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G1000 compass calibration - how?

An engineer peripherally involved in DA40/42 maintenance tells me that the compass swing is done without the need for an external reference compass.

How could this work?

Apparently there is a first step where you drive the plane through 360 degrees and it tells you if there is metal etc in the ground which would interfere with the calibration. That is really clever but I can see how that works. They have a directional gyro which will know accurately what heading you have swept through in relative terms (it can't know where North is) and they just need to monitor the fluxgate output relative to the heading output from the gyro. If there are big discrepancies during the 360 degree movement then you have steel bars, hangars, etc, in proximity.

But I can't see any way to calibrate the compass (the fluxgate) without a reference compass - unless one assumes the aircraft is not interfering at all in which case it's easy; there is nothing to do...

Administrator
Shoreham EGKA, United Kingdom

I don't think compass calibration in this context means finding out where exactly north is, it essentially means finding the deviation, which in the magnetometer case means finding the bias.

For magnetometer calibration I do something similar. I just rotate the device around all three axes, while collecting 3D magnetometer readings. After that, I try to fit a sphere of unknown radius (due to the unknown field strength at the present location), into the measurement point cloud (in the least squares sense, iteratively). The center of the fitted square is the magnetometer bias, thus the calibration. This seems to work reasonably well.

The "interference detection" could probably be done by monitoring the residual MSE after sphere fitting, but I haven't tried.

So I'd be surprised if the gyro is used at all.

LSZK, Switzerland

Yet, you still need to calibrate the compass, somehow, in an absolute sense.

Administrator
Shoreham EGKA, United Kingdom

Yet, you still need to calibrate the compass, somehow, in an absolute sense.

Do you?

Compass swinging is mostly about compensating the deviation.

I guess the absolute north tolerance should be good enough with the factory calibration of the flux gate and an aircraft that doesn't have "a certain asymmetry in aircraft dimensions" (the euphemism for severely damaged used in the official HBVOV accident report)

What you cannot factory calibrate and has a rather large impact is the sum of all the small local magnetic fields of all the aircraft installations, which is what needs to be compensated by the swinging.

LSZK, Switzerland

I was recently checking my compass and fluxgate and discovered that a GPS can be used in lieu of a compass base. I plotted the centreline of the apron (which is a disused runway) on a map to get a magnetic track. I then taxied along the centreline and was surprised to see that the GNS430 gave a track accurate within 1 deg within just a few meters roll. You can also check the GPS against known runway track when rolling out from landing.

Chose a flat open area (grass probably best as there won't be reinforcing bars buried in it) and track GPS 360deg, check compass and fluxgate, then turn right by 90deg 3 times to get the cardinal points - using the free gyro.

The fluxgate was recently moved during maintenance and needs re-indexing as there are errors.

Your iPhone will ask you to swing the phone like a jedi sword to compensate the compass. That is probably the same mechanism used for fluxgate compensation (that sounds very Star Trek like).

I did wonder what that "flick" was about. My Nokias need the same before the compass works.

Somebody out there must know how this works.

It's possible that the fluxgate is factory-assembled to a sufficient accuracy, and they do make the assumption of no nearby ferrous objects.

One could probably make that assumption in a phone (if the metal case is stainless steel) and I guess Diamond make it in their plane, because the wing is "plastic" and quite long

IF that assumption is made, then I have no problem understanding how it works. But it does have to involve a gyro, otherwise the device has no way of knowing how it is being moved. Just moving a fluxgate around by itself isn't going to yield any useful data, AFAICT.

I just cannot see any way to calibrate a compass (any compass) if there are going to be unknown nearby ferrous objects. It cannot possibly distinguish between those and the earth's magnetic field. It's very simple!

Administrator
Shoreham EGKA, United Kingdom

But Peter, the fluxgate knows the relative position of it in the local magnetic field. Of course it can't know the effect of local magnetic disturbances. A gyro is to keep it more accurate during acceleration etc.

You calibrate it so that it adjusts for anomalies and knows that the front of the plane is the front.

Compasses point North (or at least their local North) without any calibration.

EGTK Oxford

Compasses point North (or at least their local North) without any calibration.

I think that's the key - the local north can be offset by magnetic fields from aircraft structure, electrical systems, cellphones etc...

If you can establish the correct offset from local north to the local magnetic north without any aircraft present then you can derive the rest by measuring the turns on a gyro. I don't see how you can perform that initial calibration without a compass, or some previously calibrated ground markings.

EGEO

I don't see how you can perform that initial calibration without a compass, or some previously calibrated ground markings.

I agree. But some of the earlier messages were somehow implying there is no absolute reference. There is but it can obviously be affected. I think a 360 degree turn allows for those errors to be corrected as they turn with the aircraft whereas the Earth's magnetic field doesn't.

EGTK Oxford
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