Best PGN for RMC sentence?

[wjquigs11]

I'm using my SH-ESP32 to read NMEA0183 GPS data from my ICOM VHF and put it on my N2K network.
The ICOM sends RMC, GSA, and GSV. I'm really only interested in RMC (time, date, position, SOG, COG).
Which N2K PGNs would be most useful (for SignalK)? There isn't a direct correlation between RMC and any single N2K PGN, although 129026 (COG and SOG) and 129025 (position) are close. If nothing better is available, I will just send two N2K packets for each RMC, but would love to hear others' opinions.

[mairas]

Those two PGNs are definitely a good match. Are you interested in providing time as well? If so, PGN 129033 would be a good candidate.

One aspect to consider is the expected transmission frequency of the PGNs. 129025 would be normally sent 10 times a second and 129026 four times a second. You might want to stick to those rates even if you receive NMEA 0183 data more infrequently (not sure how often the sentences are sent). If you're using SensESP, you might want to have a look at how I dealt with that in HALMET example firmware: https://github.com/hatlabs/HALMET-example-firmware. I'm pretty happy how clean the PGN sending code turned out there.

[wjquigs11]

Thanks, that's good info. You have some fancy lambda functions in there that are a bit out of my league...I'm more of a 1990s-vintage C coder. But I will definitely use SensESP when I add a temperature sensor to my engine (8 horses of raw Nissan power :-)
I hadn't thought about time. There are definitely circumstances where my RPI won't be tethered and ntpd won't be getting any data, so getting time from N2K is a good idea.

[mairas]

Lambda functions aren't that complex most of the time. :-) Just unnamed functions. In many cases, you could replace them with function pointers but it's nice that you can define the function in-place.

I believe Signal K is set to sync with time received via N2K by default, so if you just generate those PGNs, you'll get the rest for free.