Time transfer with geodetic GPS receivers using code and phase observations

The comparison of high performance frequency standards (10^-14 ) asks for a method where frequency transfer can be realised after a reasonable time of observation. Using only GPS-code observations a precision of a few nanoseconds can be achieved over long baselines after one day of observation. Geodetic applications ask for receivers which are able to acquire all observables of all satellites in view. Because carrier-phase measurements provide a much higher precision than code-observations, it is possible to obtain an accuracy of a few millimetres when processing these data in an interferometric mode. Interpreting this number in the time domain leads to a theoretical accuracy of the time transfer data of some 10 ps. Two time transfer terminals, each equipped with a geodetic GPS receiver, were built to transfer time between two high-performance clocks. Because we are aiming at sub-nanosecond accuracy it is necessary to take into account influences of a variety of environment variations on the electronics involved in these systems. We present results of our test series which include temperature dependencies of receiver, receiver antenna, and the electronics involved. The tests were performed on a “zero-baseline” and a 5 km-baseline, respectively. We show that even station clocks of `poor´ quality can be compared with high precision