ZCull:

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   Eliminates horizon sensor measurements whose corresponding
   estimated measurement is a complex number because the
   |arcosine| > 1
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   Form:
   [ue, us, er, zh, Wh, zc, Wc] = ZCull( ra, dec, g, ue, us, er, der, zh, Wh )
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   Inputs
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   ra                     Right Ascension
   dec                    Declination
   g                      Cant angle
   ue                     Nadir vector
   us                     Sun vector
   er                     Earth radius
   der                    Delta earth radius
   zh                     Measurements
   Wh                     Chordwidths                     

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   Outputs
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   ue                     Nadir vector
   us                     Sun vector
   er                     Earth radius
   der                    Delta earth radius
   zh                     Measurements
   Wh                     Chordwidths  
   zc                     Culled measurements
   Wc                     Culled chordwidths                   

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