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   Simulate a momentum bias spacecraft with magnetic roll/yaw control.
   The orbit is modeled and the spacecraft controls errors with respect to 
   the LVLH frame.
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   Specification
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   The controller uses the momentum wheel to control pitch and magnetic torquer
   control for roll and yaw. The 3-axis controller is a 3-axis PD and 
   torque demand is handed to the torquers. We assume that the torquers
   allow linear actuation. For the purposes of the simulation we compute
   the dipole required to meet the instantaneous torque demand. We ignore the
   disturbance the torquers create around pitch and also the momentum unloading
   system. This script is useful for sizing the control system.

   The inertia in the three directions are Ix=120, Iy=120 and Iz=1.3. The
   satellite is in an equatorial orbit so that the field is almost normal to the orbit.
 
   The damping ratios and natural frequencies are:
 
   Tach loop:
   zeta = 0.7621
   wN= 0.5
 
   Pitch and Roll loop:
   zeta = 0.7621
   wN   = 0.009
 
   Yaw loop:
   zeta = 1.0
   wN   = 0.15
 
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