Re: PPS and NMEA same source not combined

Actually offset not drift (Offset is the actual time difference between your
clock and the true time, drift is the rate at which the offset grows, or
shrinks.

Anyway that is a very minor point. It sounds to me that your nmea is not
working at all, which is why I was asking how you deliver the nmea time to
chrony. 
What is your /etc/chrony.conf file? Ie, could you show us a copy of your
chrony.conf file?


What does (run as root)
chronyc
sources
actually say? (Ie show us a copy and paste of the output of the sources
output.)



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 ______|_    theory.physics.ubc.ca/

On Sun, 19 Mar 2023, Mike Smith wrote:

> [CAUTION: Non-UBC Email] Bill thanks for explanation of the 300ns, all very clear and no I don’t need
> it, it’s an interesting exercise however. :)
> My pps and NMEA both come from a gps hat (u-blox M8Q). 
> 
> When I found the mega drift I did restart chrony which in effect did nothing to the drift.  So on the
> surface it seems to me that NMEA is looking at the second and pps looking at sub second, which did
> surprise me a little as I would have expected the NMEA to cover the broadest time and date first
> before narrowing down on the second, hence my original question.  
> In order to get it all back in sync I had to set the date and then restart chronyd, which shortly
> after resulted in a corrected time.  At no point did chronyc sources show me any errors, all very odd.
> 
> Thanks for you time and explanations Bill you have help me understand this a lot more that I did a few
> days ago.  
> 
> Mike
>  
> 
> Kind regards, Michael A Smith.
> 
> 
> 📩 mike-/[email protected]
> My apologies for any incorrect information or typographical errors. Any opinions expressed are my own,
> and are not intended to offend. Anyone offended by anything stated, will likely have offended me, by
> being inconsiderate of my beliefs.
>  
>
>       On 19 Mar 2023, at 17:26, Bill Unruh <[email protected]> wrote:
>
>       Whetehr it is actually accurate to 300ns is open to debate. It has no way of
>       knowing what the time lag is between the GPS receiver receiving the second
>       mark, and the computer actually registring it. Length of the cable from the
>       receiver to the computer, time of the computer actually registring the arrival
>       of the leading edge of the pulse. Time required to actually read the system
>       clock. Those are all places of systematic delays. All chrony can do is to
>       measure the random delays and errors, and average them out. That gives the
>       300ns. Without another more accurate timing source you cannot test the
>       actually accuracy. But I suspect you do not care for that kind of accuracy
>       anyway.
>
>       Secondly, are you sure that your nmea actaully works and is delivering the
>       time to the computer?If you stop and restart chrony is the time delivered
>       approximately right? If you look in the chrony logs, do you see the nmea time
>       being delivered to the machine even if it is not selected?
>
>       So what does it say
>       chronyd
>       sources
>
>       How are you getting the nmea timing?
> 
>
>       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 ______|_    theory.physics.ubc.ca/
>
>       On Sun, 19 Mar 2023, Mike Smith wrote:
>
>             [CAUTION: Non-UBC Email] Well in theory no one has access apart from me, but
>             yesterday after I was messing about with it, trying to
>
>             get it all working.  Then I looked in more detail PPS working within 300ns
>             wow! but then I looked at the time and date and it was in
>
>             July 2022.  So it was accurate to the second but way out in time and date,
>             rather surprised me.  Was all very odd.
>
>              
>
>             [t_7edvEm.jpg?v=45]
>
>             Kind regards, Michael A Smith.
>
>             📲 07973 221971
>
>             🏠 24, Fifth Avenue, Portsmouth PO6 3PE UK
>
>             📩 mike-/[email protected]
>
>             My apologies for any incorrect information or typographical errors. Any
>             opinions expressed are my own, and are not intended to
>
>             offend. Anyone offended by anything stated, will likely have offended me, by
>             being inconsiderate of my beliefs.
>
>              
> 
>
>                  On 19 Mar 2023, at 16:22, Bill Unruh <[email protected]> wrote:
> 
>
>                  On Sun, 19 Mar 2023, Mike Smith wrote:
> 
>
>                        [CAUTION: Non-UBC Email] Ok Bill thanks very much for the
>             comprehensive reply, so in effect I am using
> 
>
>                        both effectively by locking my pps against an NMEA, and once it has
>             that pps takes over and NMEA
> 
>
>                        ceases to be needed.  So in effect what I am seeing is the correct
>             behaviour and I shouldn’t expect
> 
>
>                        anything different.  That’s all great to know, thank you. What
>             mitigation can I add for say someone
> 
>
>                  Someone? who else has root on your machine? If it is on the net you can
>             also
>
>                  have other network servers as backups. And nmea is still working I
>             suppose.
>
>                  And if "someone" can change the date, they can also change your
>             chrony.conf
>
>                  and remove the PPS, and do alll sorts of damage.
> 
>
>                        changing the date of the machine, as my current method keeps
>             seconds accurately but doesn’t seem to
> 
>
>                        test beyond that, so into mins hours and days?  Hope you understand
>             my question.
> 
>
>                        Many thanks, Mike.
> 
>
>                         
> 
>
>                        [t_7edvEm.jpg?v=45]
> 
>
>                        Kind regards, Michael A Smith.
> 
>
>                        📲 07973 221971
> 
>
>                        🏠 24, Fifth Avenue, Portsmouth PO6 3PE UK
> 
>
>                        📩 mike-/[email protected]
> 
>
>                        My apologies for any incorrect information or typographical errors.
>             Any opinions expressed are my own,
> 
>
>                        and are not intended to offend. Anyone offended by anything stated,
>             will likely have offended me, by
> 
>
>                        being inconsiderate of my beliefs.
> 
>
>                         
> 
>
>                             On 19 Mar 2023, at 00:41, Bill Unruh <[email protected]>
>             wrote:
> 
>
>                             NMEA is in general a terrible clock, unless what you want to
>             know is "What second is the
> 
>
>                             time at". Delivering an NMEA string takes about 1/10 of a
>             second. 9600Bd, with 10 bits per
> 
>
>                             character, and about 100 characters per string.
> 
>
>                             Compare that to PPS which delivers the "top of the second" to
>             about 1
> 
>
>                             microsecond, 100000 times better. Ie, nmea and pps are
>             completely
> 
>
>                             incomparable. The NMEA is of course critical in telling which
>             second it was
> 
>
>                             that tht PPPS pulse was telling you to the microsecond when it
>             occured. YOu
> 
>
>                             can tell PPS to use NMEA to find out what second it is (it
>             could also use
> 
>
>                             almost any other source on the net as well, but once it has
>             done so, it no
> 
>
>                             longer needs it unless something disasterous happens so that
>             chrony no longer
> 
>
>                             know what second the signal came in on (eg the computer shut
>             down for an hour
> 
>
>                             or more).
> 
>
>                             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 ______|_
>                theory.physics.ubc.ca/
> 
>
>                             On Sat, 18 Mar 2023, Mike Smith wrote:
> 
>
>                                   [CAUTION: Non-UBC Email] Hi, I have a GPS hat working
>             fine with a steady nice
> 
>
>                                   long stream of sats.
> 
>
>                                   My chrony conf file has the following refclocks 
> 
>
>                                   refclock PPS /dev/pps0 lock nmea refid PPS
> 
>
>                                   refclock SHM 0 refid NMEA
> 
>
>                                   Chrony eventually chooses my PPS as the current best
>             clock as I would have
> 
>
>                                   expected.  Unfortunately the NMEA clock always comes up
> 
>
>                                   with the ? Maybe in error when queried from chronyc
>             sources
> 
>
>                                   I can’t see why NMEA should be in error when it’s
>             essentially the same source.
> 
>
>                                    Is there something I can do to keep pps as current
> 
>
>                                   best clock and NMEA to be combined?
> 
>
>                                   Sorry if it’s a stupid question
> 
>
>                                   Thanks, Mike.
> 
>
>                                    
> 
>
>                                   Kind regards, Michael A Smith.
> 
>
>                                   📩 mike-/[email protected]
> 
>
>                                   My apologies for any incorrect information or
>             typographical errors. Any
> 
>
>                                   opinions expressed are my own, and are not intended to
> 
>
>                                   offend. Anyone offended by anything stated, will likely
>             have offended me, by
> 
>
>                                   being inconsiderate of my beliefs.
> 
>
>                                    
> 
> 
>