Recent results of Physikalisch-Technische Bundesanstalt's primary clock CS1

This paper describes the current performance of the Physikalisch-Technische Bundesanstalt's (PTB) primary clock CS1. After major reconstruction during 1995 and 1996, routine clock operation was started in May 1997. An evaluat ion of the CS1 Type B uncertainty yielded 7 . 10(-15)(1 sigma). The perform ance of the CS1 is illustrated by results of a comparison lasting 2 yr with other clocks at PTB and with the International Atomic Time (TAI) and free atomic time scales. With reference to TAI, e.g., a frequency instability of 3.9 . 10(-15) at an averaging time of 30 d was observed.