headingsource NMEA

[julled]

hey, first of all thanks a lot for building this nice OSH tracker, havent built one yet, but i am looking forward to it!

i would be interested if its possible to use heading via NMEA?
I would like to use onship high quality GPS and COMPASS to provide especially the good heading to the tracker.
Usually its hard to use trackers own compass as viable source, as the ship contains too much metal to make it a feasable source of heading.

Best

[zs6buj]

Hi, and yes you can use NMEA telemetry to determine the heading of the ship, and by deduction, where the tracker "front field" is relative to world coordinates (or a map). A full description of the technique is described in the wiki, but the short explanation is that we take two GPS readings while the ship is in motion, and use the imaginary line between them as the heading.

[zs6buj]

Since the ship may be tacking, or some yaw present due to currents, the heading may be off to some degree, but the angular error could possibly be corrected visually, or by watching the rssi of the telemetry transmission. It should remain constant thereafter.

[julled]

Hey @zs6buj thanks for these suggestions. I also thought of this approach you suggested, but as you mentioned it can introduce some error, especially if no movment is there.

I thought of instead using a proper heading provided from a mature ship compass via NMEA messages, like the GPHDT message, e.g. see here https://docs.advancednavigation.com/gnss-compass/Operation/NMEA%200183.htm

Most of the ships have a good compass so the idea was to use this info from there.

Would this be possible to integrate?

[zs6buj]

I took a quick look, and yes it is likely possible to decode $GPHDT and $GPGLL from serial or ethernet

[zs6buj]

I found this:

system
Feb 2012post #2
The $GPHDT sentence gives you the True Heading of the GPS device. The device calculates the heading by working out the deviation in successive location readings, so at low speeds it may not be particularly accurate, given the location precision of consumer GPS.

So $GPHDT might not be what you expect?

EDIT: But I see you already addressed this issue

mature ship compass via NMEA messages

I'm looking for a library to decode GPDDT, otherwise I must just parse it myself