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    Missile guidance system
    From: Bill B
    Date: 2007 Mar 20, 18:57 -0400

    I misquoted the guidance tutorial tape as "The missile knows where it is by
    knowing where it isn't."
    Following is the sound bite transcribed:
    The missile knows where it is at all times.  It knows this because it knows
    where it isn�t.  By subtracting where it is from where it isn�t, or where it
    isn�t from where it is, whichever is greater, it obtains a difference or
    deviation.  The guidance subsystem uses deviations to generate corrective
    commands to drive the missile from a position where it is to a position
    where it isn�t, and arriving at a position where it wasn�t it now is.
    Consequently the position where it is, is now the position that it wasn�t,
    and it follows that the position that it was, is now the position that it
    In the event the position that it is is not the position that it wasn�t, the
    system has acquired a variation.  The variation being the difference between
    where the missile is and where is wasn�t.  If variation is considered to be
    a significant factor, it too may be corrected by the GEA.  However, the
    missile must also know where it was.
    The missile guidance computer scenario works as follows:  Because the
    variation has modified some of the information the missile has obtained, it
    is not sure just where it is.  However it is sure where it isn�t--within
    reason.  And it knows where it was.  It now subtracts where it should by
    from where it wasn�t, or vice versa.  And by differentiating this from the
    algebraic sum of where it shouldn�t be and where it was, it is able to
    obtain the deviation and its variation, which is called error
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