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

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

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.
>