3-axis control of a university satellite for attitude acquisition departing from unfavorable initial conditions

Resumo

The design of a low cost university satellite ITASAT seeks to meet launch constraints as a secondary, piggyback payload. The dual-spin configuration has been investigated to determine the performance of the autonomous Attitude Control System (SCA) when the satellite separates from the launch vehicle in an unfavorable initial condition prior to attitude acquisition. The dual-spinner is controlled in three axes to acquire and maintain a single face pointing to the Sun by means of a momentum wheel whose rotation axis is orthogonal to that face, and a triad of air-core coil magnetotorquers that provide gyroscopic stiffness and control torques to maneuver and point the solar panel to the Sun. The SCA starts actuation as from separation, and operates in closed loop with estimates of the satellite attitude and angular velocity. These estimates are computed by an extended Kalman filter (FKE) that processes vector measurements of the Sun direction and the geomagnetic field provided by Sun sensors and a triaxial magnetometer. Starting from unfavorable initial conditions, approximately 1.5 day has been required to yield a pointing error under 10°, and less than two days to maintain the error below 0.1°. Satellite oscillations about the wheel axis have been within an acceptable margin. The duration of the maneuvers in relation to the capacity of the batteries on board have confirmed the feasibility of the dual-spin configuration. The resulting performance and complexity have been compared with those of the rigid-body spin stabilization to select an appropriate configuration for the satellite mission.

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