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    Re: Automatic deviation calculation by electronic compasses
    From: George Huxtable
    Date: 2009 Nov 23, 23:41 -0000

    Joe Schultz reminded me-"George, "flux-gate compass" is a mature
    technology."
    
    Yes, I am aware of that. In fact, flux-gates, which employ staurating
    magnetic cores,  have been used for measuring magnetic fields for far longer
    than the 25 years that Joe tells us they have been used in US Navy
    compasses.
    
    And indeed, they are not the only form of magnetic sensor that can be used
    for such purposes. More recent devices, such as those that Frank Reed has
    described, use the magnetoresistive effect, which allows them to be even
    more compact.
    
    But we were not discussing the magnetic sensor mechanism itself, but the way
    such a magnetic sensor can be arranged to self-correct for local magnetic
    perturbation.
    
    I wonder: were those "first gas-turbined frigates" that Joe refers to, built
    of steel? If so, I'm interested in what, if anything, was in place as a
    first-order correction for deviation, if these were, as Joe's words
    "self-correcting" compasses. Did they have the familiar correction magnets,
    and the hollow soft-iron spheres placed to port and starboard of the compass
    housing? I would be surprised if any electronic correction algorithm, such
    as we have been discussing, was capable of tackling the enormous deviations
    that a steel vessel gives rise to, without some such first-order
    corrections.
    
    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.
    ----- Original Message -----
    From: 
    To: 
    Sent: Monday, November 23, 2009 7:50 PM
    Subject: [NavList 10801] Automatic deviation calculation by electronic
    compasses
    
    
      First tried at the fleet level in the first gas-turbined frigates (US
    Navy) at least 25 years ago.  The objective was to get a self-correcting
    magnetic compass as backup to the gyro-compass, and explore the possibility
    of removing the then "large footprint" gyro-compass altogether.  Taken over
    by GPS/inertial with a gyro-compass backup.
    
    Today's minature electronics means you can buy a relatively inexpensive
    flux-gate compass and wear it like a wrist-watch.  Timex is one brand.  Fun
    is jogging a box (or ellipse) over a steel pipeline.  All machines have
    their limitations.
    
    Joe
    
    
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