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The direction of the measured Earth's magnetic field relative to the orientation of the device is used as a (3D) compass. This direction is used as an absolute reference in the MT's orientation estimation algorithms (heading or yaw).
A locally disturbed (warped) magnetic field causes an error in orientation that can be quite substantial. Ferromagnetic materials, permanent magnets or very strong currents (several amperes) alter the local Earth's magnetic field. Whether an object is ferromagnetic should preferably be checked by using the MT's magnetometers. It can also be checked with a small magnet, but be careful, you can easily magnetize some ferromagnetic materials, causing even larger errors. If you find that some object is magnetized (hard iron effect, this is often the case with, for example, stainless steels that are normally not magnetic), it may be possible to 'degauss' the object.
The MT contains the absolute possible minimum of ferromagnetic materials ('hard' and 'soft' magnetic materials). Nonetheless, some minor components can be magnetized permanently by exposure to strong magnetic fields. This will not damage the unit but will render the calibration of the magnetometers useless, typically observed as a (large) deviation in heading. For mild magnetization, it may be possible to compensate for the magnetization of the device by a re-calibration (magnetic field mapping). Taking care not to expose the MT to strong magnetic fields, such as close proximity of permanent magnets, speakers, electromotors, etc. will prevent magnetization.
In practice, the distance to the object and the amount of ferromagnetic material determines the magnitude of disturbance.
Distortions of the Earth's magnetic field can be divided into two kinds of effects:
Temporal or Spatial disturbances. These disturbances can be caused by objects in the environment of the motion tracker, like file cabinets or vehicles that move independently with respect to the MT. This type of disturbance is non-deterministic, and cannot be fully compensated for. However, the error caused by the disturbance can be reduced by optimally using the available sensor information and valid assumptions about the application. This is the task of the Xsens Kalman Filter (XKF) or Xsens Estimation Engine (XEE) running inside the on-board processor of the MT.
Static disturbances. These disturbances are often caused by mounting the MT to an object of which the motion is to be recorded (the MT moves with the object). The error in the magnetic field is constant, and can therefore be predicted (i.e., it is deterministic) and taken into account during motion tracking. By mapping the disturbance (warping) of the magnetic field, the errors caused by this type of disturbance can in theory be reduced to zero. The calculations and methodology required to achieve this are supplied by this Magnetic Calibration Manual. This type of correction is also commonly known as compensation for hard and soft iron effects.
Disturbances of the first type cannot be calibrated for. Instead, you may want to consider the use of Active Heading Stabilization (AHS) as an alternative. Refer to the following article for more information: Active Heading Stabilization (AHS)
Disturbances of the second type can be calibrated for. This document describes three tools that can be used to achieve this:
Magnetic Field Mapper (MFM): This is the most powerful and therefore preferred calibration procedure.
Representative Motion (RepMo): An embedded solution that can be used to achieve a fast and effective magnetic calibration in situations where a connection with a host PC is not possible.
In-Run Compass Calibration (ICC): An embedded solution that can perform a continuous on-board magnetic calibration.
The figure below presents a flow chart that can help you decide which tool to use.
Flowchart for tackling magnetic distortions.
In a non-disturbed magnetic field, we define the 3D measured magnetic field vector 
to have a magnitude (magnetic norm) 
equal to 1 and, therefore, all measured points would ideally lie on the circumference of a 3-dimensional sphere with its centre at zero. See the figure below. In the case of a disturbed magnetic field, this sphere is both shifted and warped. The calibration procedures described in this document aim to derive a function that maps the measured magnetic field vector to a sphere, and the magnetic norm to 1. See the figure below. This function is then used to create new magnetometer calibration parameters, which are stored in non-volatile memory in your MT.
The 3D magnetic field measurements 
mapped onto the surface of a sphere with its centre at 
.
The norm of the magnetic field vector before (red) and after (blue) compensation using the Magnetic Field Mapper. The black dots represent the samples used by the MFM algorithm.