A Self-referenced RAFS Ensemble Time Keeping System for GNSS Application

He Wang, Hunter Kettering, James Camparo, and Daniele Monahan

Abstract:	Atomic clock ensembling is a practical precision-timekeeping strategy that leverages existing clock technologies and available clock hardware to improve the frequency stability and operational reliability of a timekeeping system. In this paper, we present our experimental implementation and characterization of a real-time Self-referenced RAFS Ensemble Time Keeping System (SRETKS) suitable for GNSS timekeeping applications. Our goal is to examine the role of real space clocks in ensembling algorithms for precision satellite timekeeping as some issues associated with space atomic clocks (i.e., frequency jumps and phase jumps) have not been well modeled in simulation. Our SRETKS testbed consists of three GNSS Rubidium Atomic Frequency Standards (RAFS) in a vacuum chamber and an ultra-low noise VCXO. The VCXO is the master oscillator of the timekeeping system and at the same time functions as the reference clock for phase and frequency measurements of the RAFS clocks. The measured relative phase and frequency between the RAFS and the VCXO are processed on real-time using the Equal Weight Frequency Averaging (EWFA) algorithm to generate the RAFS ensemble output, which then steers the VCXO’s frequency with an optimized integration time. Allan deviation characterization results show that the self-referenced RAFS ensemble time keeping system (SRETKS) partly retains the frequency stability of the VCXO at short averaging times while achieving the frequency stability of the RAFS ensemble at long averaging times for overall improved timekeeping performance over a single RAFS system. Specifically, the SRETKS has achieved a Hadamard deviation of 2.6 ? 10-13 at 1 s, about ten times better than a single RAFS, and 1.0 ? 10-13 and 1.1 ? 10-14 at 100 s and 10000 s respectively, representing a 30% improvement over the single clock. Furthermore, our experiment demonstrates that an atomic clock ensemble can also enhance the operational reliability of a timekeeping system by effectively mitigating the impact of frequency jumps in an individual clock.
Published in:	Proceedings of the 57th Annual Precise Time and Time Interval Systems and Applications Meeting January 26 - 29, 2026 Hyatt Regency Orange County Anaheim, California
Pages:	60 - 72
Cite this article:	Wang, He, Kettering, Hunter, Camparo, James, Monahan, Daniele, "A Self-referenced RAFS Ensemble Time Keeping System for GNSS Application," Proceedings of the 57th Annual Precise Time and Time Interval Systems and Applications Meeting, Anaheim, California, January 2026, pp. 60-72. https://doi.org/10.33012/2026.20485
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