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Philips Semiconductors
Electronic Compass Design using
KMZ51 and KMZ52
2.
EARTH´S MAGNETIC FIELD
The magnetic field of the earth is the physical quantity to be evaluated by a compass. Thus, an understanding of
its basic properties is required, when designing a compass. Figure 1 gives an illustration of the field shape.
The magnetic field strength on the earth varies with location and covers the range from about 20 to 50 A/m. An
understanding of the earth´s field shape can be gained, if it is assumed to be generated by a bar magnet within
the earth, as pointed out in Figure 1. The magnetic field lines point from the earth´s south pole to its north pole.
Fig. 1 indicates, that this is opposite to the physical convention for the poles of a bar magnet (the background is
a historical one, in that a bar magnet´s north pole has been defined as that pole, that points towards north in the
earth´s magnetic field). The field lines are perpendicular to the earth surface at the poles and parallel at the
equator. Thus, the earth field points downwards in the northern hemisphere and upwards in the southern
hemisphere. An important fact is, that the magnetic poles do not coincide with the geographical poles, which are
defined by the earth´s axis of rotation. The angle between the magnetic and the rotation axis is about 11.5°. As
a consequence, the magnetic field lines do not exactly point to geographic or "true" north.
Figure 2 gives a 3-D representation of the earth field vector He at some point on the earth. This illustration
allows to define the quantities, which are of importance for a compass. Here, the x- and y-coordinates are
parallel to the earth´s surface, whereas the z-coordinate points vertically downwards.
•
Azimuth α
The angle between magnetic north and the heading direction. Magnetic north is the direction of Heh, the
earth´s field component perpendicular to gravity. Throughout this paper, Heh will be referred to as
"horizontal" component of the earth´s field. Figure 2 shows, that:
Hey
α
=
arctan
Hex
Magnetic
North
True
South
Figure 1
Earth´s magnetic field
(1)
True
North
Magnetic
South
8
Application Note
AN00022
EARTHFLD.GIF
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