I have only one time source in ntp.conf — with PPS enabled
> I want to add another two internet sources for stability but at first i want to emulate wrong data from gpsd to see how ntpd will make this time jump.
> As first step i read data from SHM when gpsd worked correctly at 14 p.m. today UTC+3 for 30 minutes
> And after that i stopped gpsd and launched my own binary at 17:30 p.m. UTC+3 writing that old data to SHM. I expected the great offset and the time jump after some time(as it was when receiver lied) but what i saw was:
> - without binary launched ntpd had no updates — it’s correct (no data — no action)
> - with binary launched ntpd had a little offset for all 30 minutes without any attempt to correct system time
> please give me some help!! Maybe i don’t understand ntpd — gpsd communication correctly or smth else
> Thank you in advance

There's another way to communicate between gpsd and ntpd, using driver
46 (gpsd-json). It probably won't help with a really bad receiver, but
it is a bit more sophisticated than the SHM memory slot, even if I say
so myself. Stale data is much less of a problem with this driver. It
might give different/better results, but I won't make promises here.


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