NavNet UDP data format

[muttnik]

The binary preamble which furuno navnet sticks in front of NMEA data in UDP packets caused problems for OpenCPN before the parsing rules were changed and it was ignored. However it seems that some (at least) Furuno plotters require this preamble for AIS data (and possibly others?) to be recognised as a poster on the kplex google group has discovered.

Does anyone have information on the structure of this preamble (or indeed other mysteries of NavNet like what the various ports are used for)? Hacking kplex to add such a preamble is do-able but I'd feel uncomfortable adding a header where I only know the meaning of one field.