Re: a const time offset ahead

On Wed, 9 Mar 2022, Lin Zhao wrote:

> [CAUTION: Non-UBC Email]
>       It would help if you gave a complete list for example of the sources command
> 
> What do you mean of "sources command"? if you mean chrony configuration: 

Run chronyc as root
then type
sources
This will list all of your sources and statistics of them.



>       ## Internet server
> pool ntp.ubuntu.com iburst maxsources 4

You have access to the network and can use the pool command? Why are you using
nmea then?

> 
> driftfile /var/lib/chrony/drift
> 
> ## Used for NTP time sync for other system (e.g. DVL, Topside, Pi...)
> allow 192.168.2.0/24
> 
>
>       ## Used for time sync Jetson from Arduino using gps NMEA string and PPS
> makestep 1.0 3
> maxupdateskew 100.0
> refclock SHM 0 poll 0 refid NMEA precision 1e-1 offset 0.0 trust
> refclock PPS /dev/pps1 poll 0 lock NMEA refid PPS precision 1e-6 prefer
> initstepslew 30


> 
>
>       How do you hook up the microcontroller, how does it send out the PPS to make
>
>       sure it is on the UTC second, How is the NMEA generated, etc.?
> 
> 1) The computer is connected to the Internet, so it's originally time synchronized UTC.
> 2) The computer and microcontroller have two part connections: 1) USB cable: used for data transmission. 2) PPS connection and UART (for pure NMEA)

PPS from what? How does the pps operate on the arduio. I presume it is the
machine that is sending the PPS to the computer. Is it run by gps?

> 3) Then I will send UTC time at the beginning of one second to microcontroller through the USB serial. So the microcontroller time is aligned with
> UTC. Obviously, serial transmission will take some time. but it should be very small.

No that is not obvious. nmea is very very slow. Thus it is probably off by
many milliseconds.

> 2) After microcontroller time is aligned with UTC, microcontroller will start to count time using hardware timer, and each one second send PPS and
> NMEA. NMEA is saved from a real GPS, but I will replace the time information that counts inside the microcontroller. 
> 3) Right now, I can compare computer time and microcontroller time, they are the same.
> 4) Now, I start gpsd and chrony, computer start time synchronized with microcontroller. After synchronization, computer time is ahead of the
> microcontroller. For example, microcontroller time is 1646779996, computer time is 1646779997. 
> 
> On Tue, Mar 8, 2022 at 11:46 PM Bill Unruh <[email protected]> wrote:
>       PPS has no "seconds" associated with it. It just blips exactly ( to withing a
>       few ns) on the second. It gets its "seconds" from nmea.
>       It would help if you gave a complete list for example of the
>       sources
>       command. It looks to me like you told it that PPS is off time (ie tried to
>       correct the PPS time rather than the NMEA time) It is the NMEA time that will
>       be off, but .2-.5 seconds usually (late) because the nmea sentence are sent
>       out after the second, are sent out when the gps is not busy, and take a long
>       time to get to the computer( it takes a while for the nmea sentence to be
>       transmitted). But you have a home grown setup, so the problem could well be in
>       there.
>
>       How do you hook up the microcontroller, how does it send out the PPS to make
>       sure it is on the UTC second, How is the NMEA generated, etc.?
>
>       Are you telling chrony that teh PPS is late?
> 
> 
>
>       William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
>       Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
>       UBC, Vancouver,BC _|_ Program in Cosmology |____ [email protected]
>       Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/
>
>       On Tue, 8 Mar 2022, Lin Zhao wrote:
>
>       > [CAUTION: Non-UBC Email]Hi,
>       >
>       > I am using a microcontroller to generate PPS and NMEA string to time sync an Nvidia Jetson
>       > computer using gpsd and chrony for the GPS-denied environment.  PPS and NMEA are verified
>       > as good. 
>       >
>       > When I started time synchronization process, chrony always like:
>       > #* PPS                           0   0   1     0    -894ms[  -894ms] +/- 1000ns
>       >
>       > I tried a lot of times, chrony always considers PPS time is ahead around 890 ms. But I
>       > checked the NMEA time data is right. When the computer is synchronized,  system time is
>       > ahead of NMEA time. Before the time synchronization, both of them are verified the same.
>       >
>       > After the 1 hour test, time is synchronized, but with this const offset.
>       >
>       > Could you tell me how to debug this? Thanks for your help.
>       >
>       > Best regards,
>       >
>       > -------------------------------------------------Lin Zhao
>       >
>       > Research Assistant @ Graduate School of Oceanography 
>       > Ph.D. Student @ Department of Ocean Engineering
>       > University of Rhode Island
>       >
>       > Room 2, Horn Lab
>       > 215 South Ferry Road
>       > Narragansett RI 02882
>       > Phone: 401-771-4661
>       > E-mail: [email protected]
>       > Personal: [email protected]
>       > Lab: https://soslab.wordpress.com/
>       >
>       >
> 
> 
>