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

TL;DR

This work tackles the challenge of achieving precise line-of-sight pointing for optical instruments on stratospheric balloon platforms by developing a control-oriented, uncertainty-aware model of the entire flight chain. It combines a multibody dynamics framework with Linear Fractional Transformations to capture parametric uncertainties and wind disturbances, and uses a robust $H_{\infty}$ design to optimize disturbance rejection, bandwidth, and actuator limits. The methodology is demonstrated on the FIREBall mission, where a flight-data–driven disturbance model and detailed LOS sensing/estimation architecture are integrated into a structured synthesis problem solved via non-smooth optimization, yielding a final controller that maintains robust stability and performance across worst-case parameter variations. The results highlight the method’s potential to generalize to balloon-borne astrophysical instruments, offering a principled alternative to empirically tuned controllers and improving pointing reliability in flight. Key mathematical constructs include the $LFT$ framework for uncertainty, the disturbance model $D(s)$ with $oldsymbol heta^{p}$ PSD matching $ ext{PSD}( heta^{p}) = |D(j\omega)G(j\omega)|^2$, and the $H_{\infty}$ objective with weighting functions $\mathbf W_d, \mathbf W_n, \mathbf W_u, \mathbf W_{e_d}, \mathbf W_{e_r}$ to shape closed-loop behavior over frequency."

Abstract

This paper addresses the lack of a general methodology for the controller synthesis of an optical instrument on-board a stratospheric balloon-borne platform, such as a telescope or siderostat, to meet pointing requirements that are becoming more and more stringent in the context of astronomy missions. Most often in the literature, a simple control structure is chosen, and the control gains are tuned empirically based on ground testings. However, due to the large dimensions of the balloon and the flight chain, experimental set-ups only involve the pointing system and the platform, whereas flight experience shows that the pointing performance is essentially limited by the rejection of the natural pendulum-like oscillations of the fully deployed system. This observation justifies the need for a model that predicts such flight conditions that cannot be replicated in laboratory, and for an adequate methodology addressing the line-of-sight controller design. In particular, it is necessary 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 dynamical model of the complete system is proposed, based on a multibody approach and accounting for parametric uncertainties with Linear Fractional Transformations. The comparison with flight data shows that the frequency content of the platform's motion is accurately predicted. Then, the robust control of the line-of-sight is tackled as a $\mathcal H_{\infty}$ problem that allows to reach the performance objectives in terms of disturbance rejection, control bandwidth and actuators limitations.

Figures (10)

• Figure 1: Sketch of the FIREBall system (not in scale)
• Figure 2: Platform's motion frequency content: model VS flight data
• Figure 3: Individual models constituting the balloon system
• Figure 4: Multibody LFT model of the flight chain
• Figure 5: Sketch of the pointing system