# Rapid Tracking Satellite Servo Control for Three-Axis Satcom-on-the-Move Antenna
## 1 Introduction  
Satcom-on-the-move (SOTM) plays an important role in satellite communications, which focuses on realizing real-time satellite communications during the motion of the carrier [1, 2, 3, 4]. Through the SOTM system, carriers such as vehicles, ships and aircrafts can realize the continuous transmission of multimedia information such as data, voice, images and videos through satellites as relays during the movement, which can play an important role in many civil and military fields. Compared with the traditional communication method based on internet service provider (ISP) base stations, the communication of the SOTM system does not rely on fixed infrastructure, so it is not limited by the coverage area of ISP base stations and it has a wider range of applications and application scenarios, such as primitive forests, deserts and open seas, where ISP base stations cannot provide coverage. The SOTM system can easily realize Internet access and communication at a lower cost and the implementation method is more flexible. The SOTM antenna usually adopts a dual-axis mechanical structure, including the azimuth axis and the pitch axis. The entire hemisphere can be scanned using these two axes. This mechanical structure has the advantages of simple design, low cost, robustness and compactness [5]. However, the dual-axis mechanical structure has an inherent shortcoming; that is, when the antenna pitch angle is 90\({}^{\circ}\), the antenna pointing is perpendicular to thehorizontal plane of the earth and the gimbal lock state is entered at this time [6]. In this case, when the carrier is moving, the antenna cannot continuously track the target satellite. The usual solution is to add a roll axis to the dual-axis SOTM antenna mechanical structure to form a three-axis SOTM antenna mechanical structure [7]. In engineering applications, system identification methods are often used for modeling, but it has the disadvantages of low applicability and poor portability. Especially for three-axis SOTM mechanical structure, a coupling relationship between the azimuth axis, the pitch axis and the roll axis increase the difficulty of modeling. For the three-axis SOTM mechanical structure, one method is to use the Newton-Euler method [8] for modeling, but the Euler equation established by this modeling method is more complicated, which makes the calculation and the processing difficulty, reducing the realization of the calculation of the kinematic relationship. At present, traditional PID control still occupies a dominant position in the design and development of SOTM servo control systems [9; 10; 11]. This is because PID control has the advantages of low model relevance, simple controller structure, mature parameter adjustment methods, convenient design and development and no steady-state error [12; 13; 14; 15; 16; 17; 18]. However, in traditional PID control, the differential link D is a non-regular system and cannot be directly applied to system development. Therefore, in engineering applications, PI controllers are often used to replace PID controllers, or differential links with filters are used to replace traditional differential links. These schemes will reduce the performance of the PID controller or increase the complexity of the PID controller. In addition, there is a contradiction between the \"rapidity\" and \"overshoot\" of the PID controller [19; 20; 21; 22]; that is, if the system output response is made to converge quickly, the system will have a large overshoot; if the system overshoot is reduced to a small value or even zero, the convergence speed of the system output response will be reduced. In the servo control system of the SOTM antenna, if the system output response converges slowly, it may result in the antenna failing to adjust to the required direction before changes in the carrier's attitude and/or location occur, necessitating a readjustment of the antenna direction; if there is a significant overshoot, it could cause the SOTM antenna pointing to deviate from the target satellite.