###### Abstract  
This paper proposes a nonlinear control method for carrying out Minimum-time satellite attitude maneuver and antenna motion which have robustness against model uncertainty. In recent years, small Earth observation satellites have been utilized and expected to maneuver rapidly in missions such as multi-target acquisition. On the other hand, small satellites need to send the observation data to ground station. Recently, small Earth observation satellites acquire high-resolution data, resulting in an increase in the time required for data communication. Thus, small satellites need to use inter-orbit communication link through Data Relay test satellite sending data from Data Relay communication (DRC) antenna. In conventional operations, the antenna motion is implemented after satellite attitude maneuver. However, this method has a time delay between the completion of the attitude maneuver and the start of data communication. The purpose of this study is to extend the time of earth observation and data communication by carrying out satellite maneuver and antenna motion concurrently. Because small satellite mounted with DRC antenna has large mass ratio of the antenna, we cannot ignore time variability of the moment of inertia of the whole system and reaction torque generated by antenna motion. Hence, in order to take the influence of the antenna motion into consideration, we combine a satellite attitude control system and an antenna drive system into one control system by governing equations and constructing the optimal control problem. We convert the optimal control problem into a NLP by discretizing the control input (a series of pulses) to minimize the final time of the total maneuver that includes the antenna adjustment. In addition, it is considered that a model uncertainty and unknown disturbance occurs in real space. Thus, we have to design feedback controller to secure robustness in model error and unknown disturbance. Accordingly in order to propose a nonlinear control method for carrying out minimum-time satellite attitude maneuver and antenna motion which have robustness against model uncertainty and unknown disturbance, we calculate a reference attitude by application of optimal control input torque to ideal satellite model and design servo controller by using state-dependent Riccati equation (SDRE) control method in order to track time-variant reference attitude. Keywords:small satellite; Data Relay Communication antenna; attitude control; Minimum-time; Nonlinear Programming problem; State-dependent Riccati equation; Robust control; nonlinear servomechanism +
Footnote †: journal: Applied Soft  
satellite system with a compact, autonomous and completely automatic concept. ASNARO is a small earth observation system that has a weight less than 500 kg, and an optical sensor with a high resolution of 0.5 m. The image performance of ASNARO is competing with other commercial, mid-sized, or large-sized satellites. In order to conduct an observation on Asia (using high frequency and high resolution), the Japanese government worked together with private companies to research and develop a satellite, and promote the launching of a small-sized satellite equipped with optical sensors. A Japanese private company, Nippon Electric Company (NEC), developed a space segment, and the ASNARO-2 satellite was launched on 18 January 2018 [1; 2; 3]. Small satellites like ASNARO have been required to maneuver rapidly in missions such as fast three-dimensional imaging, multi-target acquisition, etc. [4; 5]. Small observation satellites are required to increase the time required for data communication because it get a lot of observation data in a day. However, small Earth observation satellites are low Earth orbit (LEO) satellites. When a direct communication link is used, the contact time between a ground station and a small Earth observation satellite is limited to approximately 10 min per visible pass [6].