918-t Annual Precise Time and Time Interval (PTTI) Meeting COMPARISON OF ATOMIC FREQUENCY STANDARDS AT NIST AND PTB USING CARRIER- PHASE GPS

We have constructed a link between Physikalisch-Techniche 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 will provide a direct measurement of the frequency difference between UTC(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 X I @ using one day of averaging. This uncertainty is smaller than the combined uncertainttes of the primary frequency standards in both laboratories, and it will therefore support a near-real-time comparison of these primary frequency standards without degrading their capabilities with the noise of the transfer system The use of GPS carrier-phase for time transfer on both short (200 m) and long (2400 km) baselines has been previously reported [l-8]. In these experiments GPS carrier-phase time transfer has shown an uncertainty of 100 ps and a frequency uncertainty of 2 parts in 1015 for averaging times of a day. Initial short-term comparisons between two-way satellite time transfer (TWSTT) and GPS carrier phase on continental baselines have encouraged the use of carrier phase as a feasible time-transfer method. This paper reports o