Angular momentum-based safing attitude control: An approach and experiences of ISAS' spacecraft

For certain classes of medium to small satellites in low earth orbit, bias angular momentum which is stored in the momentum wheel(s) can be utilized to advantage to assure a reliable and robust safe-hold attitude in case of instrument failures and/or miss-commanding from the ground. The scheme can be advantageous in terms of the required resources as compared with systems with full redundancy, and does not need to be compromised with regard to the attitude control performance during the mission. In this paper, attitude dynamics and control schemes for attitude safe-hold and initial attitude acquisition of the satellite YOHKOH and ASCA are explained. YOHKOH and ACSA are scientific satellites of the Institute of Space and Astronautical Science (ISAS), the former being a satellite solar physics mission and the latter being an X-ray astronomical observatory in space. Some comments on the design aspects of such schemes will be presented. Experiences and lessons learned from operating the above satellites in orbit are presented to show the authors' above assertions.