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    Automatic deviation calculation by electronic compasses
    From: Joe Schultz
    Date: 2009 Nov 24, 10:44 -0800

    Yes, George, US Navy ships are made of steel.  Very, very special steel.  Just 
    kidding - I don't do the "look at me I'm so smart" game.  My posting (10801) 
    was in response to your "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" in post 10794.
    
    The advantage of these "electronic" compasses is that you can put the sensor 
    assembly anywhere you want - no need for "Navigator's Balls."  Find the sweet 
    spot and put it there.  Then, if needed, punch in an offset (this for the 
    high-end, i.e. expensive equipment).  Doesn't fix the three major drawbacks: 
    1) eliminating the vertical component of the magnetic field when the vehicle 
    is heeled, 2) moving in a circle to calibrate, and 3) effects of an 
    alteration to the ship's configuration, i.e. additions or deletions of 
    equipment, unless you redo the offset.
    
    1) is reduced through gimbaling (two axis systems) or by using a third axis 
    sensor and then "calculating out" the vertical component.  A third sensor 
    leads directly to "why bother?"  Three magnetic sensors and a computer or 
    three accelerometers and a computer (inertial).
    
    2) has been partially solved by an engineer at Delphi, an automotive industry 
    parts supplier - they do the automotive "electronic compasses."  With his 
    system you move in a partial circle - the electronics assumes an ellipse and 
    calculates the ellipse locii.  The locii location, in relation to the curve 
    location, determines the quadrant.  It's patented, so the rest of the 
    suppliers will have to wait it out or devise their own system.
    
    3) is not a factor in an inertial system.  "Why bother" won for US Navy ships.
    
    Joe
    
    
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