The TUBSAT-1 Attitude Control and Stabilization System

TUBSAT-1 (Technical University Berlin Satellite) is an experimental low-cost satellite being financed by the German BMFT. The dimensions and weight are determined by the NASA Gas-Program and it will be ejected from the Space Shuttle within the German spacelab mission D2 by December 19, 1991, into a 298 km circular orbit and at a 28.5° inclination. To enable a large variety of useful experiments to fly with TUSSAT, it was necessary to develop a rather precise attitude control and stabilization (ACS) system. The ACS should be low cost, flexible (in view of changing ACS modes and parameters during the mission time), minimum component number and a low power consumption. A sun/star orientation with an additional spin mode was chosen and developed. The system is based on a microcomputer, fixed momentum wheel (FMW), one magnetic torquer, one sun and two star sensors. The closed loop pitch control consists of FMW, sun and star (for the eclipse phase) sensors, achieving a pitch pointing accuracy of 0.26° for any slew maneuver by using momentum transfer from the wheel to the satellite. Control of the wheel momentum (desaturation) without affecting the pitch axis orientation can be accomplished by executing a pitch slew maneuver. Positioning the magnetic torquer (which is mounted perpendicular to the pitch axis) to interact with the geomagnetic field vector. The pitch axis reorientation maneuver due to interaction between the magnetic torquer and the magnetic field vector component can be controlled by the one axis star sensor (roll/yaw rotation). A further pitch slew maneuver of 90° is necessary for positioning the sensor (roll = yaw).