PNI SeaTRAX - Manual (Page 17)

PNI Sensor Corporation DOC#1018154 r02
SeaTRAX User Manual Page 12
5.1 Magnetic Calibration
Two fundamental types of magnetic distortion exist, hard-iron distortion and soft iron
distortion. These are discussed in the following two paragraphs, plus a discussion on how
temperature also effects magnetic distortions. For more information on magnetic distortion
and calibration, see PNI’s white paper “Local Magnetic Distortion Effects on 3-Axis
Compassing” at PNI’s website (
http://www.pnicorp.com/technology/papers ).
Hard-iron Effects
Hard-iron distortions are caused by permanent magnets and magnetized objects in
close proximity to the sensors. These distortions add or subtract a fixed component to
each axis of the magnetic field reading. Hard-iron distortions usually are unchanging
and in a constant location relative to the sensors, for all heading orientations.
Soft Iron Effects
Magnetically “soft” materials effectively bend the magnetic field near them. These
materials have a high magnetic permeability, meaning they easily serve as a path for
magnetic field lines. Unlike hard-iron effects, soft-iron effects do not increase or
decrease the total field in the area. However, the effect of the soft-iron distortion
changes as the host system’s orientation changes. Because of this, it is more difficult
to compensate for soft-iron materials.
Temperature Effects
While the hard-iron and soft-iron distortion of a system may remain quite stable over
time, normally the distortion signature will change over temperature. As a general
rule, the hard-iron component will change 1% per 10°C temperature change. Exactly
how this affects heading depends on several factors, most notably the hard-iron
component of the system and the inclination, or dip angle.
Consider the example of a host system with a 100 µT hard-iron component. This is a
fairly large hard-iron component, but not completely uncommon. A 10°C
temperature change will alter the magnetic field by ~1 µT in the direction of the hard-
iron component. Around San Francisco, with an inclination of ~60°, this results in up
to a couple of degrees of heading change over 10°C.
Consequently, no matter how stable a compass is over temperature, it is wise to
recalibrate over temperature since the magnetic signature of the host system will
change over temperature. The SeaTRAX helps accommodate this issue by allowing
the user to save up to 8 sets of magnetic calibration coefficient sets, so different
calibration coefficients can be generated and loaded at different temperatures.