19
Calibrating the Compass
Compasses operate by measuring the earth’s magnetic field and locating magnetic
north. When mounted in real platforms or devices there are other local magnetic fields
or materials that disrupt or offset the earth’s magnetic field. Sources to be aware of are
ferrous metal, electric currents and permanent magnets in electric motors. There are
two sources of magnetic field distortion: “Soft-iron” and “Hard-iron”. “Soft-iron”
effects are caused by un-magnetized ferrous materials in the area of the compass.
“Hard-Iron” effects are caused by magnetized ferrous materials near the compass.
“Hard-Iron” can be cancelled out by going through the rotational calibration
(described bellow). “Soft-iron” effects should be addressed during the design in phase
and compasses should be located away from such items to minimize impact to the
sensor. OceanServer compasses with firmware version V1.4 and later include a soft
iron calibration option.
Magnetic Field Examples:
Undisturbed Magnetic
Field
Compass Placement
Non-Ferrous Bow
Least Magnetic
Field Disturbance
Best Place for
Compass
Placement
Ferrous Material in Tail
Plus an Electric Motor
Creating Magnetic
Field Disturbance
Not an Ideal Place for
Compass Placement
