NMEA Output of COG by GP39

[jfordyce]

Hello,

I can get my GP 39 to display COG in either True or Magnetic, but it seems that the NMEA 0183 output is only in true. I have the output hooked up to a Furuno Navtex which displays the position and speed just fine, but the COG comes out in true. IS there a way to produce a magnetic output? Or is it up to the receiving machine to convert, which the Navtex can't do.
Thanks for your help.

J.E. Fordyce

 

[Johnny Electron]

Maybe is more of how your Navtex is setup to display the NMEA than the NMEA data from the GP39. The default COG/BRG refrence on the GP39 is Magnetic. As you say, it can be changed to "True". The NMEA data output follows that setting.

 

[jfordyce]

Hi Johnny, Thanks for your response. I did a little more digging on this. I am using REM 1 on the GP 39. According to the manual, unless I have a waypoint active (which I don't) the only output of COG are with the NMEA 0183 sentences RMC and VTG. According to my research, RMC contains the COG in True and also provides the variation. VTG may be in either Magnetic or True, as far as I can tell.
My Navtex 300, according to the manual, only accepts GGA, GLL, RMB and ZDA. The GP 39 only transmits RMB when a waypoint is activated, so I am not sure how the Navtex is picking up any COG. In any event, it has no way of combining the True COG and the variation to produce a Magnetic COG on its display. I am not really surprised at this, since display of navigation information is not the primary function of the Navtex.
It's all quite a mystery to me and I guess I will have to live with it. It is only a secondary display and not driving any other instruments, so not a huge problem.
Regards,
J.E. Fordyce