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Interpreting the Calibration Score
The TCM2.5/2.6 provides feedback on calibration through the calibration score, which has the
following format: “...HnVnMn.n....”
The first two numbers in the calibration score, HnVn, respectively describe the quality of the
calibration for the horizontal component and vertical component of the host system’s local mag-
netic field. Higher numbers reflect higher quality. The highest possible score is a “9”. The factors
that contribute to a good score for Hn and Vn are as follows:
•
a good, magnetically quiet location was chosen for the user calibration procedure.
•
the magnetic environment is stable; there are no large sources of changing fields.
•
the calibration data points included changes in system heading and inclination to allow for
proper measurement of the horizontal and vertical field vectors.
•
there are no significant soft-iron distortion effects.
The last number in the score, Mn.n, describes the magnitude of local field. Larger numbers
denote strong local fields. Small local fields are preferable, since less correction will be necessary,
and they utilize less of the magnetometers’ dynamic range. A magnitude score greater than 30
indicates strong magnetic fields at the TCM2.5/2.6 location; you should consider alternative
mounting locations. Any score less than 10 is very good.
Note:
The calibration score values mostly provide a qualitative estimation. For example, a good
score would be H9V9Mn.n. A poor score as anything less than H9V9Mn.n. A poor V score gen-
erally indicates that you need to tilt more during the calibration. A poor H score indicates you did
not turn two full circles or that you turned too quickly.
Soft Iron Effects
The TCM2.5/2.6 can calibrate for hard iron effects, or local fields that can be modeled as static
fields such as those created by permanent magnets. Hard iron distortions are significant in most
systems. There is another class of iron effects, soft iron, that are an amplification of magnetic
fields created by highly permeable materials, such as ferrous metals. The TCM2.5/2.6 does not
compensate for soft iron effects. Soft iron effects, however, are generally far weaker than hard
iron effects in most systems, and can be more readily defeated by choosing a suitable location to
mount your compass module.
In some systems, however, it may be difficult to avoid large masses of ferrous metal that may
create non-trivial soft iron effects, such as an armor plate in a tracked vehicle. In these instances,
try to locate the module as far away from the ferrous metals as possible. Soft iron effects
decrease with distance by an inverse square relation so even modest separation can be effective.
Other Limitations
As discussed, the TCM2.5/2.6 models local disturbances as a static magnetic vector contribution
to the earth’s field. Any local fields, which are not static, will create errors. You cannot calibrate
for anomalies that are not fixed with respect to the compass. For example, you may know that the
TCM2.5/2.6 will be used in close proximity to other vehicles. You cannot calibrate for the effects
of these other vehicles, as they will be moving with respect to the TCM2.5/2.6. This is a limitation
universal to all compasses. Consider, therefore, the TCM2.5/2.6’s position relative to any
potential sources of field that will not be static: magnetic cargo or payloads that may be placed in
close proximity, fans or other electrical equipment that may be turned on and off, and so on.
The TCM2.5/2.6 can calibrate for any environment that creates a magnetic field that does
not exceed the dynamic range of its magnetometers.
