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Compassing Using Magnetic Sensors
People have used Earth's magnetic field for centuries as a navigational aid. The earliest compasses were simply needle
shaped pieces of naturally occurring lodestone floating in a bowl of water. How does compassing work today?
A more modern approach utilizes magnetometers to measure the magnetic
field directly. This application note will
discuss various approaches to compassing using magnetic sensors and their suitability
to real world applications. We will not discuss
magnetic sensor technology (the various types of sensors), but rather a systems
level analysis of magnetic compassing schemes.
Earth's Magnetic Field
The center of the earth generates a magnetic field, making the earth into a huge
bar magnet. The magnetic field flows out
of the magnetic South Pole, loops around the earth, and flows back in through the
north magnetic pole. The magnetic field is
a three dimensional structure. The magnetic
field will have a vertical component, commonly referred to as V, and a horizontal
component, H. The horizontal component is
what we use for compassing; it is the horizontal component of earth's magnetic field
that points to North. The vertical component
just complicates things. The angle the magnetic
field makes with horizontal is called the dip angle.
The dip angle gets larger as you get closer to the magnetic poles.
The larger the dip angle, the smaller the horizontal field, which makes compassing
more difficult. Compassing is easiest near
the equator where the dip angle is small.
Magnetic compassing gets progressively more difficult as you go higher in latitude.
 
Remember that the magnetic field points to the
magnetic North Pole. The magnetic
north pole is different from the true
North Pole. The true North and South Poles
are defined by the axis about which the earth spins.
The magnetic poles are defined as being where the magnetic field is vertical. The magnetic poles and true poles happen to
be different in different places. So when
you point to the magnetic North Pole, you could be pointing in a different direction
than if you were pointing to the true North Pole.
The difference between magnetic North and true North is called declination. The declination will vary depending on your
position on the earth. Fortunately for us,
people have mapped out these differences, and many references exist to tell you
your declination as a function of position.
For the rest of this note, when we refer to North, we should understand we are really
talking about magnetic north.
size="+1">Two-Axis Magnetometer
The simplest compass instrument contains a two-axis magnetometer (or two single-axis
magnetometers) designed to measure earth's horizontal magnetic field.
Let's call the two axes X and Y. We'll
arrange them so that X is pointing "forward" and Y points to the left.
Let's keep the sensors level so that we are only measuring the horizontal
magnetic field, H. Now if we are facing magnetic
north, we will measure +H in the X axis and 0 in the Y axis.
Now if we turn 90?/span>
to our right, we are facing east; we measure 0 in the X axis and +H in the Y axis.
Another 90?/span>
to the right, and we are facing south; X measures ?, Y measures 0.
Finally, west; X measures 0, Y measures -H.
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