Leveraging Low-Altitude Constellations in Ad-Hoc Lunar PNT System Distributed Spacecraft Autonomy

This paper investigates leveraging a low-altitude Lunar Position, Navigation, and Timing (LPNT) constellation comprising 100 small satellites at an altitude of 100 km to provide reliable, ad-hoc navigation services for future lunar exploration missions. Autonomous operations of this large-scale swarm rely on two-way Inter-Satellite Link (ISL) measurements, utilizing pseudoranges and relative velocities among satellites. We describe four localization methodologies of the swarm assets utilizing Centralized Extended Kalman Filter (CEKF) and Decentralized Extended Kalman Filter (DEKF) algorithms. We evaluate their localization performance to quantify the positioning accuracy for both satellite swarm assets and lunar surface users, computational efficiency, and robustness to communication failures. We numerically assess four candidate localization methodologies (Redundant CEKF, Leader-Follower CEKF, Distributed CEKF, and DEKF) to address challenges posed by limited ISL measurements and computational resource constraints inherent in small satellite platforms. Simulation results indicate that Redundant CEKF can achieve the highest position accuracy with a median satellite position error of 371.2 m and a lunar surface user localization error of 16.69 m, although it incurs significant computational and bandwidth requirements. Leader-Follower and Distributed CEKF approaches demonstrate balanced trade-offs, achieving slightly degraded position accuracies of 419.6 m median satellite position error but offer more efficient computational and network resource utilization. The DEKF method shows the lowest computational demands but exhibits higher sensitivity to communication failures, with a median satellite position error of 541.7 m. This research explores critical trade-offs among positioning accuracy, computational complexity, and communication overhead, providing valuable insights into the operation and scalability of distributed spacecraft autonomy for future lunar navigation missions.