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
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Kind regards, Michael A Smith.
📩 mike@xxxxxxxxxxxxxxxxxxxxx
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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 <unruh@xxxxxxxxxxxxxx> 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 |____ unruh@xxxxxxxxxxxxxx
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@xxxxxxxxxxxxxxxxxxxxx
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 <unruh@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx
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 <unruh@xxxxxxxxxxxxxx> 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 |____ unruh@xxxxxxxxxxxxxx
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@xxxxxxxxxxxxxxxxxxxxx
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.
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