First experiments on application of Rb fountain frequency standards for TA(SU) time scale maintenance

At previous PTTI meeting, PTTI'17, we mentioned that VNIIFTRI has got four Rb fountain frequency standards, which run as frequency standards, not clocks. These instruments are not intended for secondary presentation of the time unit the second SI. These are indeed very stable frequency reference without frequency drift inherent, for example, to H-masers. During last year output frequency of these standards has been calibrated against operational CsF02 primary fountain frequency standard and now one may use these instruments as time and frequency unit keepers. This report will present preliminary results of the first experiments on application of Rb fountain frequency standards for maintenance the national atomic time scale TA(SU). According internal regulation TA(SU) is independent atomic time scale with the unit of time the second SU, reproduced by operational primary Cs fountain frequency standards. This time scale by-turn is maintained by ensemble of continuously operating high stable hydrogen masers. On other hand, each H-maser demonstrates more or less predictable frequency drift the value of which may be quite comparable to daily H-maser frequency stability. That is why one has times to times re-evaluate H-maser output frequency parameters. The more often one will re-evaluate H-maser output frequency parameters the more stable and accurate (in sense unit of time the second SU) time scale will be produced. The key problem is interruptible Cs standard mode of operation. Typically, Cs standard produces two or three sessions per month, but sometimes happens gaps for some months. In such cases availability of more or less continuously operating stable and none drifting frequency reference such as Rb fountain frequency standards may be a solution of the problem. The paper presents comparisons of some scenarios of TA(SU) maintenance: a. purely based on H-maser ensemble and primary Cs fountain frequency standards data and b. based on H-maser ensemble and Rb fountain frequency standards data.