Modeling of stratospheric balloons and robust line-of-sight pointing control

This paper investigates state-of-the-art modeling and control techniques for the robust line-of-sight pointing control of an optical payload on-board a stratospheric balloon, to meet stringent pointing requirements in the context of astronomy missions. Previous experience shows that the pointing performance of such systems is essentially limited by the rejection of the natural pendulum-like oscillations of the flight chain. This observation justifies the need for a model that accurately predicts such flight conditions that cannot be replicated in laboratory, and for a systematic methodology addressing the line-of-sight controller design to reject these flexible dynamics excited by wind disturbances. Moreover, it is sought to ensure robust stability and performance to the parametric uncertainties inherent to balloon-borne systems, such as complex balloon’s properties or release of ballast throughout the flight, especially since experimental validation is limited. In this paper, a first dynamical model of the complete system is proposed, based on Lagrangian mechanics; the comparison with flight data show that the frequency content of the platform’s motion is accurately predicted. Then, a multibody approach is applied to derive a second model taking into account the parametric uncertainties with the Linear Fractional Transformation representation. Finally, the control of the line-of-sight is tackled as a robust, structured ℋ_2/ℋ_∞ problem that improves the performance with regard to traditional PID controller. A μ -sensitivity analysis is also proposed to identify the most sensitive parameters and verify the performance.