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   Determine ground station visibility for spacecraft position(s).
   Has a built-in demo.
   See also SatelliteVisibility, EarthRot, LatLonAltToEF
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   Form:
   visible = GroundStationVisibility( rGroundStation, jD, rSC, horizonAngle )
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   Inputs
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   rGroundStation   (3,m)   Earth fixed location of station(s)
   jD               (1,n)   Julian date
   rSC              (3,n)   Position(s) of spacecraft
   horizonAngle     (1,1)   Minimum elevation to ground station

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   Outputs
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   visible          (n,m)   Matrix of visible flags

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