Magnetotorquer-only Attitude Control System Robust to Wide Range of Initial Conditions for Low-cost Spin- Stabilized Itasat Satellite

Abstract. Air-core magnetotorquer-only attitude control system (ACS) for the low-cost, spin-stabilized ITASAT satellite is here described. ITASAT’s ACS has been tested by simulation with synthetic measurement data. The proposed closedloop ACS system is based on 3-axis attitude and angular rate estimates provided by an extended Kalman filter processing vector measurements from a Sun sensor and a 3-axis magnetometer. For improved performance, such estimates are preceded by in-flight estimation of the residual magnetometer bias. The 3-axis estimation and purely magnetic control approach circumvents the use of a passive device for nutation damping, hence aiming at rigid-body satellite dynamics that present more benevolent conditions for adequate ACS performance. The design of the nutation damping ring-aided, magnetically controlled ACS nowadays flying in the Brazilian SCD (Data Collection Satellite) satellite series was conducted under the assumption of a dedicated launch that should deploy the payload at the proper orbit, and spin-stabilized with a small initial attitude error. Moreover, magnetically-actuated attitude control systems previously described in the literature often handle first the spin axis pointing maneuver, then follows spin rate control. The ACS here described, on the other hand, can be initiated upon separation from either a 3-axis controlled launcher, or a spinning one, and simultaneously handles spin rate, spin-axis pointing, and nutation damping by activating the magnetotorquer that minimizes an asymptotic stability condition criterion. Only one magnetotorquer is active at any given time. Air-core magnetotorquers are employed for fast, accurate magnetotorquer switching to avoid nutation angle excitation when the ACS is acquiring the desired 40rpm spin rate and spin axis direction orthogonal with respect to the ecliptic plane prior to the onset of the spin-stabilized operational phase. ITASAT is a rigid body with 73.6kg, dimensions 700x700x650 mm, and principal inertia moments of 6.5 kg·m about its principalspin-plane axes and 8.0 kg·m about its spin axis. Its target orbit, as defined so far, is circular at 750-km altitude and a 25o inclination angle. Disturbance torques included in the simulation were those caused by gravity gradient and ITASAT’s residual magnetism. Investigated here were various initial spin-axis pointing errors with respect to the desired ecliptic plane normal, and initial spin rates of 0rpm and 120 rpm, respectively corresponding to the separation from a launcher’s 3axis controlled last stage and from a launching vehicle with a spin-stabilized last stage – such as the Brazilian-made VLS (Satellite Launch Vehicle). The simulation results have indicated that proper attitude acquisition occurred in both orbit injection conditions. Injection by a 3-axis controlled launcher called for about 5 days of ACS maneuvering, when the angular momentum pointing error magnitude remained under 3o and spin rate error to 1rpm, whereas staedy-state nutation angle remained below ±2o.