An overview on GNSS carrier-phase time transfer research

global navigation satellite system (GNSS) carrier phase observations are two orders of higher accuracy than pseudo-range observations, and they are less affected by multipath besides. As a result, the time transfer accuracy can reach 0.1 ns, and the frequency transfer stability can reach 1×10 −15 with carrier phase (CP) method, therefore CP method is considered the most accurate and promising time transfer technology. The focus of this paper is to present a comprehensive summary of CP method, with specific attention directed toward day-boundary clock jump, ambiguity resolution (AR), multi-system time transfer and real-time time transfer. Day-boundary clock jump is essentially caused by pseudo-range noise. Several approaches were proposed to solve the problem, such as continuously processing strategy, sliding batch and bidirectional filtering methods which were compared in this study. Additionally, researches on AR in CP method were introduced. Many scholars attempted to fix the single-difference ambiguities to improve the time transfer result, however, owing to the uncalibrated phase delay (UPD) was not considered, the current studies on AR in CP method were still immature. Moreover, because four GNSS systems could be used for time-transfer currently, which was helpful to increase the accuracy and reliability, the researches on multi-system time transfer were reviewed. What’s more, real-time time transfer attracted more attention nowadays, the preliminary research results were presented.