fluxgate compensation (that sounds very Star Trek like)
I thought this was ‘Back to the future’.
I have just heard this from another pilot:
I think it’s brilliant 
That's a brilliant post, Tom.
Sphere fitting software... I recall fitting a polynomial to some points, but fitting a sphere is serious maths.
Now... where were you when we were discussing in-flight compass calibration using a GPS?
Let's denote the 3D vector of the earth magnetic field with BM, and the local magnetic field generated by all aircraft objects (at the location of the magnetometer) with BL.
The projection of BM to the north-east plane points north.
When you rotate the aircraft around all three axes and record the measurement of the magnetometer, and have an aircraft with BL=0, you get a sphere around the origin, whose radius is the total magnitude of the earth magnetic field at the given point on the earth.
Now if you have a BL!=0, the magnetometer measures BL+BM. Thus, you get a sphere around BL, since BL rotates with the aircraft. Thus, the sphere fitting procedure I described above will find BL, and thus the calibration.
It doesn't work if you have local disturbances that do not rotate with the aircraft, though.
But see, you don't need to know how fast you rotate the aircraft.
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.
Exactly, but how does the system which is being rotated know the angle through which it is being rotated?
The fluxgate itself cannot tell the difference between (a) the field being bent on a particular bearing and (b) the speed of the rotation changing as you pass through that bearing.
But with a directional gyro, the system can work it out because it knows the relative heading.
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.
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.
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.
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!
