Combining the fountain clock NIM5 and optical clock Sr1 to maintain a time scale independently running at NIM

Abstract Steering a flywheel clock with primary and secondary frequency standard (PSFS) has emerged as an important timekeeping framework for maintaining highly precise time scales. However, the inherent downtime of existing PSFS severely degrades the performance of the time scale，such as its stability, accuracy, and reliability. To alleviate the limitation, this paper proposes a multiple-PSFSs-steered time scale generation method. As a conceptual proof, data post-processing method is used to combine the fountain clock NIM5 and optical clock Sr1 to generate an independent time scale, which is continuously operated for 676 days at National Institute of Metrology (NIM). By fully exploiting the uptime of multiple PSFSs, incorporating both measurement data and associated uncertainty information into the steering process, along with optimized predictor-filter designs, the performance of the PSFS-steered time scale is significantly enhanced. The results indicate that the time difference of the time scale relative to Coordinated Universal Time (UTC) remains within ±4 ns, and its stability reaches 5.33×10 -16 for the averaging time of 30 days. Moreover, the time uncertainty of the time scale has been substantially reduced with the integration of the optical clock.