[ntp:questions] ntpd - gpsd communication

William Unruh unruh at invalid.ca
Thu Jun 4 20:59:57 UTC 2020

On 2020-06-04, ah at avtodoria.ru <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)

What is the wrong data? Ie, how much of an offset does the wrong data
What does wrong data mean? Do you mean that the PPS is wrong? that of
course has no time attached so can be wrong by only up to 1 second.
Or do you mean that the time reported by the NMEA sentences is wrong?
How wrong? 1 sec? 100 days?
> 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

Note that if it really is your GPS that is misbehaving, why not just buy
another board. They are cheap. The Adafruit gps is only about $50.
Your tearing out your hair is probably worth more than that.

> Thank you in advance

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