[chrony-users] GPS time

[scs@xxxxxxxxxx (Steve Summit)]

I have two sets of questions about chrony, but they're sort of
independent, so I'm going to ask them in separate messages.
This one is about GPS timing.

We have a system with a specialized set of timing requirements
(see other message) which have proved difficult to achieve,
so we're building a simulation capability to let us monitor
and validate the system's performance under various unusual or
extreme conditions.  Naturally enough, part of that simulation
capability is a GPS simulator.  It simply synthesizes NMEA
sentences in the obvious way, but giving us control over the
messages emitted, both their content and their timing.

The simulator seems to mostly work: gpsd connects to it (via TCP),
and the cgps utility displays its data properly.  But, for
whatever reason, chrony is *not* happy with it.  The moment I
switch from the real to the simulated GPS receiver, "LastRx" as
reported by 'chronyc sources' begins to increase, and after a
minute or two chrony concedes that it is no longer synchronized.
If I switch from the simulated GPS receiver back to the real one,
LastRx almost immediately goes to 0, and chrony regains sync.

Other than LastRx, there are no statistics reported by chronyc
sources, sourcestats, or tracking that are obviously different,
that would suggest why the simulated input is being rejected/
ignored.  I don't think there's a problem with the timing,
latency, or jitter of the synthesized messages, but those
are certainly possibilities to consider.

I suspect that the ensemble of NMEA strings generated by the
simulator (currently just $GPGGA and $GPZDA) may be incomplete
as far as chrony is concerned.  So my specific questions are:

* Is there a list of NMEA sentences (and fields) which chrony requires?
* Is there any logging or other debugging I can enable to find
  out exactly why chrony is rejecting the simulated GPS feed?
* Where in the source tree is chrony's NMEA parsing and validation?
  Once there I should be able to answer the other two questions
  for myself.

Thanks for any hints or suggestions anyone can provide.

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