# Autonomous Pointing Control of a Large Satellite Antenna Subject to Parametric Uncertainty
###### Abstract  
With the development of satellite mobile communications, large antennas are now widely used. The precise pointing of the antenna's optical axis is essential for many space missions. This paper addresses the challenging problem of high-precision autonomous pointing control of a large satellite antenna. The pointing dynamics are firstly proposed. The proportional-derivative feedback and structural filter to perform pointing maneuvers and suppress antenna vibrations are then presented. An adaptive controller to estimate actual system frequencies in the presence of modal parameters uncertainty is proposed. In order to reduce periodic errors, the modified controllers, which include the proposed adaptive controller and an active disturbance rejection filter, are then developed. The system stability and robustness are analyzed and  
which the antenna pointing control concepts were described, and an on-board autonomous control scheme, including acquisition and autotrack modes, was proposed. An active disturbance rejection control for the antenna pointing control of a large flexible satellite system was proposed in [12], and the inner and outer loops of the control system were studied to improve pointing accuracy and rotation speed. The attitude control system on the Engineering Test Satellite-VIII, which has two large deployable antennas, was studied in [13]. The phase stabilization control for low mode frequencies and small damping, and the gain stabilization control for higher mode frequencies were respectively designed. Classical control design techniques were proposed for the Tracking and Data Relay Satellite antenna. The loop shaping provides good single loop stability margins, and multiloop stability margin analysis verifies stability robustness against sensor parametric uncertainties, modal frequency shifts and gain/phase variations [14]. An Ho control approach, to achieve pointing control design of a flexible spacecraft antenna, was addressed in [15]. A proportional-derivative (PD) plus structural filter was designed to improve pointing accuracy and suppress vibration, and the frequency-domain method was used to analyze system performance [16]. All the above-mentioned works utilize satellite attitude maneuvers to perform large antenna pointing control. Meanwhile, an APM-based approach, to perform pointing control of LSA, was proposed in [17; 18]. The APM, which can drive antenna maneuvers and then correct the pointing errors, is installed on the satellite body. If the antenna is relatively small and its structural frequency is high, the APM-based approach can be used for a high-performance antenna pointing control. However, if the antenna is large with high moments of inertia and low structural frequency, the fundamental frequency of the whole satellite-antenna system will be mainly driven by the antenna. In this case, the bandwidth of APM-based pointing control loop is then not much higher than that of the satellite attitude control loop, which will thus lead to complicated attitude dynamic coupling problems. In this scenario, satellite attitude stabilization could be significantly disturbed during APM maneuvers. Correcting the pointing error of the LSA optical axis by means of the satellite attitude control is therefore more practical [19; 20; 21; 22]. The main factors that influence on-orbit pointing precision of LSA are twofold [16]. On the one hand, the LSA has very large dimensions and low stiffness, and the structural vibrations that may arise due to disturbances could seriously affect pointing and even system stability. On the other hand, the flexible satellite and LSA are subject to weight loss, thermal radiation and temperature variation in space. The modal parameters, which are uncertain or even time-varying, cannot not be known exactly. Besides, the thermal deformation error due to solar radiation, installation error and the deployment error also affect LSA pointing. All the above issues make it difficult to achieve high precision for LSA optical axis pointing.