I can't help you with the wiring of the usb gps antenna to you Raymarine unit, but one concern does come to mind. USB devices are typically powered at 3 to 5 volts dc. If you supply power from your ship's 12vdc system will you blow the antenna or will you need a resistor to reduce the power from 12vdc to the appropriate vdc?
More to think about
D & D
s/v Bruadair
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