In a fixed installation, your LD-250 antenna points North. In a mobile installation (car, truck, boat, etc) you need a compass or GPS to tell the LD-250 what direction your antenna is pointed.
In a boat or other specialised application where the direction of motion is not always forward a compass is required.
LDCMP-1 Fluxgate Compass comes prewired and ready to plug into your LD-250