Re: use gpsd and chrony time sync with fake gps

[Gary E. Miller]

Yo Lin!

On Wed, 10 Nov 2021 21:56:15 -0500
Lin Zhao <crazymumu0804@gmail.com> wrote:

> Thanks for providing this mail-list. I am using microcontroller as a
> fake GPS in our project for time sync in gps-denied environment.

Odd.  That could go wrong many ways.

> The microcontroller generates PPS and NMEA messages to Nvidia Jetson
> computer running Ubuntu 18.04. The PPS is stable as tested in
> Oscilloscope. 

What is your definition of "stable"?  You realize PPS is not a "message"
but a Pulse (Per Second).

> The NMEA message is copied from a real gps record.

You at least have to change the time stamps.

> I followed the tutorial in Feeding chrony from GPSD <http://Hi,
> Thanks for providing this mail-list. I am using microcontroller as
> fake GPS in our project for time sync in gps-denied environment. The
> microcontroller generates PPS and NMEA message to Nvidia Jetson
> computer which running Ubuntu 18.04. The PPS is stable as test in
> Oscilloscope. The NMEA message is copied from a real gps record. I
> followed the tutorial in>,

Is that just a repeat of your openeing?

> */etc/chrony/chrony.conf*:
> 
> > pool ntp.ubuntu.com           iburst maxsources 4
> > pool 0.ubuntu.pool.ntp.org iburst maxsources 1
> > pool 1.ubuntu.pool.ntp.org iburst maxsources 1
> > pool 2.ubuntu.pool.ntp.org iburst maxsources 2
> > driftfile /var/lib/chrony/drift
> > allow
> > # set larger delay to allow the NMEA source to overlap with
> > # the other sources and avoid the falseticker status
> > refclock SOCK /var/run/chrony.ttyTHS0.sock refid GPS precision 1e-1
> > offset 0.9999
> > refclock SOCK /var/run/chrony.pps1.sock refid PPS precision 1e-7  
> 
> *Start commands:*
> 
> > sudo killall -9 gpsd chronyd
> > sudo chronyd -f /etc/chrony/chrony.conf
> > sleep 2
> > stty -F /dev/ttyTHS0 speed 9600
> > sudo gpsd /dev/ttyTHS0 -F /var/run/gpsd.sock  

https://gpsd.io/ubxtool-examples.html#_sudo_sudont

You forgot the "-n" option to gpsd.  It is essential for timekeeping.

Why do you have both the serial port, AND a socket connection (-F)?

What is:  /dev/ttyTHS0 ?  Is it a "tty" like driver?  Does it support
KPPS?

> *cgps -s *result:
> [image: cgps.png]

You don't use cgps to debug tim.  What does ntpshmmon (run as root)
show?

> *chronyc sources -v* result:
> [image: chronyc.png]

No reach.  No idea why.

> *sudo ppstest /dev/pps1* result:
> [image: ppstest.png]

So you do have a real PPS, not a "PPS message".

> Everything looks good,

Not to me.

> gpsd can parse the NMEA right,

How do I know that?

> PPS is showing good.

I don't see that.  I pretty sloppy PPS.  No idea how you think it is getting
into gpsd.

> But chrony is not working.

Yes.

Send us the "-D 4" log from gpsd.  Something like this:

As root:

 # gpsd -nND 4 /dev/ttyTHS0 > tmp.log 2>&1

Let that run a minute, then send tmp.log here.

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

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

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