# High-Precision Pointing and Tracking System Design for Near-Space Balloon-Based Optical Observation
## 2 Methods and Analysis
### Primary Platform Stable Pointing System Design  
Different from other spacecraft, the balloon-based observation platform carrying scientific research instruments is suspended below the balloon through a 10-100 m long sling and flexible structure, and it relies on the buoyancy of the balloon to achieve lift and high-altitude suspension flight. During the flat flight process, slight turbulence in the airflow will cause the balloon to experience varying wind pressures, resulting in a random force that causes the balloon to slowly rotate at a speed of approximately 0.01-0.1 revolutions per minute (rpm), which will cause changes in the orientation of the platform. Meanwhile, the long rope can cause the platform to undergo complex swinging movements, and due to the weak and stable airflow in the near space, the oscillation motion will gradually attenuate, and the average amplitude will eventually decrease to 0.01\({}^{\circ}\) at a stable state [31]. These disturbances must be suppressed otherwise they will cause an adverse impact on the accuracy of the attitude control of the system. The swing disturbance will directly change the attitude of the platform, thus affecting the pointing accuracy of the scientific instruments. The directional interference not only changes the posture of the platform but also influences the twisting and flexible structure of the lifting rope [32].  
Therefore, in order to ensure the pointing accuracy of optical observation instruments towards the target, the observation platform needs to adjust the azimuth angle and ensure the stability of the azimuth angle. In order to eliminate the azimuth change caused by the periodic rotation of the balloon, this paper adopts a momentum flywheel. The acceleration or deceleration of the momentum flywheel can change its angular momentum, and the change in the flywheel's angular momentum will enable the observation platform to obtain the control torque required to drive its rotation, achieving the purpose of azimuth control. In order to prevent reverse torque caused by the twisting of the lifting rope during the directional control of the observation platform and avoid saturation of the flywheel speed, a reverse-twisting device needs to be designed between the lifting rope and the observation platform to reduce the twisting effect. The primary platform stable pointing system is shown in Figure 4 and includes a flywheel, a flywheel torque motor, a torque decoupler, a flywheel control module, a torque decoupler control module, an inertial navigation module, a sun sensor, and a control module.  
The simulation results show that when the pitch/roll attitude measurement accuracy is less than 0.05\({}^{\circ}\) and the azimuth attitude measurement accuracy is better than 0.2\({}^{\circ}\), a stable pointing accuracy of \(\pm\)1\({}^{\circ}\) can be achieved through the high-altitude balloon-based attitude adjustment mechanism using a momentum flywheel and torque decoupler device. This accuracy is high enough to provide a stable direction for the second platform.  
Figure 3: Workflow of the stable pointing and tracking system.