Development of NPL Rb fountain frequency standard

The first characterization of the distributed cavity phase frequency shift of the NPL Rb fountain frequency standard is reported. It is shown how the symmetric part of the distributed cavity phase shift can be used to set up the vertical angle of the fountain. The estimated uncertainty of the distributed cavity phase frequency shift of 1.25×10⁻¹⁶ reduces the total type B uncertainty of the Rb fountain down to 2.4×10⁻¹⁶.