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Using an Airmar Heading Sensor
Garmin recommends customers use a Marine Heading Sensor with the Garmin Marine Chartplotter.
A third-party heading sensor like the ones made by Airmar connects to a chartplotter using NMEA 0183 or NMEA 2000. Below are links for the cables used to connect the Airmar heading sensor to the Garmin Chartplotter or Garmin Radar.
By adding a heading sensor, the chartplotter will have the ability to provide a true heading at all times. Without a heading sensor, the chartplotter will rely on a GPS heading that will only be accurate when the boat moves. With GPS heading only, and while the ship is not moving, the chartplotter cannot determine its true heading.
Suppose the Garmin chartplotter connects to a Garmin Radar and adds the heading sensor. In that case, the chartplotter can now use MARPA (mini-automatic radar plotting aid), which helps to avoid collisions.