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About calibrating a fluxgate compass by making 360-degree turns.

This method presumes that the course of the vessel is changing at a very
steady rate with time. If there was a substantial wind, it would spoil that
approximation.

All the instrument has to go on is its indicated course as a function of time.

First step would presumably be to work out how long it takes for the
indicated course to returm to its original rading. That is then the time
for a 360-deg turn. So now the times can be converted to the course angles
of the vessel in degrees, except there is an unknown offset, because we
don't know the starting angle. If there was a button that you could press
to inform the system when the course was passing through North, that would
provide the unknown offset, but,in general, there's no such button.

The deviation can be represented by-

A + B sin course + C cos course + D sin 2*course + E cos 2*course

(ref. Fanning, steady as she goes, HMSO 1986, appendix, p.435.

A is the steady, unknown error, that the system has no way to measure, a
constant difference all round the horizon. It could be that the fluxgate
sensor has been twisted with respect to the fore-and-aft line of the boat.
That would never be discovered in a turn-in-a-circle correction procedure.
Assume for now that the alignment of the body of the fluxgate unit with the
vessel's fore-and-aft direction is exact.

If the compass has been installed symmetrically on the centreline of the
vessel, with any soft-iron components disposed symmetrically about it,
there can be no other A component. But if those conditions have not been
met, it's possible for an A component to arise from induced magnetism.
This, too, would be undiscoverable from the turn-in-a-circle correction
procedure. This could happen, I suppose, in an unsymmetrical vessel such as
an aircraft carrier.

Rick Emerson states-

>We finally put the fluxgate in a locker about even with the mast and along
>>the port side of the hull but various fittings in the galley and coming
>>from water tanks under the settees caused problems during calibration
>runs.   >It took four tries to make things come out right...

That state of affairs (the compass being mounted off  from the centreline)
would worry me somewhat as it could produce an error that was not
detectable by the calibration procedure. However, it may be an unavoidable
compromise if ferrous components are unsymmetrically acattered around the
inside of the boat.

The swinging procedure for a fluxgate compass should be capable of
correcting for all the coefficients that depend on course and on 2*course,
but not for any A component that doesn't vary with course.

Other than A, all the other components should average zero over 360
degrees. Fanning says that normally the A and E components turn out to be
negligible.
He adds that the B and C components are the result of the permanent
magnetism, and the D and E components are the result of the induced
magnetism.

If the vessel is making a long voyage, there's a good case for
recalibrating after a major change in latitude.

George Huxtable.


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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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