Comparing primary frequency standards at NIST and PTB

We have constructed a link between the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany and the National Institute of Standards and Technology (NIST) in Boulder, Colorado using carrier-phase Global Positioning System (GPS) receivers. The link provides a direct measurement of the frequency difference between hydrogen maser H2(PTB) and UTC(NIST), and can also be used to provide a direct comparison between the primary frequency standards at the two laboratories. Based on our previous work with this method, we expect to be able to realize the frequency comparisons with an uncertainty of about 2×10^-15 using several days of averaging. This uncertainty is smaller than the combined uncertainties of the primary frequency standards in both laboratories, and it therefore supports comparison of these primary frequency standards without degrading their capabilities with the noise of the transfer system