NTPsec flags my GPS as a falseticker

[Nick Burkitt]

Hi Hal.

>It could be a bug in our code, but that seems like the sort of thing that
>would get noticed.
Even if I stop gpsd, which kills both shared-memory segments, and reduce 
my ntp.conf file to this:

refclock shm unit 0 refid GPS

I still see this error (in stdout, since there's no log file):

2020-03-19T22:13:50 ntpd[23312]: CONFIG: configpeers: Ignoring duplicate 
'127.127.28.0'

Is it possible to define a reference clock (using the refclock command) 
without provoking this error?

>ntpd does scan a directory, something like /etc/ntp.d/, but I thought that
>added a log message if it found anything.  gpsd may have put something there.
Nothing unusual in /etc/ntp.d:

-rw-r--r--  1 root root  577 Mar 19 22:21 ntp.conf
-rw-r--r--  1 root root 1437 Sep 27  2018 use-country-pool
-rw-r--r--  1 root root  652 Mar 19 22:16 use-gpsd-gps-shm
-rw-r--r--  1 root root  379 Sep 27  2018 use-gpsd-json
-rw-r--r--  1 root root  631 Mar 19 22:20 use-gpsd-pps-shm
-rw-r--r--  1 root root  178 Sep 27  2018 use-minimal-logging
-rw-r--r--  1 root root  416 Sep 27  2018 use-no-remote-configuration
-rw-r--r--  1 root root  465 Sep 27  2018 use-performance-logging
-rw-r--r--  1 root root 1246 Sep 27  2018 use-pool

>There is probably a way to tell gpsd how to find the PPS device.  My guess
I believe the official method is to include it on the command line. :-)
>
>would be that it looks for /dev/pps0 when it opens the first serial port and
>uses if if it finds one, else does the ldattach dance to make it.
gpsd doesn't find /dev/pps0 unless I specify it on the command line.
>
>  If nothing else, there is a HOWTO on setting up a Raspberry Pi and that has a
>separate PPS via GPIO.
>https://www.ntpsec.org/white-papers/stratum-1-microserver-howto/
Since I'm using a hardware interrupt, rather than a GPIO input, the 
pps-gpio driver isn't useful. But what it does is trivial, and my kernel 
driver does the same thing.

But - since there's nothing to indicate that either PPS or gpsd has a 
problem...

>>  I'd like to understand why that is. So far, I've  managed to improve things a
>>  bit by adjust the GPS offset
>
>That's the mindist stuff in my second message.  I forgot to include this line
>from my ntp.conf
>
>tos mindist 0.250                      # default is 0.001 (1 ms)
And that seems to be the magic incantation that has been missing from my 
ntp.conf until now!
250 ms is probably more generous than I need - I don't see offsets 
greater than about 15 ms, but it probably doesn't hurt

I think the mystery is finally solved!
Thanks,

-Nick
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