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.

This paper presents a combined theoretical and experimental method for noise suppression in the repetition frequency (f_r) locking of erbium-doped fiber optical frequency combs (OFCs). This study proposed a novel mathematical model to bridge the noise relationship of f_r between the free-running and locked modes, and analyzed this relationship from two perspectives: the additional phase noise and the frequency stability. In addition, to integrate theoretical modeling with experimental validation, this study designed f_r locking strategy that uses a phase-locked loop (PLL) with PFD + PIID (a phase frequency detector and a proportional, first-order integer, second-order integer, first-order differential controller). Under synchronization of the f_r with a microwave reference (REF), this study achieved OFC additional frequency stabilities of 2.81 × 10⁻¹⁵@1 s and 8.08 × 10⁻¹⁹@10,000 s at 200 MHz fundamental frequency locking and 4.25 × 10⁻¹⁶@1 s and 1.91 × 10⁻¹⁹@10,000 s at 1200 MHz harmonic locking. The simulated and experimental results are in good agreement, confirming the consistency of the theoretical model and experiment. This work provides a reliable theoretical model that can be used to predict stability for OFC locking and significantly improves the additional frequency stability of OFCs.