PGN 129539 and 129540 required to have GPS info displayed in N2K Garmin instruments GMI20, GNX20.

[dpazz]

In my config GPS derived info (position etc.) is forwarded to other instrumentation by Signalk server. Everything is ok with NMEA0183 devices (thru the Signalk to NMEA183 plugin) but NMEA2K instruments (in my case GMI20 and GNX20) aren't displaying GPS related info or displaying it "flashing", just to mean "GPS signal lost" (as is readable in error log of GMI20). I have two hypotesis about this behavior: 1 - the frequency of GPS info update (PGNs 129025, 129026, 129029) is too low (1hz in my case), because Garmin original GPS antennas have higher update frequency (but I'm not confident on this hypotesis) 2 - to consider "valid" the GPS position,COG,SOG info received the Garmins pretend to have also the info about satellite constellation used by GPS itself to output position info and (perhaps) the DOP info. I think that implementing PGN 129539 and 129540 could fix the issue.

[dpazz]

Probably solving the Tigrecito #80 issue could be that also this one will get solved....Maybe...

[jacques-cd]

@dpazz - I've got exactly the same issue and working through debugging them. I started making some code changes (see my comment in #80 ) but from the testing I've done so far I'm pretty confident the main problem here is in line with your hypothesis 1.

I confirmed this by making raw recordings of a mate's setup which has a garmin chartplotter and the same GMI20, GNX20 displays as we have. In the recording I saw the chartplotter sending updates like clockwork, and some of them much faster. I looked at n2k spec, and no surprise they were all in line with that 😅

Here's what I saw:

• 129025 - update every 100ms
• 129026 - update every 250ms
• 129029 - update every 1s
• 129540 - update every 1s

I set the update time in config for 129025 and 129026 and suddenly GPS data started showing on displays. I saw that it did still glitch. I did a raw capture of what was being send on my network (updates glitching) and found that even though I set update time to 0.1s and 0.25s - there was a lot of jitter and sometimes the messages were sent quite late. I'm not sure how to fix that.

Then I found #27 - which seems to address exactly this! The name says "limit", but perhaps it's just hard setting them? I think this might be a big part of what's needed to fix this for garmin compat.