A5023/24
10 of 19
4
Calibration
4.1 Why the compass needs calibrating
When any compass is installed the magnetic characteristics of the whole
installation affect the way the Earth’s field reaches the detector inside the
device. Each individual installation will be different and so the A5023 and
A5024 are equipped with an auto-calibration routine which corrects for
these installation distortions.
4.2 Starting calibration
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A NMEA-0183 command can be sent to initiate the process
–
section 5.2.9
OR
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Switch 1 can be pressed.
There are two compass calibration techniques in common use:
1
Constant angular velocity
2
Double rotation without constant velocity.
The Autonnic compasses use the first method. The process consists of
rotating the whole installation in the Earth’s field so that both permanent
magnets and induced magnets are corrected. The method used is the
single turn at a constant rate and this rate should be in range 60 to 180
seconds for the full 400 degrees needed. The whole arrangement is turned
at constant angular velocity because the method relies on applying a
correction when the heading measured is uneven with time.
The second method is not used by us. Autonnic has evaluated the method
which uses a mathematical model of an ideal compass and treats errors as
arising from distortion due to magnetic objects. The ideal error plot of the
field vector would be a circle with no offset from the origin. Permanent
magnets (‘hard’) have the effect of shifting the centre from the origin and
induced magnets (‘soft’) modify the circle into an ellipse. The results of
