Mission Design for the CubeSat OUFTI-1

CHAPTER 7.ATTITUDE CONTROL SYSTEMWe can see that the couples trend in the updated case is almost the samethan in the non-updated.7.3 Attitude control hardwareThe main question in the OUFTI-1 project is if we really need an attitudecontrol system. In fact, the use of omnidirectional antennas doesn’t require aspecific orientation of the satellite respect to the earth. In theory, there wouldbe only one position to be avoided: the ideal antennas gain diagram shows infact that the only case that prevents the communication is when the antenna ispointing directly to the ground station. In the real world, as the presence of theCubeSat structure between the antennas avoids the perfect dipole, the gain isnon-null even along the antennas direction. Anyway it will be really small andprobably not enough to guarantee the communication. In this case we wouldlike to avoid these undesired positions.Another problem connected to the attitude control system is that the rotationrate of the satellite shouldn’t be too high. In fact, an high rotation ratecombined with the satellite speed on the orbit, could generate an huge speedrespect to the ground station with the consequent doppler effect. Event if it canbe corrected on earth, we cannot accept a too high value to guarantee a goodcorrection. Zero angular velocity is also undesired because of the risk of havingthe antennas pointing towards the earth and because of thermal behavior witha side continuously in sunlight.Excluded all the active ACS devices as inertia and momentum wheels becausethey require a power that is not available and because basically we do not needto control the satellite but only to avoid some angular positions, we have twopossible choices: leave out any attitude control system or choose a passive system.The KISS philosophy would push us towards the first one but the need ofa good communications level makes us think it out.A possible ACS would be a Passive Magnetic Attitude Control System (PMACS).It has been used before on other CubeSats as Delfi-C3 and in XI-IV, respectivelyat the University of Delft (The Netherlands) and at the University ofTokio (Japan).Generally, it consists of a strong permanent magnet and hysteresis material onone or two axes to damp rotation. The only rotation left is therefore the oneabout the longitudinal axis of the magnet and the hysteresis material dampthe rotations about the others. As the magnet would align itself with the earthmagnetic field (almost always parallel to the earth surface on the OUFTI-1 orbitas the inclination is low), we could in this case place the magnet on a directionthat prevents the antennas to be pointed towards the earth.Otherwise, we could use only hysteresis material on all the three axes: in thiscase, they wouldn’t try to align themself to a precise direction but they wouldGalli Stefania 71 University of Liège