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1/6 of a degree. At the same time the FG1 would be also sending the NMEA data
to a precision of 1/10 of a degree.
Clock
Data
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 FG1 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.
-
O R
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 FG1 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 FG1. Technician 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
Duration of data is x16 clocks each
480µs
high=low=30 µs. Total = 480µs.
100ms minimum
Fig 4
7 of 12
Frequency of data packets is set by
the interval command. Ma = 10Hz
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