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   Load a spherical harmonic gravity model from GSFC/University of Texas at Austin.
   The coefficients returned from this file are unnormalized.

   This routine works with GEMT1, JGM2 and JGM3. JGM3 is the most accurate
   earth gravitational model currently available. JGM2 is used by Vallado
   in his book. GEMT1 is also available within the toolbox by typing
   LoadGEM. The gravity files all have the suffix '.geo'.

   NOTE
   ----
   All these models are publicly available, however, the proper
   authorizations to obtain the GEM-*, JGM-1, and JGM-2 models
   should be directed to NASA/GSFC (Steve Klosko, internet:
   klosko@ccmail.stx.com (Steve Klosko)).  The request for other models 
   should be directed to UT/CSR (shum@csr.utexas.edu).

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   Form:
   gravityModel = LoadGravityModel( action, fileName )
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   ------
   Inputs
   ------
   action          (1,:) 'get url' or 'load file'
   fileName        (1,:) fileName

   -------
   Outputs
   -------
   gravityModel    (1,1) Data structure
                         .name (1,:) Model name
                         .mu   (1,1) Gravitational constant (km^3/sec^2)
                         .a    (1,1) Model earth radius (km)
                         .c    (n,n) Cosine coefficients
                         .s    (n,n) Sine coefficients
                         .j    (1,n) Zonal harmonics

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