Re: NTP via tcp NMEA feed

[Gary E. Miller]

Yo Nick!

On Fri, 14 Oct 2022 19:11:32 +0100
Nick Taylor <nicktaylor@dataskill.uk> wrote:

> >> You may have seen that we are working with using tcp NMEA feeds
> >> for a device that we built - basically certain customers have
> >> existing GPS devices and wish to use feed from that instead of
> >> separate antenna...  
> > Why?  
> Clients don't want separate antenna - or to interfere with their 
> existing with splitter. Understandable in part because some trucks
> are autonomous and every connection adds potential point of failure.

Oh, right, you described that before, the clients are all at the same
location.

> >> - the only success we had was using the shm feed and with normal
> >> setup ntpshmmon shows normal time feed coming through nicely which
> >> we can then link into chrony.  
> > There is no NTP in NMEA.  The best you can ge is to take the time
> > from the NMEA, and that will have a lot of jitter.
> >  
> I should have made it clear, we aren't after high accuracy time just
> few seconds here or there acceptable just to correct for gradual
> clock drift

Then just take the time from $GPRMC, which is what gpsd does, and then
puts it in a SHM, that you see with ntpshmmon.

> >> Is this expected behaviour or a bug?  
> > I'm not sure what you are expecting, but what you see is normal.
> >  
> I was hoping that NMEA contained enough time info that the dest gpsd 
> could still obtain time and broadcast on the shm segment as readable 
> with ntpshmmon.

Which is eacatly what it does.

> It seems that my hopes unfounded and we may need to forego this wish.

Lost me?

> We have fallback time sources via htpdate or using last resort of
> picking up time from our https servers and using that.

If you have even intermittent internet, then use NTP protocol.

> >> Is there any way to still get
> >> NTP from a tcp NMEA feed??  
> > Nope.  Totally different protocols.  NTP on the net has a handshake,
> > NMEA does not. NTP in hardware, is a dedicated wire, NMEA in
> > hardware is serial bits.  NTP can get you (almost) to 1 ns, NMEA is
> > lucky to get you to 1 second.  
> As I say accuracy not important here but I'm kind of coming to 
> understand that my expectation of time just appearing into dest gpsd 
> feed via NMEA and appearing in shm is not going to happen!!

gpsd has worked that way for a long time.  So nothing needs to happen
in gpsd.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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    "If you can't measure it, you can't improve it." - Lord Kelvin

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