External magnetometers can help your navigation system avoid the dynamic interference around the center of the aircraft. If you want to connect an external magnetometer to your HoverflyGPS unit (and we strongly recommend that you do), connect the magnetometer to the "Ext Bus" port in the following arrangement:
Upper Row: Signal (SDA) - Power - Ground
Lower Row: Signal (SCL) - None - None
Take extra caution that you make these connections correctly! Failure to do so can short out your HoverflyGPS!
See this article for more information on external magnetometers: