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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Automatic deviation calculation by electronic compasses
From: Antoine Cou�tte
Date: 2009 Dec 11, 08:11 -0800

Dear Joseph,

Thanks for your nice comments which have been reviewed from (well) Above and
have been subsequently approved and most appreciated !

Since you are quite familiar with Mathematics as applied to Physics, and since
we (supposedly) keep instructing ourselves about FLUX GATES and/or related
items, I will offer at last one more development here since we are starting
going off track.

Sorry for the (simple) math hereafter (last time !! promised again !) But I
could not resist it for so many days !

It concerns FLUX GATES as mounted on Aircraft (and probably on Ships too).
THESE ARE EXTREMELY CLEVER DEVICES, which puzzled me when first learning
about them for my ATPL exams.

Simply stated, they work as follows.

An Aircraft pure iron sensor is maintained horizontal and always aligned with
the Aircraft (Ship ?) Centerline.

With :

- "Mu" being the magnetic permeability of the iron rod, and

- "H" (oriented vector) the Horizontal Component of the Earth Magnetic Field, and

- "Mh" (oriented vector) Magnetic Heading of the iron rod direction

- "B" (oriented vector also) the Earth induced Magnetic Induction in
the Rod (i.e. the "flow rate" of H through the rod)

We have the following simple equation :

B = Mu H cos(Mh)

B itself is not detectable per se with just such a device.

However the first derivative of B with respect to time, i.e. "d/dt B", is
easily detectable since it immediately translates into a electric voltage
"E", as per the well known formula :

E = k d/dt B (with k being a constant)

The NICE TRICK here is that the FLUX GATES inventor has not forgotten that B
is a vector. Accordingly its derivative - and accordingly E - is the result
of both any change in its direction (which is the Magnetic Heading of B), or
any (possibly achievable ??? ) change in its intensity.

Can we possibly change anything regarding B, without any undesirable or even unsurmountable effect(s) ?

WE CANNOT AND MUST NOT CHANGE THE DIRECTION OF B, since we are looking for its
direction itself which is (closely related to) the A/C Magnetic Heading.

Change what then ???

- H cannot be changed, obviously ...

- only would remain a (technically and adequately feasible ?) change in Mu.

The clever physical implementation of this pure mathematical idea is that the
system constantly modifies the Magnetic Permeability "Mu" of the iron rod
through alternative currents around it arranged in such a way that they are
"magnetically undetectable" outside the Flux Gate in order to not perturb all
other Aircraft Avionic systems.

Just vary the Magnetic permeability of an iron rod, and you can get an
accurate reading of its magnetic heading ... Well done and congratulations to
whoever had both this great idea and could accomplish its very delicate
electrical implementation !!!

That's all Folks !

Antoine M. Couette

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