Re: NTP via tcp NMEA feed

[Nick Taylor]

Yo Gary
> > 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.
> > Additionally often Internet connectivity is firewalled back so we
> > drop back to using GPS time rather than proper NTP
> VPNs solve that easily.
Often we are limited to https connection to IP of our server load 
balancer only
> > Now I notice that I don't seem to see NTP coming through from NTP
> > feed
> Uh, what is an NTP feed?  I assume you mean NMEA feed.
Yep typo
> > - 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
> > 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.

It seems that my hopes unfounded and we may need to forego this wish. We 
have fallback time sources via htpdate or using last resort of picking 
up time from our https servers and using that.
> > 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!!

Many thanks guys for your input

Nick