# Rapid Tracking Satellite Servo Control for Three-Axis Satcom-on-the-Move Antenna
## 3 Modified ADRC Servo Control Design
For the SOTM antenna, when the aperture of parabolic antennas is small, the energy of the antenna beam relatively diverges, resulting in a wider antenna beam. Therefore, although it is easier to locate and track satellites with the small aperture of SOTM parabolic antennas, their communication quality is generally relatively poor. On the other hand, for large-aperture of parabolic antennas, due to the larger aperture size, the energy of the antenna beam is more concentrated, resulting in a narrower beam. While the narrow beam of large-aperture of SOTM parabolic antennas makes satellite acquisition and tracking more challenging, the concentrated antenna energy during steady-state satellite tracking with SOTM antennas enables large-aperture of parabolic antennas to achieve better communication quality. In the field of SOTM antenna applications, from the perspective of communication quality, if the installation size of the moving carrier allows, a system design using larger aperture antennas is generally preferred. For larger-aperture SOTM antennas, if the SOTM antenna servo control system faces a contradiction between rapidity and overshoot, it may lead to the following issues: When the servo control system controls the antenna to quickly converge to the target direction, it may produce a large overshoot. At this point, once the SOTM antenna has aligned with the satellite, the presence of overshoot causes the antenna direction to keep adjusting, leading the antenna away from the optimal position. This deviation can result in reduced communication quality in the SOTM system and may even lead to interruptions in satellite communication if the antenna strays beyond the satellite beam coverage range. Additionally, because the carrier's attitude and location may continuously change, the presence of the aforementioned overshoot further increases the difficulty of tracking satellites with the SOTM antenna. On the other hand, when the servo control system has a small overshoot or no overshoot, the antenna direction often requires a longer time to converge to the target direction. If the carrier's location and altitude change little or not at all, the impact of this situation will be relatively minor. However, the SOTM system requires providing stable satellite communication services to users while the carrier is in motion. In a certain control cycle, due to the poor rapidity of the control system, when the servo control system has not yet adjusted the antenna direction to converge to the target direction, the carrier's altitude and location may change, generating a new target direction for the SOTM antenna. This will cause the SOTM antenna to never converge to the ideal target direction, reducing the satellite communication quality of the SOTM system and even leading to interruptions in SOTM satellite communication. From the perspective of disturbance suppression, there are many sources of disturbance during the operation of the SOTM system, such as: environmental and atmospheric changes causing signal attenuation and multipath effects, changes in the motion carrier's attitude and location and inherent vibrations, electromagnetic interference from the SOTM system itself, the moving carrier, or the surrounding environment, nonlinear characteristics introduced by gear backlash in servo transmission devices, production and installation errors between different batches of products, etc. The internal and external disturbances of the above-mentioned SOTM system generally have characteristics of being random, unpredictable and having no fixed features, making it difficult to effectively suppress these disturbances through conventional schemes. Active Disturbance Rejection Control (ADRC) was proposed by Professor [PERSON] of the Chinese Academy of Sciences in the 1990s, based on the principle of invariance. The core idea of ADRC is to treat the controlled object as a series integral type and to consider everything outside the series integral type object in the control system as the system's equivalent total disturbance.