In this instance you should connect green to the same busbar as GPS black.
The radar has three different ground connections for different reasons:
- Ship's battery negative - this wire will carry the radar current back to the battery, which will have high current pulses when it's transmitting so will have a lot of noise and may be above 0V when the radar is operating due to voltage drop in the cable.
- Nav-Aid data ground - this wire is the voltage level against which the Nav-Aid data signal will be measured. It should be clean and steady, so needs to be separate from the above.
- Ship's ground - this will go to the outer metal shielding of the radar. It needs to be well grounded to the ship for RF purposes.
The GPS is a much lower power device and doesn't really need all that, so it has a single ground for both signal and power.
To make it "Alan Proof" can I confirm :
Radar Black wire to battery negative (via negative busbar) As already connected.
Radar Green (Nav-Aid neg) to battery negative (via negative busbar)
Radar Braid to ships ground plate.
Yes it could be a question of semantics and is the only logical (pun?) explanation. I've come across this before where the manual states what you should be connecting to rather than the purpose of that connection.
NMEA data transmission works at very high impedance so its very unlikely there will be issues even if you make a mistake.
Take Giants advice if you wish, it won't do any harm, but not necessary since NMEA - is already strapped to battery negative at GPS.
Do you have an electronic compass outputting the NMEA 0183 HDM sentence BTW?
Just a simple illuminated (+/- wired) compass.
The GPS is my 'electronic' compass
Thanks for the guidance.