Raspberry Pi NTP config with fudge factors

[Gary E. Miller]

Yo Frank!

On Thu, 5 May 2016 20:53:32 -0400
Frank Nicholas <frank at nicholasfamilycentral.com> wrote:

> > On May 5, 2016, at 8:48 PM, Gary E. Miller <gem at rellim.com> wrote:  
> >> 
> >> I don’t know enough to answer regarding “coarse time”.  Here’s my
> >> output (FreeBSD-10.3-RELEASE):  
> > 
> > yeah, that's a problem with refclock #20, important data is
> > hidden.  
> 
> What was missing from my ntpq -p output?  I’ve included it again
> below: 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

Notice the jitter?  3 microSec?  Clearly PPS.

All the others are remote chimers.  No entry for the NMEA input.

> > When you hide important data from the user, as #20 does, you have
> > over simplified.  
> 
> Please help me understand what’s missing.

Note from my refclock #28:

root at raspberrypi2:/usr/local/src/GPS/gpsd/gpsd# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
-SHM(0)          .GPS.            0 l   27   64    3    0.000   41.271  30.843
*SHM(1)          .SHM.            0 l   26   64    3    0.000    0.249   0.045

SHM(1) is my PPS, like your GPS_NMEA(0).  I just restarted it, so the
jitter is 45 microSec, pretty good, but not the 4 microSec I expect.

You have no correspondence to my SHM(0).  Notice the jitter of 30
milliSec.  That is time based on the first NMEA sentence from the GPS.
Offset is 41 milliSec.  I could chnage my fudge to zero that out, but
after a few minutes it would be -40 milliSec.


> >>> 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).  
> > 
> > Yes, except you'll need to disconnect the PPS to see it…  
> 
> I don’t follow you here…

When you disconnect the PPS, your GPS_NMEA(0) can no longer report PPS
base time, so it will report the NMEA based time.  To not so closely 
related times, mushed into one.  If you GPS stop PPS when it does not
have a good signal the GPS_NMEA(0) will jump wildly.  ntpd will mark
it a false ticker and then you got shit.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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