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    Re: Automatic deviation calculation by electronic compasses
    From: George Huxtable
    Date: 2009 Nov 23, 11:25 -0000

    Thanks to Lu for sending a copy of the instruction manual for the KVH
    electronic compass. That contained a couple of surprises (to me), about its
    self-compensation ability. One was that it didn't care whether the circles
    that it required the vessel to make during the compensation procedure were
    indeed circles, and ellipses were acceptable. Another surprise was that if
    it discovered the vessel making a 360-degree turn over a prescribed period
    of time (unspecified) it would perform a new compensation procedure, quite
    In [10794]. I had written- "It's always possible that I've misunderstood the
    technology, and the makers have some clever trick up their sleeve that I've
    missed. If so, no doubt some list member will put me right."
    After further thought, I've now convinced myself that the makers are,
    indeed, using another trick, and that the compensation procedure does NOT
    depend on precise rate-of-change of heading, to deternine the errors. And
    that I had, indeed, misunderstood the technology. In which case, my
    objections, expressed in earlier postings, would be quite invalid.
    Let me try out this argument on listmembers. From its manual, it seems that
    the KVH compass has a traditional gimbal mechanism, on which floats a
    sensor, which can measure two components of the horizontal magnetic field.
    Presumably, these will be the horizontal component along the vessel's
    heading line, taking ahead as positive, and the horizontal component athwart
    that direction choosing (arbitrarily) to port, say, as positive.
    From the ratio of those two components, it can work out the angle between
    the vessel's heading and the total horizontal field, the magnetic heading,
    just as a traditional compass does. And from the vector-sum of those
    components, it can work out the total magnitude of the horizontal field,
    which a traditional compass cannot do.
    The total horizontal field is made up of-
    1.  The Earth's horizontal field, which varies with place and (slowly) with
    time, but can be taken as constant at a particular place and time.
    2. The magnetic field caused by magnetised material in the vessel and its
    contents. This arises from two sources-
        2a. Permanently magnetised material, giving rise to a constant magnetic
    field which is "attached" to the boat, and turns as the boat turns. The word
    "hard" magnetisation is often attached to this.
        2b. Magnetism induced in the steel of the vessel by the Earth's magnetic
    field, which is mostly along the fore-and-aft axis of the vessel, but
    alters, in magnitude and relative direction, as the heading changes,
    someimes attributed to "soft" steel. This is a real bugbear of steel
    vessels, which require special compensation, but not so much for others, and
    to simplify matters we will pretend that it doesn't exist here, and the only
    correction is that of 2a..
    As the boat turns through 360 degrees, the total horizontal field, then,
    will be the constant field of the Earth, combined with the rotating
    deviation-field of the boat. When those two point in the same direction, and
    so add, the total field will be greatest. When the heading changes by 180
    degrees, they will subtract., and the total field will be least. If the boat
    had no magnetism, so zero magnetic deviation, that would correspond to the
    total magnetic field being equal at all headings.
    So, by analysing the changing magnitude of total horizontal field, as the
    vessel turns, the amount and direction of magnetic perturbation (the
    deviation) can be assessed. There is no requirement for constant
    rate-of-change, all that's needed is for all possible angles to be explored
    sufficiently slowly to allow disturbances such as waves to be filtered out.
    Having worked out that deviation vector, its components can then be used to
    correct the total ahead and athwart components, by subtraction, before
    finding the heading from their ratio.
    It now seems likely, to me, that that's the way the job is done, and wish to
    withdraw my earlier adverse comments about the procedure.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
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