[ntp:questions] ntpd - gpsd communication

ah at avtodoria.ru ah at avtodoria.ru
Thu Jun 4 22:10:02 UTC 2020


> What is the wrong data? Ie, how much of an offset does the wrong data
> imply?
> 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?

We have a lot of computers with different hardware(we can mean it as different generations of equipment). Among this equipment there are different receivers, the last portion of them are stable but the first is not. I have already requested logs with gpspipe -R from system administrators, but we lost those with wrong behaviour and now we are waiting the new event. Try to believe me please... 
The typical behaviour in case of time jump is that suddenly we have time in gpspipe jumped forth or back for [some seconds - some years]. Ntpd can't believe to that jump but because this source is the only after some time (about 5-10 minutes) ntpd hardly set system time to time in SHM. About 10-15 minutes system time is wrong but ntpd characterisitics are good(offset is less than 1 ms, little jitter etc). We have situation named 'ntpd has been cheated'. After that period of time there is reverse jump in gpspipe -R log. And ntpd again slowly starts to believe in another jump. After all situation becomes OK.

> 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.

I agree with you it is the best solution. But managers solved to try to fix it by programmers hands at first. So here am i ))
I want to add two another sources to fullfill the complect of three generals)) but at first i want to understand how to cheat ntpd as gpsd regularly does. I think i did all i need. But it looks like i did something wrong in that test. Ntpd eats my data but doesn't touch system time. And i can't understand why



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