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This paper addresses the issue of coupled rotational axes in quaternion-based attitude control systems, which can lead to undesirable interactions between guidance and control, particularly during lunar landing missions like Chandrayaan-3. They propose a novel quaternion-based decoupling method that allows for independent control of the thrust axis. The method effectively mitigates guidance-control interactions and ensures accurate attitude command generation for lander attitude control.
Decoupling thrust-axis attitude control with quaternions prevents guidance-control conflicts, enabling more precise lunar landings.
India’s 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’s 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’s 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.