Dave,

I am not shure, but I have the impression that You told that you have a
Garmin GpsMap276C too, it is correct? Remenbering what that unknow guy
told, that all the DSC ready garmin gpsploter already have support to
AIS, and as You have a NASA or other AIS receiver, could You make a
test to see if the Map276C can receives the AIVDM message? Pheraps it
has the function to plot the ships position based on the AIVDM message,
buried into the DSC function. What You think?

Regards

pascal