Answers are grounded in this paper and cite their sources.
The frequencies of the cesium fountain primary frequency standards at the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt have been compared. Two-way satellite time and frequency transfer and GPS carrier-phase were the principal frequency-transfer techniques used to make the comparison. For the 15-day interval in which both fountains were in operation the frequencies were compared with an additional uncertainty due to the comparison process of only 6.2 /spl times/ 10/sup -16/. The two standards agree within their stated one-sigma uncertainties of /spl sim/ 1.7 /spl times/ 10/sup -15/.
No takes yet. Share an insight, caveat, or question.
Parker et al. (2002) studied this question.