GPSSatellite

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   Gives the location of the GPS satellites at JD.
   Choice of Earth-fixed or ECI reference frames.
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   Form:
   [r, v] = GPSSatellite( jD, coord )
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   ------
   Inputs
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   jD            (1,1)   Julian Date
   coord         (1,:)   'ef' or 'eci'

   -------
   Outputs
   -------
   r             (3,24)  Satellite locations
   v             (3,24)  Satellite velocity

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   Reference: Kayton, M. and W. R. Fried. (1997.) "Avionics Navigation Systems."
              John Wiley & Sons.
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