Analysis of the impact of measurement noise on the TWSTFT stability

The Two-Way Satellite Time and Frequency Transfer (TWSTFT) is widely used by the time institutes to compare ground atomic clocks and contributes to the production of the Temps Atomique International (TAI) calculated by the BIPM. In the past, TWSTFT links have frequency stability of 1×10^-15 at one day [1], and since the last three years, two-way links became more and more noisy and the frequency stability has been degraded which has motivated the present work. In order to improve the performance of the TWSTFT technique, we focused on the principle parameters that impact the measurements stability and accuracy. In completion to the previous work [2] on the measurement noise in TWSTFT, we analyzed the impact of the changes of reference clock, satellite and satellite transponder on the two-way link instability over the period of the last five years. In addition, we studied the contribution in measurement noise on the two-way links of the following parameters: the chip-rates of the pseudo random noise modulation of the transmitted signal, the power of the transmitted and received signals. We also use the satellite simulator to determine the dependence of the chip rates on the delay difference of the OP ground stations. We completed the work in analyzing the variation of the short term residual noise fluctuations [3] and the carrier to noise ratio of the received signals in function of the previously listed parameters. Different results are presented in this paper and possible issues for improvement of the two-way links are proposed.