HAngleUKF:

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   Angle measurement from a fixed height object.
   For use by UKF.
   This assumes that the target is a fixed height off the ground.
   This is d.h and the target position is d.xTarget. x(1) must
   be the position.
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   Form:
   theta = HAngleUKF( x, d )
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   Inputs
   ------
   x         (:,1)  Input
   d         (1,1)  
                    .h       (1,1) Height of target
                    .xTarget (1,1) Position of target

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   Outputs
   -------
   Theta     (:,1)  Output

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