Current status of the French Atomic Time scales TA(F) and UTC(OP)

Abstract

The LNE-SYRTE in Observatoire de Paris (OP), Paris, France, is the National Metrology Laboratory in charge of the French Time and Frequency References. In that frame, two atomic time scales are currently generated: the French Atomic Time TA(F) and the real time prediction UTC(OP) of the future UTC. TA(F) is a common frequency reference based on commercial clocks disseminated in nine French institutions or companies. It has been recently improved both in terms of accuracy and long term stability thanks to the steering on the LNE-SYRTE Primary Frequency Standards (PFS) which realize the definition of the SI second. During the last three years, the TA(F) scale unit interval stayed globally close to the SI second on the Geoid, as published monthly by the Bureau International des Poids et Mesures, within a 1.8 × 10−15 limit, exhibiting virtually no drift. This result was made possible thanks to the four LNE-SYRTE PFS, which include three fountains currently in operation. On the other hand, a new UTC(OP) is under development. The signal from one Hydrogen Maser is steered in frequency on an ensemble clock based on OP industrial Cesium standards. In addition, to steer the timescale towards UTC, we apply to the ensemble clock a frequency correction derived from a comparison to the laboratory PFS and from the departure of TAI from the SI second: the goal here is not to be aligned with the SI second, but to stay as close to UTC as possible. This new UTC(OP)_Maser is aiming at staying within a 30 ns departure from UTC. First results obtained from the current operational system are showing that this should be feasible. Current tests are mostly about reliability issues, an essential step before this new time scale will replace the former system.