New Quaternion Control Method Enabled Chandrayaan-3's Precise Lunar Landing
A novel decoupling technique solved guidance-control interaction during the historic soft landing.
The Chandrayaan-3 mission's historic soft landing near the lunar south pole relied on a sophisticated navigation, guidance, and control (NGC) system. While quaternion-based control laws are preferred for their singularity-free representation, they inherently couple all three rotational axes. This coupling can cause undesirable interactions between guidance and control during large rotations about the thrust axis, complicating attitude commands. The paper, authored by Aditya Rallapalli, Suraj Kumar, and colleagues, proposes a novel decoupling method that isolates thrust-axis control, preventing such interactions and ensuring proper attitude command generation. This innovation allowed the lander to precisely align its thrust vector with the required acceleration profile computed by the polynomial-based guidance scheme.
The method addresses a fundamental challenge in spacecraft attitude control: the shortest-path property of quaternions can lead to unintended rotations when large thrust-axis maneuvers are needed. By decoupling the thrust axis from the other two axes, the new approach maintains independent control authority. The paper (6 pages, 7 figures) was presented at the Indian Control Conference 2025 and is now available on arXiv. It provides a practical solution for future lunar missions and any spacecraft requiring high-precision attitude control during powered descent.
- Novel quaternion decoupling method eliminates coupling between all three rotational axes during attitude control.
- Method was validated through Chandrayaan-3's successful soft landing at the lunar south pole.
- Paper published in Indian Control Conference 2025, authored by ISRO researchers and academics.
Why It Matters
This control innovation improves landing precision for future lunar missions, reducing guidance-control interaction risks.