April 2004, Issue 165

Mini Rover 7
Electronic Compassing fo Mobile Robotics

MAGNETIC COMPASSING

The Earth’s magnetic field is created deep in its iron core by a regenerative magnetic field generator that’s sometimes referred to as the geodynamo. This iron core has a liquid outer section and a solid inner section. The flow of electrical current in the turbulent liquid iron outer section creates the magnetic field.

The simplest description of the Earth’s magnetic field spatial pattern is that of a dipolar field with magnetic flux emanating from the South Pole and converging at the North Pole. The Earth’s magnetic field pattern is a little more complex than a simple bar magnet model. As previously mentioned, the Earth’s geodynamo is constantly moving. Presently, the magnetic poles are tilted about 11° away from the geographical poles, and they are not at exactly at opposite sides of the world either. The magnetic North Pole is located in northeastern Canada, and the magnetic South Pole is located in the Antarctic Ocean south of Australia.

The geographical North Pole is also known as true north. The angular difference between the true poles and the magnetic poles at a given location is called the declination angle. Depending on your location, true north could appear to either the east or west of the magnetic North Pole.

The Earth’s spherically shaped geodynamo produces a magnetic field as shown in Figure 1. Note that the field lines are not horizontal to the Earth’s surface, except at the Earth’s magnetic equator. Unlike the Earth’s straight geographical equator, this one meanders but is located in roughly the same area. At the magnetic poles, the field lines are vertical. The angular vector of the magnetic field with respect to the horizontal plane at any given location is known as the dip angle, or inclination angle. The density of the magnetic field also varies around the world. The magnetic field density is approximately two times as dense at the magnetic poles as it is at the equator.

Figure 1—As you study the Earth’s geodynamo magnet field pattern, note how the field lines are not horizontal to the surface except along the equator. The rotating dynamo and coil in the center represent the Earth’s magnetic field being generated by its ever-flowing iron outer core.

The magnetic field vector is sometimes referred to as having two separate components, a horizontal component and a vertical component. At the magnetic equator where the magnetic field is horizontal, the field has no vertical component. At the magnetic poles, the field is purely vertical and has no horizontal component. At places where the inclination angle is 45°, the horizontal and vertical components are equal.

The U.S. Geological Service (USGS) and the Nation Oceanic and Atmosphere Administration (NOAA) maintain web sites that have global maps and on-line programs that chart declination angles, field intensities, dip angles, and much more. I will focus on the horizontal component of the magnetic field, because that is the portion that contains the heading information that I wish to measure.