A new algorithm for the French atomic time scale

The Bureau National de Metrologie, Laboratoire Primaire du Temps et des Frequences (BNM-LPTF) Time Section is in charge of generating and disseminating the French atomic time scale TA(F). The latter is obtained in deferred time (1 month) as a weighted average of individual atomic clock readings using an algorithm which aims to optimize the long-term stability of the resulting time scale. With the GPS time transfer capabilities and the new commercial caesium clocks, the quality of the timing data has changed dramatically. Consequently, the time-scale algorithm was not adapted to assign a weight to the new clocks in relation to their quality. So a new algorithm (TAC) has been built, with more sensitive statistical processing than the previous one. Tests have been carried out on real clock data covering the last few years; they have shown an improvement in the stability of the time scale. Since 1 January 1997 at 00:00:00 UT, the time scale TA(F) has been obtained by the algorithm TAC. Details of this new algorithm are presented.