Frequency comparison of five commercial cesium standards with a NASA experimental hydrogen maser

Recent long-term phase comparisons between five commercial cesium beam frequency standards and a NASA experimental hydrogen maser, NX-1, have yielded a value for the frequency of the hydrogen maser of 1 420 405 751.7767±0.0024 hertz which is 6.77×10
^-12
±1.66 lower than that given by Vessot et al. in 1966.