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 A5022 is equipped with an auto-calibration routine which
corrects for these installation distortions.
4.2 Starting calibration
-A NMEA-0183 command can be sent to initiate the process.
OR
-Switch 1 can be pressed to start it.
There are two compass calibration techniques in common use:
1 Constant angular velocity
2 Double rotation without constant velocity.
The A5022 uses 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 in the A5022. Astra Yacht 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 our investigation show that there is always a residual error due to the fact that
real magnetic objects do not behave exactly according to the model outlined. Astra Yacht
is aware that the need for a constant angular velocity is onerous but the benefit is that it
provides a very low residual error – less than 0.5°.
Note that the calibration is different if the unit is mounted upside-down.
Calibration must always be done with the unit in its final mounted position. The essential point
of the calibration is remove the influence of external magnetic components in the vicinity
– the nearby pieces of iron and steel –so it is essential that these remain unchanged after
calibration. If any significant iron component is altered – such as the installation of new
equipment or replacement of equipment such as winches or engines – then the
calibration must be re-done.
