B&G Zeus 3S COG misaligment issue

[Jukka Auramo]

Help needed. We have an issue with incorrect cog data displayed on MFD. Using Zeus internal gps as data the cog is about 13 degrees off from correct (measured from various other gps devices that are not connected to the same nmea200 network).
We are currently in French Polynesia were the magnetic inclination happens to be the same 13 deg, but this should not have are impact on the cog readings, only heading readings which we can offset in the rc42 compass.
Trouble shooting help?
Regards,
Jukka
s/y Katharsis
Oceanis 45

[Pandor]

I’m going to make the leap that you are interpreting something incorrectly. COG is derived directly from successive GPS plots. It isn’t possible for the COG to be incorrect if the GPS is tracking correct lat lons. Therefore, the fact that your perceived error matches the local variation should tell you all you need to know. Is your plotter set up to display magnetic or true degrees?