Realization of a free-running timescale based on a CsF1 cesium fountain clock and multiple active hydrogen masers

Abstract

It is important to realize a stand-alone timescale with high-quality stability in short- and long-terms that can function accurately without any external inputs. A free-running timescale (FreeAT) was calculated as a weighted average of five active hydrogen masers and has operated freely for more than 400 d in the Beijing Satellite Navigation Center (BSNC). In the weighted average algorithm, a cesium fountain clock (CsF1) with a typical Type-A uncertainty of 4.50 × 10−16 for averaging times of 30 d was used as the frequency reference to estimate the frequency drifts of hydrogen masers. Additionally, the time difference between the rapid realization of Coordinated Universal Time (UTCr) and FreeAT was obtained indirectly by utilizing the timescale signal comparison between the BSNC and National Institute of Metrology via an optical fiber link. The overlapping Allan deviation of FreeAT with respect to UTCr is 8.69 × 10−16 for averaging times of 15 d, and its frequency difference and drift relative to UTCr are −1.20 × 10−16 and −2.08E-18/day, respectively. Therefore, the combination of a fountain and maser ensemble can generate a high-performance time scale.