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    Re: Automatic deviation calculation by electronic compasses
    From: George Huxtable
    Date: 2009 Nov 22, 23:05 -0000

    Lu wrote-
    
    "First of all, I have no personal experience with auto-correcting electronic
    compasses, so what I write is hearsay. "
    
    Well, that makes two of us, then. Perhaps a list member will join in who has
    that experience, who can tell us if makers of these instruments emphasise
    any precautions in how the auto-calibration should be done.
    
    And "In fact, isn't that the concept of a "harbor" where one is sheltered
    from the wind and weather?". No, not in my view. It's where shelter can be
    found from waves, to provide a secure berth. Some harbours also offer
    shelter from wind, that's true.
    
    And "So I don't see making a 720-degree turn at a constant angular velocity
    to be anywhere near the problem you fear it to be."
    
    Well, how demanding are the requirements for the constancy of that
    rate-of-turn? Let me offer my own estimates.
    
    Let's take a vessel which actually has zero deviation at its compass (though
    nobody is aware if that), and put it through such a procedure.
    
    Let's propose that we choose a rate-of-turn of 60 degrees per minute of
    time, so allowing 6 minutes for a complete turn. It doesn't matter whether
    the rate-of-turn is exactly that value, as long as it's constant, but it
    helps to put some round-numbers to the problem.
    
    Then, if that rate is held throughout, the heading will change with time
    just as expected, at 1 degree for each second of time, and the device will
    deduce, correctly, that there's no correction required for deviation.
    
    But now, let's assume that some external factor (most likely, a bit of a
    breeze) causes that rate of change to differ, as the circles are rounded.
    Not by much, by just one part in 60, say, in a cyclic manner. Say the
    rate-of-turn is increased, when heading North, to 61 degrees per minute, and
    when heading South, reduced to 59 degrees per minute, varying sinusoidally
    as the vessel makes its turns. It could be the result of changing speed of
    the vessel, or changing radius of curvature, or both acting together. The
    compass doesn't know that there is such a change, and all it can do is to
    introduce an entirely spurious correction for presumed deviation to null it
    out. And my estimate is that the resulting "correction" will amount to 1
    degree clockwise at one heading, 1 degree anticlockwise at the opposite
    heading. Does that seem right?
    
    Those resulting errors would then become built-in to the compass,
    misaligning its readings by up to 1 degree either way, until the next time
    such a correction procedure is run. It doesn't matter what rate of turn is
    chosen, variations by 1 part in 60 will give rise to compass errors of 1
    degree.
    
    I suggest that such an inbuilt error, of amplitude 1 degree, is the maximum
    any compass-user would put up with. So I ask Lu if he would be happy to
    guarantee that when his boat makes such circles, his rate of turn is exactly
    constant to within 1 part in 60? That's a very demanding requirement, in
    practice. Have any tests been made of the achievable constancy of that
    rate-of-turn under various trial conditions?
    
    Do compass makers put any emphasis into their instructions to users, about
    the importance of choosing the right conditions under which such
    calibrations should be made?
    
    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.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
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