Space-based Autonomous Timescale Realization for LEO Constellations Enabled by Low SWaP-C Heterogeneous Clock Ensembles and Inter-Satellite Links

Low Earth Orbit (LEO) mega-constellations are emerging as a critical infrastructure for the global Internet of Things (IoT). To support ubiquitous connectivity and coordinated sensing, nodes within LEO constellations require precise time synchronization based on a unified and stable timescale. While Global Navigation Satellite Systems (GNSS) achieve highly stable timescales through extensive ground high-performance atomic clocks, this paradigm is ill suited to emerging LEO constellations, which are characterized by large satellite populations, limited mission lifetimes, and stringent Size, Weight, Power, and Cost (SWaP-C) constraints. To address these limitations, this study proposes a space-based autonomous timescale realization architecture designed for LEO constellations, leveraging low SWaP-C heterogeneous clock ensembles and Inter-Satellite Links (ISLs). Each satellite carries two Chip-Scale Atomic Clocks (CSACs) and two micro-Rubidium Oscillators (mROs), with a total volume of 136 cm³ and a power consumption of less than 1 W. At the satellite level, an onboard Harmonic Extended Ensemble Timescale Filter (HE-ETF) generates a weighted composite timescale that mitigates periodic environmental disturbances. At the constellation level, a standard ETF ensembles satellitelevel timescales to form a LEO constellation timescale (LEOT) with enhanced stability. Ten-day simulations demonstrate that the HE-ETF recovers up to approximately 86% of the midterm stability degradation induced by periodic environmental disturbances. At the constellation level, the stability of the realized LEOT is further improved by more than 35% relative to the satellite-level timescale. Overall, this architecture offers a cost-effective and autonomous solution for resilient, GNSS-denied synchronization, essential for the sustainable operation of future large-scale satellite IoT infrastructures.