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    Re: Automatic deviation calculation by electronic compasses
    From: Lu Abel
    Date: 2009 Nov 22, 17:31 -0800

    I can understand your concern about the influence of wind on rate of
    turn; at the same time I've been out often (and frustratingly) enough
    when there hasn't been wind on the water so I think there would be a lot
    of other factors that would enter my mind around allowing a digital
    compass on my boat well before "can I find a calm spot to make my
    compensating rotations?"
    KVH is a leader in electronic compasses, here's a link to the manual on
    one of their compasses.   https://www.kvh.com/pdf/540042F_Azi1000.pdf
    Instructions on the auto-compensation feature start on page 15.
    They require a circle taking at least two minutes, so a fairly slow one,
    but nowhere near as slow as the one you used (quite legitimately) in
    your back-of-the-envelope calculations.   I rather suspect that the more
    quickly one turns, the more the boat's angular momentum compensates for
    any heading-dependent influences on its rate of turn.
    Besides some fairly complete instructions on auto-compensating, I was
    pleasantly surprised (although in retrospect I shouldn't have been
    surprised) that the compass allows one to electronically adjust the
    "lubber line" to the axis of the boat.   So no need to fiddle with
    aligning the sensor.  I also note that the compass can compensate for
    "soft iron" errors as well as "hard iron" errors (for those unfamiliar,
    the latter are the ones that create the sinusoidal pattern in a plot of
    deviation vs heading; the former cause lumps and bumps in the sinusoid
    (harmonics, to those familiar with Fourier series))
    KVH makes a number for compass products for marine use.
    Their digital hand-bearing compass has received rave reviews.   One can,
    for example, shoot a number of "sights" from a pitching boat and then
    have the compass average them.   It's also blindingly expensive.
    KVH also makes servo-controlled satellite TV dishes that one can install
    on an RV or even a boat and watch live TV while pitching and turning
    down a road or the waves.   I see them on a lot of high-end boats
    (although I personally can't understand why anyone would want to watch
    TV with the splendor of the sea surrounding them, guess that's also why
    I'll never be a passenger on something like that new cruise ship).
    George Huxtable wrote:
    > 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@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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