EllipsePropCirc:

Path: FormationFlying/Collision

% Function finds the propagated state uncertainty ellipsoid
   and the disturbance ellipsoid. For circular orbits.
 
 Note: The state is in the form [X;Y;Z;Xdot;Ydot;Zdot]
 where true Hills   
                    X,Xdot - radial direction
                    Y,Ydot - velocity direction
                    Z,Zdot - cross-track

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   Form:
   [StateQ,DistQ,Phi,Binput] = EllipsePropCirc(P,Q,DT,wn,hills);
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   Inputs
   ------
   P      (6,6)   Initial state uncertainty ellipsoid
   Q      (3,3)   Initial disturbance uncertainty ellipsoid
   DT      (1)    Time horizon of the prediction (DT = Tf - T0)
   wn      (1)    Orbit rate (rad/sec)
   hills   (1)    Flag for state type, 1 = true Hills  

   Outputs
   --------
   StateQ   (6,6)  Propagated state uncertainty over DT
   DistQ    (6,6)  Propagated dynamics uncertainty over DT
   Phi      (6,6)  Transition matrix, expm(A*DT) 
   Binput   (6,3)  Input matrix

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   Reference:   M. Campbell and B. Udrea, "Collision Avoidance in 
   Satellite Clusters", IEEE American Control Conference, 2002.
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