Re: [chrony-users] a const time offset ahead

[ Thread Index | Date Index | More Archives ]

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

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 |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_

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: linzhao@xxxxxxx
Personal: crazymumu0804@xxxxxxxxx

Mail converted by MHonArc 2.6.19+