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From: | Martin Boissonneault |
Subject: | Re: Clarifications about PPS SHM content |
Date: | Fri, 20 Mar 2020 17:35:25 -0400 |
User-agent: | Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 |
Hi Gary,
I noselected the GNSS serial refclock. I don't think I can optimize it better on my rPi:
Skew- Kurt- Name Mean Units ness osis Local Clock Time Offset -1.672 ns -4.39 12.12 Local RMS Time Jitter 277.112 ns 28.36 220.4 rPi 3 B+, Raspbian Buster, NTPsec 1.1.8, GPSd 3.20, DS3231 RTC, uBlox MAX-M8Q GNSS (fixed, GPS only, SBAS integrity only)
I just wanted to tank you for the help in getting those results. I subscribed to the ntpsec users list, so any further questions about it will be there now.
Again, Thanks
Martin
Yo Martin! On Thu, 19 Mar 2020 13:44:31 -0400 Martin Boissonneault <address@hidden> wrote:Thanks for confirming my intuition. This thought came from watching ntpshmmon. The answer caused the question ;-)Good data does that.Follow-up question: I recall someone said that all time sources have an influence over the stability of the NTPd time, and if you had one unstable time source connected (like the GNSS serial shm?), the whole stability was affected even if you have a better, more precise clock selected.Well, sorta, sometimes. NTP decision making is PhD level obfuxcation. ntpd will only use info from a source is it is part of the best cluster. So a bad pool server will usually be ignored, but it may, every once in a while, yield a good time and be included in the cluster, then be quickly thrown out. Within the cluster, the bext chier is picked, and gets the "*". That is then filtered and then used to discipline the PLL that adjusts the system clock. IFF a bad chimer gets selected will it throw around the system clock. This can happen, during startup, when you have only a PPS and a GPS chimer, with little jitter history. ntpd flips a coin, and it it selects the GPS first, less than optimal things happen. This is why you put a short poll on the PPS, and put it first in the ntp.conf, so it gets selected first, if possible.Considering my GNSS serial SHM has been (x) and is stratum 11, do you think it is negatively affecting my stability and should be commented out?I am unaware how stratum plays into NTP selection algorithms. Best to ask that on their mailing list.I know my ntpviz graphs would look better without the serial shm ;-)Easy: noselect. RGDS GARY --------------------------------------------------------------------------- Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703 address@hidden Tel:+1 541 382 8588 Veritas liberabit vos. -- Quid est veritas? "If you can't measure it, you can't improve it." - Lord Kelvin
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