Performance of PTB's caesiurn fountain CSF 1

CSF1, a caesium atomic fountain frequency standard, has been developed and operated since the end of 1999 at the PTB. In an ongoing process, its uncertainty u/sub B/ has been evaluated and varies between 1/spl middot/10/sup -15/ and 2/spl middot/10/sup -15/, depending on the mode of operation and the duration of the evaluation. Local comparisons with PTB's conventional primary clocks, CS1 and CS2, and with NIST-F1 using GPS carrier phase and TWSTFT, revealed agreement within the respective uncertainties u/sub B/ and the comparison uncertainty. Five measurements of the TAI scale interval with CSF1 have been performed, each lasting 15 days. Use of CSF1 for the realisation of. a free atomic time scale TAF(PTB) is discussed.