An electronic compass, such as the ones made by Brunton Compasses, uses a patented magnetic sensor technology that was first developed by PNI, Inc. for the U.S. military. This technology is called " magneto-inductive" and is the largest advancement in compass technology since the fulxgate was invented 60 years ago. The magneto-inductive technology is able to electronically sense the difference in the earth's magnetic field from a disturbance caused by external elements such as ferro-magnetic materials and the magnetic field generated by automobile electrical systems. Most digital compasses have an embedded micro controller that subtracts the automobile magnetic field (the distortion) from the stronger earth magnetic fields resulting in a highly accurate compass reading.
The performance of a compass will greatly depend on its installation location. A compass relies on the earthâ€™s magnetic field to provide heading. Any distortions of earth's magnetic field by other sources such as a car massive iron components should be compensated for in order to determine an accurate heading. Sources of magnetic fields in any automobile include permanent magnets mostly in its audio speakers, motors, electric currents flowing in its wiringâ€”either dc or ac, and ferro-magnetic metals such as steel or iron. The influence of these sources of interference on an electronic compass accuracy can be greatly reduced by placing the compass far away from them. Some of the field effects can be compensated by way of calibrating the compass for a defined location in terms of magnetic interference. However, it is not always possible to compensate for time varying magnetic fields; for example, disturbances generated by the motion of magnetic metals, or unpredictable electrical current in a nearby power lines. Magnetic shielding can be used for large field disturbances from motors or audio speakers. The best way to reduce disturbances is distance. Also, never enclose the compass in a magnetically shielded metallic housing.
Each calibration method is associated with a specified physical movement of the compass platform in order to sample the magnetic space surrounding the compass. The Hard and Soft iron distortions will vary from location to location within the same platform. The compass has to be mounted permanently to its platform to get a valid calibration. A particular calibration is only valid for that location of the compass. If the compass is re-oriented in the same location, then a new calibration is required. It is possible to use a compass without any calibration if the need is only for repeatability and not accuracy.
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