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In comments to Is the Garmin G1000's heading indicator based on a Magnetic compass?, it's claimed that the GMU44 magnetometer is not a magnetic compass.

I don't understand this claim, as the device is a magnetometer, which senses the Earth's magnetic field, meaning it's aligned to the magnetic poles rather than geographic poles¹, and it will be affected by local magnetic anomalies (unless stabilised using gyro correction).

Is there some definition of "magnetic compass" that excludes such magnetometers, and if so, how are they categorised?


¹ possibly with corrections in the processor, just as you have offset adjustment with a mechanical compass

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The GMU44 is a magnetic compass. Any device that measures the direction of earth's magnetic field is called compass, including electric ones. There's also the term magnetometer, which is a device that measures the strength of a magnetic field. A multi-axis magnetometer measures strength and direction, which means you can use it in both roles. In the G1000, the GMU44 is used in the compass role.

That said, the heading display in a G1000 is primarily a gyroscopic instrument, but slaved to a magnetic compass (the GMU44) to prevent heading drift.

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    $\begingroup$ Do you have a source for your last statement? I think you are definitly right but never found a description of this. $\endgroup$
    – U_flow
    Mar 14, 2023 at 9:33
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As I explained in my previous answer, the magnetometer can detect its orientation in the Earth’s magnetic field by measuring the electrical signals from the pickup coils in a flux gate. A magnetic compass makes use of a bar magnet, generally suspended in a liquid such as kerosene, and attached to a card which is referenced to a fixed lubber line on the casing of the compass. The bar magnet swings freely, and so naturally aligns itself with the magnetic field of the earth. A user can reference the compass card in reference to the lubber line and deduce magnetic heading based on this.

Magnetic compasses should not be confused with a gyro compass or a heading indicator. These are independent units which maintain a preset zeroed indication using a gyroscope and must be periodically aligned with a reference, such as a magnetic compass, to compensate for accumulated deviations due to gyroscopic precession.

Of the two, a magnetometer is a little more precise, and that it is not subject to lead/lag or compass swing errors during turning and acceleration and less affected by magnetic deviation as the flux gate can be placed well away from electronic equipment. Conversely, a magnetic compass does not require electric power to operate.

And technically speaking, a magnetometer isn’t even a compass at all. It’s just a piece of equipment used by an integrated flight deck or conventional HSI (which are also NOT magnetic compasses) to provide directional orientation with magnetic north.

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    $\begingroup$ You've just describe the difference between a mechanical and electronic compass - which are both magnetic compasses (unlike the gyro unit, which is not). $\endgroup$ Mar 14, 2023 at 13:11
  • $\begingroup$ Think you need to go back and re-read the definition of what a compass is. $\endgroup$ Mar 14, 2023 at 22:09
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    $\begingroup$ A compass is a thing that tells you what direction is "north". Doesn't matter if it uses a gyroscope, coiled wires, or a cart with cleverly-designed gearing. $\endgroup$
    – Mark
    Mar 15, 2023 at 1:25

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