Analysis of two way satellite time and frequency transfer system

The quality in time metrology depends not only on the performances of the atomic clocks but also on the capabilities of the time comparisons. Since GPS signals are available, time transfer method via GPS has become the main means. However, its precision of a few nanoseconds is barely enough for comparing the commercial clocks. It is thus insufficient for comparing the primary standards. Over the past several decades, the performance of atomic clocks has improved on average by an order of magnitude every seven years. To keep up with the development, new means is strongly demanded for comparing these devices. Two way satellite time and frequency transfer (TWSTFT) method is one such tool. Advantages of the two-way method are that one does not need the positions of the stations or the satellite with high accuracy, and that many sources of errors are substantially reduced. In this paper, the accuracy of TWSTFT in the Asia-Pacific region is estimated. The factors affecting the accuracy are mainly satellite motion, the Earth´s atmosphere and the ground station equipment. The estimated accuracy is about 1.5 nanoseconds. Comparisons of several time scales in the Asia-Pacific region by means of the TWSTFT and GPS common-view (GPS-CV) methods shows that the time transfer stability of the TWSTFT method is better than that of the GPS-CV method for sampling periods of less than 20 days.