Raspberry Pi NTP config with fudge factors

[Frank Nicholas]

> 
> Easy with the 28 refclock.  Not sure on the 20.  DO you even see the 
> coarse time with ntpq -p?

I don’t know enough to answer regarding “coarse time”.  Here’s my output (FreeBSD-10.3-RELEASE):

[2.3-RELEASE][admin at burns.springfield.com]/root: ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .GPS.            0 l    6   16  377    0.000   -0.002   0.003
*tock.usshc.com  .GPS.            1 u   26   64  377   32.138    4.606   0.901
+clock.isc.org   .GPS.            1 u   21   64  377   70.356   -0.027   0.995
-tick.apple.com  .GPS.            1 u   47   64  227  147.752   40.539 163.016
-ntp.wdc1.us.lea 199.102.46.72    2 u   47   64  377   49.799   20.090   3.552
-leeloo.scurvyne 173.162.192.156  2 u   47   64  377  142.869   16.063  34.157
-2001:19f0:8000: 132.163.4.101    2 u    2   64  377   65.329   -7.336   0.499
+67.212.117.78   128.138.141.172  2 u   46   64  377   38.889   -4.778   0.341

>> Regarding 28 ref clock, that is SHM?  Why that rather than NMEA (20)?
> 
> gpsd handles a lot more GPS types than #20.  So let gpsd handle the GPS.
> gpsd passes the time it determines to ntpd using the SHM (shmctl) 
> interface.

So the type 28 (SHM) is coming from gpsd on the same box?  I still like KISS (no need for other GPS type info)...

> The long term goal is to kill off #20 as no need to maintain 2 NMEA
> parsers.

Understood.  I still like KISS...

> time1 adjusts the PPS derived time, time2 adjusts the NMEA derived time.

So for type 20 (NMEA local) & PPS, time2 would be the correct place to put the fudge factor (assuming time1 (PPS) is dead nuts).

>> What’s a good way to “tune” my NTP config?
> 
> Compare it to another ntpd server and see which has the least jitter/wander.

Got it.

>> What’s a good way to monitor my NTP performance?
> 
> I stare at the results of:
> 
> root at pi2:~# watch ntpq -p
> 
> Watch how you local refclock(s) compare to some others.

Got it.

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