Safe Landing on Small Celestial Bodies with Gravitational Uncertainty Using Disturbance Estimation and Control Barrier Functions

TL;DR

This paper tackles safe autonomous soft landing on small celestial bodies under gravitational-uncertainty. It integrates an Extended High-Gain Disturbance Observer (EHGO) to estimate $w(x)=U_{\Delta}'(r)^{\top}$ with a nonlinear disturbance-canceling feedback-linearizing tracker and a Control Barrier Function (CBF) based minimum-intervention safety layer to enforce state and input constraints. The approach enables aggressive reference trajectories to be tracked while formally guaranteeing safety during the maneuver, demonstrated on simulations with both ellipsoidal and irregular asteroid models using fuel-optimal offline references. The results suggest this framework is suitable for onboard autonomous SCB missions, offering robust performance under significant gravitational-model uncertainty.

Abstract

Soft landing on small celestial bodies (SCBs) poses unique challenges, as uncertainties in gravitational models and poorly characterized, dynamic environments require a high level of autonomy. Existing control approaches lack formal guarantees for safety constraint satisfaction, necessary to ensure the safe execution of the maneuvers. This paper introduces a control that addresses this limitation by integrating trajectory tracking, disturbance estimation, and safety enforcement. An extended high-gain observer is employed to estimate disturbances resulting from gravitational model uncertainties. We then apply a feedback-linearizing and disturbance-canceling controller that achieves exponential tracking of reference trajectories. Finally, we use a control barrier function based minimum-intervention controller to enforce state and input constraints through out the maneuver execution. This control combines trajectory tracking of offline generated reference trajectories with formal guarantees of safety, which follows common guidance and control architectures for spacecraft and allows aggressive maneuvers to be executed without compromising safety. Numerical simulations using fuel-optimal trajectories demonstrate the effectiveness of the controller in achieving precise and safe soft-landing, highlighting its potential for autonomous SCB missions.

Figures (10)

• Figure 1:
• Figure 2: Control architecture consisting of an extended high-gain disturbance observer, a feedback-linearizing and disturbance-canceling control, and a control barrier function-based controller to ensure optimality and safety during trajectory execution.
• Figure 3: Spacecraft trajectory during the landing maneuver on the ellipsoidal asteroid, where landing is achieved.
• Figure 4: Normalized disturbance estimate error for Ellipsoidal asteroid case. The error exponentially converges to a set whose size changes as the spacecraft approaches the asteroid.
• Figure 5: State and input constraints remain positive, ensuring safe execution of the soft-landing maneuver.

Theorems & Definitions (5)

• Definition 1
• Proposition 1
• Proposition 2
• Theorem 1
• Theorem 2