Munin Attitude Control Subsystem (ACS)
Passive magnetic attitude control is ideal for Munin since the main instrument (MEDUSA) then will have fixed aperture angles relative to the magnetic field. Each aperture will have its own dedicated pitch angle range. Magnets have been used many times before, e.g. in the AMSAT microsatellites. Magnets are simple and cost-, mass- and volume effective.
Munin will be affected by torques from different sources. The mass and size of a satellite decides which torque will be the dominant one and affect the satellite more than others. In Munin’Äôs case the magnetic torque generated by the Earths magnetic field tends to be the most dominating.
When the satellite is in orbit there will be disturbances from an external magnetic field, the Earths magnetic field. The direction of the field is well known at low altitudes and is in the most northern parts of the Earth downwards (towards the Earth). If the satellite would have some magnetic properties, it could align itself in the direction of the field.
How magnets interact is well know but the science behind it might not be as clear. In any case the magnets have the ability to repel or attract each other. When the magnets have the same pole towards each other they repel and when they have the opposite poles they attract. If one would think of the Earth as an giant bar magnet, then a another bar magnet, attached to Munin, would try to point towards the direction of the opposite pole of the Earth, thus creating a torque on the satellite.
An advantage of using the magnetic solution is that there are no moving parts for the ACS. The disadvantage is that the solution is less effective at higher orbits.
The main reason for the oscillations (librations) of Munin is the initial separation forces and the Earths magnetic field. The field lines are directed outwards from the magnetic north pole and into the magnetic south pole. On its travel around the Earth Munin will be spinning two revolutions every orbit.
Munin will be using a set of six hysteresis rods for libration damping. The main reason for choosing such a type of damper is because it have been successfully used before and will not take up too much space, and it is simple and low cost.
More information about attitude control systems can be found at: SAPPHIRE ADC SUBSYSTEM