Attitude Control of a Spacecraft with a Strapdown Inertial Reference System and Onboard Computer

Book excerpt: A strapdown inertial reference system and an onboard digital computer can be used to change the attitude of a spacecraft over large angles in an arbitrary direction. A study was conducted with an advanced Orbiting Astronomical Observatory (OAO) using a three-axis control law previously proven globally stable. The control system can reorient the spacecraft to an arbitrary inertial attitude with a single command by operating the three momentum wheels simultaneously. Reorientation, therefore, becomes a simple extension of a hold or pointing mode. The time required for reorientation with this system is considerably shorter than the time required for a series of single-axis slews. The spacecraft attitude and control law are continuously updated by the onboard computer's using information provided by the strapdown inertial reference system. The use of a computer and inertial reference system with characteristics of systems presently under development demonstrates the feasibility of orientation with such a reference system. The basic system is not limited to the OAO but may be adapted for other three-axis-stabilized spacecraft.