  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\times10^-16 reduces the total type B
uncertainty of the Rb fountain down to 2.4\times10^-16.
