LLAToECEF:

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   Compute ECEF position from latitude, longitude, altitude.
   Assumes spherical planet.
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   Form:
   rECEF = LLAToECEF( lla, rP );
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   Inputs
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   lla         (3,:)   Latitude [rad], longitude [rad], altitude [km]
   rP          (1,1)   Radius of planet (for Earth: 6378.14) [km]
                       

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   Outputs
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   rECEF       (3,1)   ECEF position vector [km]

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