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    Re: Gyroscope vs. Fluxgate compass
    From: Brian Whatcott
    Date: 2002 Feb 5, 17:40 -0600

    At 07:37 AM 2/5/02, John LeRoy wrote:
    > >>>  [bw] Instead, bigger airplanes used a flux gate sensor ...
    > >> Actually on modern "big" airplanes like the747-400 I used to drive the
    > >> navigation system has no magnetic compass at all!
    > >
    > >
    > >> John LeRoy
    > >> M/V Traveller
    > >
    >  there is a whisky compass, but its not part of the navigation system.
    >It is highly unlikely that the aircraft will lose both GPS systems and all
    >three FMCs.
    >John LeRoy
    Wandering probably too far afield now... but the electronic flight instrument
      system (EFIS) aboard the 747-400 displays several compass roses on
    the tubes, and INS does not inherently produce magnetic
    orientation unless it senses the motion of an inertial axis 'fixed in space'
    wrt to a fixed body in the Earth frame - [much as Sperry's gyrocompass did ]
    and applies the variation appropriate to  some supposed lat/lon to derive
    the mag heading.
      I seem to recall that's how an INS system initializes, but I haven't reviewed
      that system recently - I expect this is the 'compassless' attribute John has
      in mind.  Pity that INSs are very very dear (and drift with time unless
      resynced with other inputs!)
    Brian Whatcott
       Altus OK                      Eureka!

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