Re[2]: NTPsec reports excessive jitter from GPSD/KPPS SHM

[Nick Burkitt]

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.

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

Where does SHM(2) come into the picture?

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
 sample NTP2  1583636734.999550084  1583636734.998374918  1583636735.000000000 0 -20
 sample NTP0  1583636735.073310114  1583636735.070379191  1583636734.999650531 0 -20
 sample NTP2  1583636736.000680522  1583636735.998374849  1583636736.000000000 0 -20
 sample NTP0  1583636736.074692156  1583636736.072042286  1583636735.999650317 0 -20
 sample NTP2  1583636736.999562295  1583636736.998374742  1583636737.000000000 0 -20
 sample NTP0  1583636737.076126896  1583636737.073286765  1583636736.999650104 0 -20
 sample NTP2  1583636737.999548505  1583636737.998374666  1583636738.000000000 0 -20
 sample NTP0  1583636738.077508030  1583636738.074253077  1583636737.999649891 0 -20
 sample NTP2  1583636738.999552552  1583636738.998374608  1583636739.000000000 0 -20
 sample NTP0  1583636739.076370830  1583636739.073640798  1583636738.999649678 0 -20
 sample NTP2  1583636739.999556191  1583636739.998374933  1583636740.000000000 0 -20
 sample NTP0  1583636740.073544785  1583636740.071383503  1583636739.999649465 0 -20
 sample NTP2  1583636740.999522148  1583636740.998375213  1583636741.000000000 0 -20
 sample NTP0  1583636741.073857233  1583636741.071042563  1583636740.999649252 0 -20
 sample NTP2  1583636741.999549024  1583636741.998375206  1583636742.000000000 0 -20
 sample NTP0  1583636742.073080020  1583636742.070756753  1583636741.999649038 0 -20
 sample NTP2  1583636742.999555768  1583636742.998375219  1583636743.000000000 0 -20
 sample NTP0  1583636743.075747919  1583636743.072900466  1583636742.999648825 0 -20

Thanks,

-Nick

------ Original Message ------

From: "Gary E. Miller" <address@hidden>

To: address@hidden

Sent: 3/7/2020 7:16:18 PM

Subject: Re: NTPsec reports excessive jitter from GPSD/KPPS SHM

Yo Nick!

 

On Sun, 08 Mar 2020 01:49:00 +0000

"Nick Burkitt" <address@hidden> wrote:

 

> Thanks - does the order matter? I've been using

 

Not really.

 

> gpsd -n /dev/pps0 /dev/ttyACM4

 

That should be fine. But I admit I never do it in that order.

 

> and seeing those large PPS jitter numbers in NTPsec

> NTPsec doesn't seem to be able to find SHM(1) at all

 

More likely gpsd is not finding it. ntpshmmon will tell you.

 

Oh, I forgot. When you do it "/dev/ttyAMC4 /dev/pps0" then

the GPS is on SHM(0) and PPS is on SHM(2).

 

The way you did it, the PPS is on SHM(0) and the GPS on SHM(2).

Your 1.4 micro second jitter is pretty good. We used to think

that 1 micro scond jitter was as good as you could get.

 

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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