Research on the Influence of the Residual Effects of TWSTFT on the Triangular Closure Difference

The triangular closure (TC) method of two-way satellite time and frequency transfer (TWSTFT) can be utilized to conveniently and quickly evaluate the time transfer performance between ground stations without additional equipment. To expand the application of the TC method and make it applicable to inclined geosynchronous orbit (IGSO) satellites, we analyze the influence of various residual effects of TWSTFT on the TC difference (TCD). Relying on the two-way comparison network of the National Time Service Center of the Chinese Academy of Sciences, Beijing, China, TC experiments were carried out using a geostationary orbit (GEO) satellite (CHINASAT-12) and an IGSO satellite (BDS I1-S). The results show that the Sagnac effect and ionospheric delay can be ignored without affecting the TCD, whereas the equipment delay and geometric path asymmetry effect cannot be ignored. Specifically, as a systematic error, the equipment delay has a great impact on the TCD and must be corrected. In contrast, the geometric path asymmetry effect caused by the relative motion of satellites and ground stations is small in magnitude for GEO satellites and can be ignored. However, for IGSO satellites, the geometric path asymmetry effect can reach the nanosecond level; thus, it also has a great impact on the TCD and cannot be ignored. Using the correction method proposed in this article, subnanosecond time synchronization exactness can be achieved in TC experiments based on both types of satellites. Therefore, we have successfully expanded the application of the TC method to IGSO satellites. Theoretically, the TC method can also be applied to medium Earth orbit and other satellites.