I've been recently doing simulations for attitude control of a 3U cube satellite. In my simulation, actuators include 3-axis magnetic torquer and a reaction wheel. Even if the actuator saturation is not considered, unexpected yaw rotation happens whenever the satellite passes through near the North pole. At first, I thought this came from the weak Earth magnetic field near the North pole, but it is okay near the South pole.
Has anybody experienced the same problem? I hope somebody can give me a clue.
What is the inclination of the orbit and altitude? Do you consider atmospheric drag ?
Thanks Stavros,
Our target orbit is Sun synchronous orbit with altitude 700km and Inclination 98.193 °. Also, we take into account disturbances such as atmospheric drag, gravity gradient torque, and solar radiation pressure.
Near the poles magnetic control becomes ineffective, because the magnetic field lines become parallel to the yaw axis (nadir/zenith axis),. If the satellite has a CoM to CoP offset perpendicular to the yaw axis, atmospheric drag or solar pressure will cause a yaw disturbance torque that cannot be countered by the magnetorquers.
Thanks a lot for your interest to my question, Willem and Stavros. I totally agree with Willem's thought, but the strange thing is unexpected yaw motion does not happen near the South pole.
Remember the geographic north and south poles are not the same as the magnetic poles, so it may be that your polar orbit close to the geographic north pole is also closer to the vertical field lines with little magnetic yaw control torque possible, but at the south pole the magnetic field lines does allow some magnetic torque capability in yaw.
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