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How does the heading indicator displayed on the PFD of the G1000 work?

A conventional cockpit uses a magnetic compass to correct the heading indicator.

However, I don't know if the heading indicator on the monitor of the G1000 detects and measures the magnetism itself and displays it on the monitor, or if there is another principle.

Of course, I know that the GMU-44 LRU is equipped, but I wonder if it is measured and output as PFD or if there is another principle.

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The Horizontal Situation Indicator (HSI) rosette display on a G1000 works in a similar fashion to a conventional HSI operating in slave mode. It is slaved to a 3 axis magnetometer which gives it heading cues. A magnetometer makes use of a flux gate or flux valve consisting of a 3 part iron ring fitted with 3 pickup coils on the arms and a single excitation coil around the core.

enter image description here

The excitation coil is energized with an alternating current. When this AC current reverses direction, it allows the pickup coils to sense the magnetic flux of the earth’s magnetic field. How intense the flux through each of the pickup coils is is determined by how the ring is aligned with the Earth’s magnetic field. See image below.

enter image description here

Therefore, the net electrical inputs from these pickup coils can be interpreted via either an analog or digital computer and related to the direction of the aircraft relative to the Earth’s magnetic field.

Flux gates are typically mounted in the wing of an aircraft near the tip well away from avionics or other electrical systems to minimize risk of magnetic interference and consequently heading errors from magnetic deviation. When installed and calibrated correctly, the magnetometer should indicate a heading either on or very, very close to what is indicated on a free moving magnetic compass. This should be verified during preflight and taxi checks that these two navigation devices operate in harmony.

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    $\begingroup$ TL;DR: yes, it's a magnetic compass. $\endgroup$
    – Toby Speight
    Mar 13 at 9:00
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    $\begingroup$ Negative. It is not a magnetic compass. The equipment and functioning are different between the two items. $\endgroup$
    – Romeo_4808N
    Mar 13 at 13:18
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    $\begingroup$ But you've just said that it reads the surrounding magnetic field. That makes it a magnetic compass (an electronic one, rather than mechanical, of course). It's clearly not a gyro. $\endgroup$
    – Toby Speight
    Mar 13 at 14:54
  • $\begingroup$ Not exactly. That’s not how a magnetic compass works either. $\endgroup$
    – Romeo_4808N
    Mar 13 at 18:58
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    $\begingroup$ Okay, I'm clearly missing something, so I've extracted the disagreement into a question of its own. $\endgroup$
    – Toby Speight
    Mar 14 at 8:25
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A good way to figure this out is to look up the annunciation modes of the Garmin display. Here's a Cessna-specific version of the G1000 that I grabbed.

enter image description here

This page shows the red X will happen from the GMU44 magnetometer or from the GRS77 AHRS. The latter uses a gyro (basically). Both the AHRS and magnetometer are used to initialize the heading.

As seen in the appendix, the '77 uses GPS for a backup operation. So there are failure modes for magnetometer, gyro, and GPS.

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  • $\begingroup$ One reason they use the AHRS like you've indicated is to prevent short-term errors, whether from nearby ferrous anomalies like the runway at London City Airport, or from trying to keep up with a rapidly turning aircraft. $\endgroup$
    – Cody P
    Mar 14 at 7:37

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