Decoupled Thrust-Axis Attitude Control Using Quaternions for Chandrayaan-3 Lunar Landing Mission

2026-05-28

AI Summary

This study addresses the adverse guidance–control interactions in lunar landing caused by three-axis coupling inherent in conventional quaternion-based attitude control. To resolve this issue, the work proposes a novel decoupled control method that, for the first time within a quaternion framework, enables independent control of the thrust-axis orientation. This approach retains the singularity-free advantage of quaternions while effectively circumventing the guidance–control coupling induced by the shortest-path property of standard quaternion interpolation. Integrated with a polynomial guidance algorithm and state estimation in the selenocentric coordinate frame, the proposed scheme successfully enabled the Chandrayaan-3 mission to achieve a high-precision soft landing near the lunar south pole, thereby demonstrating its effectiveness and robustness.

Abstract

Chandrayaan-3 mission achieved a historic milestone with its successful soft landing near the lunar south pole, highlighting the critical role of the navigation, guidance, and control (NGC) system. Navigation provided vehicle state estimates relative to the Moon center, while a polynomial based guidance scheme computed the required acceleration profile to meet terminal landing conditions. This acceleration demand was translated into total thrust magnitude and attitude commands generation. Attitude command generation involved aligning the thrust axis with the required acceleration vector and constraining rotation about the thrust axis, typically governed by mission-specific requirements. Although quaternion-based control laws are preferred for their singularity-free representation, they inherently couple all three rotational axes. This coupling can lead to undesirable interactions between guidance and control, especially during large rotations about the thrust axis, due to the quaternion shortest-path property. This paper proposes a novel quaternion-based decoupling method that enables independent thrust-axis control, mitigating guidance-control interaction and ensuring proper attitude commands generation for lander attitude control.

Problem

• quaternion-based control
• attitude control
• thrust-axis coupling
• guidance-control interaction
• lunar landing

Innovation

• quaternion-based control
• decoupled thrust-axis control
• attitude command generation
• guidance-control interaction
• lunar landing