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    Re: Index corr., Octant as dipmeter
    From: Chuck Taylor
    Date: 2004 Nov 23, 10:01 -0800

    --- Alexandre Eremenko wrote:
    
    > >  > in the same way our vision is.>
    
    I have some experience with naval gunfire during the
    Vietnam war.  When firing from a ship at sea at a
    fixed target ashore, you need to know the location of
    the target and your own location with precision.
    Target location usually came from spotters on the
    ground or in the air.  Within sight of land, nothing
    beats an optical fix (from bearings of landmarks taken
    with a gyrocompass and a pelorus).  A radar fix can be
    quite good if the topography is suitable (for example,
    rocky headlands).  Don't forget that radar gives you a
    bearing as well as a range.
    
    Once you know your location and the location of the
    target, you can compute range and bearing to the
    target.  Then, in order to hit the target, you need to
    compute azimuth and elevation.  That is called the
    fire control problem and involves the calculus
    (integration if I recall correctly).  That was done
    using mechanical analog computers (lots of gears).
    The forward motion of the ship also needs to be
    factored in, as does the wind, and also the roll of
    the ship.  Mechanical gyroscopes were used to
    compensate for the latter, so that the gun could be
    fired at exactly the right instant.
    
    The spotters then provided any necessary corrections.
    This is called "walking it in".  I once heard a story
    about an incident in the Korean War, in which U.S.
    Army troops 10 miles inland called for gunfire support
    from a U.S. Navy battleship offshore to destroy a
    bridge.  The Army spotters called in the coordinates
    and the first round was 100 meters long.  The second
    round was 100 meters short.  The Army spotters, who
    were used to working with Army artillery, started to
    give the battleship instructions to "walk it in".  The
    battleship responded, "No, we are ready to fire for
    effect."  The next round destroyed the bridge.  (A
    battleship fired a 2,000 pound explosive projectile.)
    
    Best regards,
    
    Chuck Taylor
     48d 55' N
    122d 11' W
    
    
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