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    Re: Deviation Card with GPS
    From: George Huxtable
    Date: 2006 Jul 30, 03:59 -0500

    Gary LaPook asked [993]-|

    | My car has an electronic compass installed and the directions for
    | correcting it for deviation (of course the car manual didn't use
    | technical terms) has you pushing a calibration button and then
    | the car in a slow circle. How does the compass figure out the
    | from just the data it can capture while the car is driven in a

    and Red responded [994]-

    "I am guessing that the compass reads deviation versus time (the rate
    of turn
    must be constant) and then it "remembers" the min/max and when the
    repeates itself. Something in the pattern must be perceived as "most
    north" and
    from there, you just need to note the other readings versus time, i.e.
    between "most north" readings would have been south, etc. and you
    divvy those up
    into a lookup table.

    Just guessing. Maybe they've got something simpler and more elegant."


    Yes, they've got something simpler and more elegant than Red's guess.

    I have long been sceptical about such claims, that one could calibrate
    a fluxgate or other electronic compass against deviation caused by its
    surroundings within the vessel, simply by travelling in a circle (or
    perhaps two). I had thought, as Red did, that the presumption was made
    that the true bearing would be changing steadily with time, and I
    didn't see how, in any sort of vehicle or vessel, that could be made
    to hold good to nearly sufficient accuracy. Now I know better; because
    the basis of the method is in fact quite different.

    Bill mentioned Richard Langley's article, referred to in [894] as-

    ""Getting Your Bearings: The Magnetic Compass and GPS":
    Mostly about electronic compasses. Nothing in it that the cognescenti
    on the
    list don't already know but it might be of interest to some."

    and for me, that paper has been the key to understanding what was
    going on. With Fred Hebard, I recommend it strongly to anyone keen to
    learn about the state of the art, in modern-day compasses. Together
    with one of the references therein, "Applications of Magnetic Sensors
    for Low Cost Compass Systems", by M J Caruso

    Here's how I see it-

    It depends on the fact that a such a magnetic sensor, to measure field
    direction in the horizontal plane, is actually a combination of two
    such sensors at right angles.  One measures the horizontal component
    of the magnetic field lengthwise, parallel to the centreline of the
    vessel, Hl, the other measures the same thing, but crosswise, athwart
    the vessel, Hc.  You can think of these two sensors being mounted on a
    gimballed platform to keep them horizontal (though there are
    alternatives to that, as a "strap-down" configuration, which those
    articles deal with, but I won't, here). To keep things even simpler,
    think about a situation at the magnetic equator, where the field is
    horizontal only, with no dip to worry about. Hl and Hc can be positive
    or negative, depending on the way the vessel points with respect to
    the horizontal field H.

    From those two components, Hl and Hc, we can derive a magnetic course
    C, from arctan Hc / Hl. That course is the only information, that an
    ordinary needle-compass would provide, but now we have extra
    information, because we know those two components of the field
    strength. A needle-compass tells us nothing about field strength, just
    the direction.

    First, let's assume that there are no sources of deviation. No iron
    anywhere around, so the compass tells the truth  about the strength
    and direction of the Earth's horizontal field. Then Hl will vary as H
    sinC , and Hc, as H cosC. If we plot Hl against Hc, the result is a
    true circle, centred on the origin, radius H.

    Next, assume there's some permanent magnetism somewhere aboard (what
    is often, somewhat incorrectly, attributed to "hard" iron). That
    implies that at the position of the sensors there's a constant
    perturbing field, with horizontal components which we can call  Hl'
    and Hc', which do not change as the vessel's heading alters, and which
    would be the same even if there was no field from the Earth.. Now the
    two sensors indicate a total field (H sin C + Hl'), and (H cos C +
    Hc'). If we plot these two values against each other now, we still see
    a circle, but it's no longer centred on the origin; the centre is
    displaced, to a position Hl', Hc'.

    The compass has to have associated with it a simple computer or
    microprocessor, which can record the pattern of outputs from the two
    sensors as the magnetic course changes through at least 360 degrees in
    a calibration run. These are, effectively, "plotted" internally,
    against a nominal course angle derived from the arc tan of their
    ratio. The whole range of angles should be passed through, but those
    angles are NOT related to the time within the test run, which does not
    rely at all on the turn rate being held constant. It's a simple
    business for the computer to analyse that information, to determine
    the offsets of the circle, Hl' and Hc', and from then on  to subtract
    those offsets from the transducer outputs, and compute a true heading
    direction based on those corrected values.

    There's another source of compass deviation, relating to induced
    magnetism (often known as "soft" iron), which is actually caused by
    the effect of the Earth's magnetic field on the iron or steel, and
    would vanish if that field were removed. That effect does not displace
    the centre of the resulting (Hl. Hc) circle, but lengthens it, in one
    direction, into an ellipse. The computer should have no difficulty in
    assessing, separately, the results of permanent and induced magnetism,
    and allowing for them appropriately.

    I can imagine one problem that might arise when making a calibration
    run in that way. It's the horizontal components that need to be
    measured, and so (except near the equator) it's important that the
    measuring platform is kept truly horizontal by gimballing (or else by
    some form of tilt-compensation). If, in a calibration run, a vessel
    was taken through its range of heading angles by steering in a circle,
    and not by stationary swinging (for example, by ropes between
    pontoons), then the "horizontal" level of the gimballing would be
    affected by the centrifugal affects of the turn. Vessels really ought
    to be instructed to travel sufficiently slowly while making the turn
    that such accelerations are quite negligible. AA tight, very
    slow-travelling, turn is better than a big circle travelling fast,
    over the same time period.  A good procedure might be to to attach to
    a mooring buoy, then push the stern slowly around in a circle, with
    the dinghy. This could be a particular difficulty when calibrating an
    aircraft compass, which presumably must be done on the ground, rather
    than in the air, for that reason.


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

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