I have two Helix 12 MSI GPS G3N units on my bass boat. One on the bow and one at the console. They are linked with a Ethernet cable. My external heading/gps sensor is mounted at the bow (see photo below) and plugged into the bow unit. My console unit is has connected to an external AS GRP receiver that is mounted at the transom. When i'm at the front of the boat fishing and on the trolling motor moving in a direction the heading line is correct but the boat icon is way off even 180 at times from the direction I'm heading in. See attached pic. Please help!