Studies on Instabilities in Long-Baseline Two-Way Satellite Time and Frequency Transfer (TWSTFT) Including a Troposphere Delay Model

Abstract

Two-way satellite time and frequency transfer (TWSTFT) is one of the leading techniques for remote comparisons of atomic frequency standards. Frequencies can be compared with an uncertainty in the 10-15 range at an averaging time of 1 day, and time scale differences can be compared at the nanosecond level. These achievements are due to the fact that many delay variations of the transmitted signals cancel out in TWSTFT because of the reciprocity of the propagation path. However, reciprocity is not completely fulfilled. Especially, on long baselines, phase variations with amplitudes of 1 ns and more seem to limit the time and frequency transfer uncertainty. Following previous studies, we report on the progress of examining the TWSTFT link between Asia and Europe. We continue studies on the impact of the environmental temperature, the satellite motion, and the ionosphere on the phase variations. For the first time, a quantitative estimation of the differential troposphere delay for TWSTFT in the Ku-band is provided. Although all investigated candidates have indeed an impact on the measurement stability (from a few picoseconds caused by the troposphere to a few hundreds of picoseconds caused by the satellite motion), the overall observed effect is not completely understood at present. In summary, we estimate the individual contributions to the total measurement uncertainty.