Re: ✘ Add NEO-M9N NMEA logfile.

[Fred Wright]

On Mon, 13 Jul 2020, Gary E. Miller wrote:

> Your recent commit message:
>
>    2cb46914 by Fred Wright at 2020-07-13T14:32:42-07:00
>    Add NEO-M9N NMEA logfile.
>
>    This is intended primarily as a sample of xxGSV sentences with four
>    simultaneous GNSSes.  This is currently handled poorly, and the
>    included .chk file reflects that.  The latter will need to be updated
>    once the code is fixed.
>
> Looking at the chk file for all of 5 seconds and looks good to me.
>
> What do you see wrong?

It's the known problem with multiple GSV groups.  At one point, a special 
kludge was added for the specific case of GPGSV/GLGSV (and only if they 
appear in that order), but it doesn't work for the more general case. 
The result is that each update in the GP/GL/GA/GB case produces three 
successive SKY reports, with increasing sets of satellites, restarting on 
each update cycle.  I'm pretty sure the reason it's only three instead of 
four is due to the GP/GL coagulation.

The effect is obvious with any of the clients that display constellation 
info.  To see it with the logfile:

$ GPSD_HOME=. ./gpsfake -S test/daemon/ublox-neo-m9n-nmea.log

Then just run one of the relevant clients (though in trying it for this 
email I stubled across a previously unknown crash in xgps).  The display 
is very spastic, as many of the satellites keep coming and going on every 
update.

It gets more complicated with multiple signals.  Although each satellite 
is reported separately for each signal, it really only exists once, and 
with only one el/az.  The only per-signal aspect is the SNR, but the 
current architecture isn't designed for that.  Although the earlier issue 
is specific to NMEA, I believe this is a more general issue, and not one 
that's so easily fixed.

> I must admit I never run a u-blox in NMEA mode.

If switching to u-blox mode worked properly, I might never have noticed 
this problem.  If you're using full u-blox mode with a u-blox 9, you must 
have confgured it with ubxtool, since neither GPSD's built-in 
reconfiguration nor "gpsctl -b" is sufficent with a u-blox 9.  But 
ordinary users shouldn't have to resort to ubxtool to get reasonable 
behavior.

I was figuring on fixing the GSV problem before fixing the mode-switching 
problem, since the latter makes it convenient to test the former.  But I 
also added the logfile as a hedge, and of course one can always put the 
receiver in NMEA mode and launch gpsd with -b to get NMEA behavior.

Fixing the multi-signal problem may be too ambitious for this release, 
considering it impacts almost everything in the chain.  Though a cheap fix 
that reports a mean SNR across signals might be doable.  Of course most 
users don't have multi-signal receivers, anyway.  The only dual-frequency 
receiver I have is GPS/GLONASS only, so it's not a fully general test 
case.

Fred Wright