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Abstract

In a magnetic compass of the kind in which a sensor derives coordinates of a sensed magnetic field, automatic correction of hard-iron and soft-iron distortions is achieved by storing a set of measured values of the magnitude of the sensed magnetic field for different discrete directions of the alignment axis of the compass relative to an estimated origin. If measured values have been accumulated for more than a predetermined number, but not all, possible directions, values are interpolated for the remaining directions to provide a set of measured and interpolated values defining a substantially elliptical locus of the sensed magnetic field vector (E.sub.T.) New coordinates for the estimated origin are derived and the measured values adjusted to take account of changes in the estimated origin. The heading is corrected for displacement of the origin due to hard-iron effect and for ellipticity of the locus due to soft-iron effects. Correction of soft-iron effect may be achieved by determining the phase rotation angle (.OMEGA..sub.e) of the elliptical locus relative to the alignment axis of the compass; deriving the ratio (E.sub.R /E.sub.L) between a pair of counter-rotating vectors, one (E.sub.L) representing the earth magnetic field component and the other (E.sub.R) soft-iron distortion, their being the sensed magnetic field vector (E.sub.T) and their phase difference .phi..sub.R being twice the rotation angle; and deriving from the ratio (E.sub.R /E.sub.L), the phase difference (.phi..sub.R), and the estimated origin headings corrected for said magnetic field distortion.

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