SPIGuidance:

--------------------------------------------------------------------------
   Guidance law for Solar Polar Imager using locally optimal control.
   Decrease the semi-major axis to 0.48 AU then increase the inclination to 60
   degrees. This function has a persistent variable for switching phases.
   The phase is reset to 1 when the input time is 0.
   The cone and clock angles follow McInnes' convention.

   Since version 7.
--------------------------------------------------------------------------
   Form:
   [q,alpha,delta] = SPIGuidance( t, x, d, env )
--------------------------------------------------------------------------

   ------
   Inputs
   ------  
   t         (1,1)      Time
   x         (6,n)      Heliocentric state
   d          (:)       Data structure
                         .mu  (1,1)   Gravitational parameter
   env        (:)       Environment structure - unused

   -------
   Outputs
   -------
   q        (4,n)       Inertial to body quaterion
   alpha    (1,n)       Cone angle
   delta    (1,n)       Clock angle

--------------------------------------------------------------------------

Children:

Common: Quaternion/U2Q
Math: Linear/Cross
Math: Linear/Mag
SC: BasicOrbit/RV2A
Sail: Coordinates/AnglesMcInnesToU
Sail: Guidance/LocallyOptimalTraj