Improvements and New Evaluation of NIM4 Caesium Fountain Clock at NIM in 2005–2006

The NIM4 caesium fountain clock has been operating stably and sub-continually since August 2003. We present our improvements on NIM4 in 2005–06 and the most recent evaluation for its frequency shifts with an uncertainty of 5 × 10 −15 . A 203-day comparison between NIM4 and GPS time shows an agreement of 2 × 10 −14 . Finally the construction of the NIM5 transportable caesium fountain clock is briefly reported.