Re: Fwd: Re: NTP with only GPS/PPS flags PPS as falseticker

[Martin Boissonneault]

Hi Nick,

My issue was sending to the list, so you got the email directly long before I re-sent it to the list. In the process of fixing my problem, I had to create a new email account, so I lost all the threading. So, yes, I might have missed your last reply.
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I know my configuration works on my hardware (with a physical RTC), but it looked like some parts of it could be useful to you. It was tuned over a year, but I had to rebuild that Pi moving it from Stretch to Buster (now offset ~100ns, jitter ~300ns).

In my rebuilding process, I traced back my changes, re-evaluated them to weed out bad or outdated advice, and, document in a reproducible way, my configuration (also, create a bunch of reconfiguration scripts). One of those elements was giving a lower (as in bigger number) stratum to the refclock shm unit in charge of the serial time string. From your output, both refclock shm units have the same stratum (0), and NTPd has to figure why the serial one is always less accurate than the other. I'd think it takes time for NTPd to figure this out, and until it has some sort of baseline to determine who is a falseticker, both are suspicious.

To help that process, I use the stratum 11 for the serial time string, which should make it undesirable if there are better chimers available (usually stratum 1-3) from the network. Your PPS shm unit, being stratum 0, should be regarded as desirable above all the other ones and be used instead of trying to use the serial one while online. I recommend you try setting the serial shm to stratum 11 and report back.

One issue to look for is the built-in SNTP-sync, look at ntpviz's graphs for recurrent time spikes.

# Disable the built-in time sync system
# https://www.freedesktop.org/software/systemd/man/systemd-timesyncd.service.html
sudo timedatectl set-ntp false

Tip: If you use pool servers, I recommend you find what are your country's official NTP timeservers and use two in the mix. They will help weed out pool falsetickers, and NTP is smart enough to get enough pool servers to get time right, pick the bests, and drop unneeded pool servers after. The official servers will be kept for quality checking, but their poll will get far apart.

After two days, I have:

• kernel PPS (o),
  
• shm PPS (+),
  
• shm serial GNSS (x),
• server machine_from_LAN ( ),
• server time.nrc.ca (+), #Canada official NTP server #1
  
• server time1.chu.nrc.ca (-), #Canada official NTP server #2
• pool server something (+)

I have nothing else used!

I will try to remember to comment out the kernel PPS line in my ntp.conf and test it offline tonight. That should be close to your software setup.

Martin

On 2020-03-19 11:36, Nick Burkitt wrote:

Hi Martin.

OK, I have issues sending to the address@hidden maillist, this is my 3rd attempt. It should be OK now, I juggled my email configuration by creating a dedicated email instead of the default catch-all. -- Martin
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Thanks - but this looks like a duplicate of your email from last week. I replied yesterday - perhaps you missed it?

-Nick