Generating a Real-Time Time Scale With an Ensemble Clock and a Primary Frequency Standard

The robustness and reliability of a reference time scale is becoming more and more crucial in many applications, particularly in global navigation satellite systems (GNSS) and in critical infrastructures with time synchronization needs. This paper proposes two steering algorithms aimed at generating a real-time time scale, the first based on a primary frequency standard (PFS), and the second on an ensemble of clocks. Extensive tests on real data from the Italian National Metrology Institute (INRIM) cesium fountain ITCsF2 and atomic clocks have been carried out. The results show that the PFS algorithm has higher performances, but it requires a cesium fountain, a prototype standard currently available in a few timing laboratories only, whereas the ensemble clock algorithm uses commercially available clocks. A special emphasis is put on the system robustness: data pre-processing and possible combinations of the proposed algorithms have been tested in order to cope with outage periods, while maintaining good performances in terms of stability and accuracy of the resulting time scale. Such algorithms can be used in several applications where a stable time reference is needed, such as for the generation of a local real-time realization of UTC, as well as for any GNSS reference time.