Re: NTPsec reports excessive jitter from GPSD/KPPS SHM

[Gary E. Miller]

Yo Nick!

On Sun, 08 Mar 2020 03:33:17 +0000
"Nick Burkitt" <address@hidden> wrote:

> Hi Gary.
> 
> Sorry, I'm having a little trouble keeping up.
> 
>       remote           refid      st t when poll reach   delay
> offset jitter
>   SHM(0)          .GPS.            0 l    8   64  377   0.0000
> -65.3223 1.4824
>   SHM(1)          .PPS.            0 l    -    8    0   0.0000
> 0.0000 0.0019
> 
> SHM(0) shows a 1.4 millisecond, not microsecond, jitter.

Yeah, you are right.  Give it some time.  And remember, last you sent
an ntp.conf, did you not have noselect on SHM(0?

> SHM(1) has a reach of 0, so no information.

Correct, gpsd is not sending a SHM(1) when started with:
        gpsd -n /dev/pps0 /dev/ttyACMx

> Where does SHM(2) come into the picture?

As I said, it is the GPS serial time.  Not the order of your
command line.  /dev/pps0 is first, so it shows as SHM(0).  /dev/pps0
has no secondary time, so SHM(1) is unused.  /dev/ttyACMx is second
on the command line, thus becomes SHM(2).  It has no secondary (PP@) so
SHM(3) is unused.

> Here's what ntpshmmon says.
> 
> root@MPM-4006:~# ntpshmmon -n20
>   ntpshmmon: version 3.20
>   #      Name Seen@                Clock                Real
> L Prc
>   sample NTP2  1583636734.047149990  1583636733.998375333  
> 1583636734.000000000 0 -20
>   sample NTP0  1583636734.080901499  1583636734.077254944  
> 1583636733.999650744 0 -20

Yup.  NTP2 is PGS, and NTP0 is PPS.

Be sure to noselect SHM(2) and prefer SHM(0)

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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