Re: [chrony-users] PPS and NMEA same source not combined

[Mike Smith <mike@xxxxxxxxxxxxxxxxxxxxx>]

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 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.

 

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.