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This paper addresses the lack of a general methodology for the controllersynthesis of an optical instrument on-board a stratospheric balloon-borneplatform, such as a telescope or siderostat, to meet pointing requirements thatare becoming more and more stringent in the context of astronomy missions. Mostoften in the literature, a simple control structure is chosen, and the controlgains are tuned empirically based on ground testings. However, due to the largedimensions of the balloon and the flight chain, experimental set-ups onlyinvolve the pointing system and the platform, whereas flight experience showsthat the pointing performance is essentially limited by the rejection of thenatural pendulum-like oscillations of the fully deployed system. Thisobservation justifies the need for a model that predicts such flight conditionsthat cannot be replicated in laboratory, and for an adequate methodologyaddressing the line-of-sight controller design. In particular, it is necessaryto ensure robust stability and performance to the parametric uncertaintiesinherent to balloon-borne systems, such as complex balloon's properties orrelease of ballast throughout the flight, especially since experimentalvalidation is limited. In this paper, a dynamical model of the complete systemis proposed, based on a multibody approach and accounting for parametricuncertainties with Linear Fractional Transformations. The comparison withflight data shows that the frequency content of the platform's motion isaccurately predicted. Then, the robust control of the line-of-sight is tackledas a $\mathcal H_{\infty}$ problem that allows to reach the performanceobjectives in terms of disturbance rejection, control bandwidth and actuatorslimitations.

Robust line-of-sight pointing control on-board a stratospheric balloon-borne platform