To resolve the dilemma in any post-processing strategy, i.e., the difficulty of monitoring the real-time time and frequency signals in a timely manner, real-time GPS time and the frequency transfer have recently become trending topics. Unfortunately, data interruption occurs when conducting real-time time transfer, sometimes from unexpected reasons. In this study, to ensure the stability and precision of real-time time transfer, an adaptive prediction model and a between-epoch constraint receiver clock model are applied as the mathematic models. The purpose of prediction is to solve the ambiguity from re-convergence when the data reappear. Moreover, compared to the conventional method, the between-epoch constraint receiver clock model is employed in this study to consider the correlation of epoch-wise clock parameters to avoid wasting useful information. The simulation data and real data are compared to verify the performance of the new approach. The simulation data for 165 days are designed with random daily interruptions of 10, 30, 60 and 90 min. Real data from 12 days is captured from the incomplete data in routine observation records. Ignoring the simulation data and real data, the investigation of six stations shows that the results with the between-epoch constraint receiver clock model were smoother than those with a white noise model. With an adaptive prediction model and the between-epoch constraint receiver clock model, the simulation results illustrate that the average root mean squares (RMS) values of all the stations are significantly reduced, i.e., by 66.03% from 0.43 to 0.14 ns, by 64.91% from 0.44 to 0.15 ns, by 57.47% from 0.43 to 0.18 ns, and by 51.67% from 0.44 to 0.21 ns for the 10, 30, 60 and 90 min data interruptions, respectively. The stability of all the stations is improved by at least 50%. The improvement increases to 100% for short-term stability. The real results show that the stability of four links is boosted by at least 5%. The model proposed in this paper is more effective in producing short-term stability than long-term stability.

The National Time Service Center (NTSC), Chinese Academy of Sciences (CAS) has developed a method for Common-View time transfer using Geostationary Earth Orbit (GEO) satellite (GCV) applicable the time and frequency transfer at distant stations. This method is independent of GNSS time and frequency transfer, as well as Two-Way Satellite Time and Frequency Transfer (TWSTFT). A master clock at the time laboratory transmits pseudorandom code signals to a GEO telecommunication satellite, and the time signals is retransmitted by the satellite. Receivers at the time laboratory and user locations coincidentally receive the time signals, and the offsets between the user clocks and the master clock of the time laboratory are determined with high precision if the precise coordinates of the user locations are known. For this technology, only a parabolic antenna with receiver devices and a demodulator are required at each user station, but the coordinates and precise orbits need to be obtained in advance. The key features of GCV include a. continuous coverage of the signal from GEO communication satellites; b. differential observations to reduce the effects of orbit error and the imprecision of the propagation delay model; c. using a Very Small Aperture Terminal (VSAT) to enhance high Signal ratio to noise to obtain precise ranging accuracy and anti-multipath ability; and d. utilizing the C band or Ku band to decrease the impact of the ionosphere. Experiments based on the TWSTFT network of CAS showed that the performance of GCV was at the same level as that of TWSTFT, and the RMS of the residuals of GCV was less than 1.5 ns with respect to TWSTFT.

Two-way satellite time and frequency transfer (TWSTFT) using satellites in inclined geo-synchronous orbit (IGSO) is fundamentally more difficult than using satellites in geostationary orbit (GEO), due to the fact that IGSO satellites are constantly moving relative to a ground station, which brings in large residual effects. In this paper, we designed a protocol and an algorithm to correct several factors that affect the precision of TWSTFT using IGSO satellites. We used two ground stations, each equipped with sending and receiving channels, to conduct real-time measurements of the distance and the speed of an IGSO satellite, and data collected were used to make appropriate corrections. We tested this concept by conducting TWSTFT experiments using an IGSO satellite (BEIDOU I1-S), and compared the results with that obtained using a GEO satellite (CHINASAT 12). Indeed, the individual components that contribute to the residual effects of TWSTFT measurements using the IGSO satellite were several orders of magnitude greater than that of the GEO satellite. By using our modified protocol, TWSTFT results with an IGSO satellite were well accorded with that of using a GEO satellite; the standard deviation of residuals was less than 0.4 ns, well within the limit for the measurement uncertainty of TWSTFT. Therefore, we have expanded the use of IGSO satellites for TWSTFT. In principle, it should also be applicable to medium earth orbit and other satellites.

High-precision time synchronization between ground stations is essential for the precise measurement and determination of satellite orbits. Among the numerous time synchronization methods available, two-way satellite time and frequency transfer (TWSTFT) technology is widely used because of the approximate symmetry of its signal transmission path, such that most of the error terms can be offset to obtain subnanosecond time transmission accuracy. A C-band two-way satellite comparison network has been constructed at National Time Service Center (NTSC), Chinese Academy of Sciences for orbit determination and to compare the times between ground stations. The gradual increase in the number of ground stations has created a problem in need of urgent resolution: the facile and rapid evaluation of the time transfer performance between ground stations by a two-way satellite comparison network without the use of additional equipment. We solve this problem by applying the TWSTFT triangle closure method and the precise point positioning time transfer method to the NTSC two-way satellite comparison network and conduct joint observations between terrestrial stations to evaluate the network time transfer performance. We select the Xi’an, Kashi and Sanya stations of the NTSC two-way satellite comparison network to carry out verification tests. The results show that the NTSC two-way satellite comparison network can achieve subnanosecond time synchronization accuracy. The accuracy of time synchronization between stations can be checked conveniently and quickly without the use of additional equipment. This study presents the most recent developments for the NTSC two-way satellite comparison network and serves as a reference for evaluating the time transfer performance of similar networks.

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 triangular closure difference (TCD). Relying on the two-way comparison network of the National Time Service Center of the Chinese Academy of Sciences, 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 paper, 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.