RESUMO
The National Time Service Center (NTSC), Chinese Academy of Sciences (CAS), Xi'an, China, has developed a method for common-view time transfer using Geostationary Earth Orbit (GEO) satellite (GCV) applicable to the time and frequency transfer at distant stations. This method is independent of Global Navigation Satellite System (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 are 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 1) continuous coverage of the signal from GEO communication satellites; 2) differential observations to reduce the effects of orbit error and the imprecision of the propagation delay model; 3) using a very small aperture terminal (VSAT) to enhance high signal ratio to noise to obtain precise ranging accuracy and antimultipath ability; and 4) 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.
