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    Re: Fwd: Marine Electronics Issue #18: Fluxgate Compasses
    From: George Huxtable
    Date: 2002 Sep 22, 21:51 +0100

    I hope it's not too late to add a postscript to this thread, about fluxgate
    compasses,  particularly the Autohelm design, and stir the pot a bit.
    
    Such a fluxgate compass resolves two components of magnetic field, with
    respect to the plane that the sensor is held in. To determine the direction
    of the horizontal component of the Earth's field, it is essential that the
    sensor is held in a horizontal plane, with no tilt. In parts of the world
    with a large angle of dip (that is, where most listmembers live and sail),
    the resulting bearing error from a residual tilt can be considerably
    greater than the tilt error itself. It is therefore essential that some
    sort of levelling or gimballing is provided.
    
    As an aside, previous discussion on this list about fluxgate compasses
    centred mainly on a different arrangement which contained three sensing
    elements, which were not gimballed but "strapped-down" to the vessel, in
    which case any tilt of the vessel had to be sensed separately and included
    in a computation. We are not discussing that type here, but a much simpler
    arrangement.
    
    A traditional on-board needle-compass is usually mounted in gimbals, and a
    further degree of self-levelling is achieved by allowing the card to tilt
    on its point-bearing to maintain a horizontal plane. A gimballed fluxgate
    compass has only the the first of these levelling devices, so it needs to
    be gimballed more freely and damped better and balanced more accurately
    than the gimbals on a traditional compass, to achieve the same level of
    accuracy. A hand-held fluxgate such as the Autohelm, which has no internal
    gimballing whatsoever, relies on the observer to act as an active gimbal
    mounting, and he is expected to take positive steps to hold the plane of
    the instrument parallel to the horizon, both in his side-to-side and his
    up-and down direction, while the measurement is made.
    
    This presents few problems in daylight, in calm conditions. But put
    yourself in the position of a navigator going out on deck on a black night,
    on a heeling vessel, when it's rough enough so he has to hang on and is
    never really sure which way is up, to take a bearing of a 15-second light
    flashing intermittently above the wave crests. A familiar situation to most
    of us, I reckon. Somehow he has to hold his compass parallel (in both
    directions) to a horizon he can't see. An impossible task, it seems to me.
    I would welcome reports from users who have tried to take bearings with an
    Autohelm under such conditions.
    
    In my view, the design of the Autohelm makes matters worse in that its
    display is hidden from the navigator while bearings is being taken. In that
    way, the extreme sensitivity of the instrument to tilt error is concealed
    from the user. Me, I would prefer to see what was going on. Wouldn't you?
    
    Matters are improved somewhat in that the Autohelm compass allows a number
    of bearings to be taken in quick succesion and then averaged, which will
    certainly do something to reduce random scatter. However, in that "dark
    night" scenario, there might well also be a systematic error, a tilt bias
    the same way each time, which the averaging process will do nothing to fix.
    
    I have tried the Autohelm personal compass, but for the reasons given above
    do not own one. It's quite cheap, and convenient in other respects. So I
    would be interested to learn from those that do, whether my prejudices
    about it, as stated above, are based on reality or not.
    
    George Huxtable.
    
    ------------------------------
    
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    
    
    

       
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