![]() ![]() As for the internal coincidence accuracy comparison method, the accuracy is judged based on internal errors that generally refer to the fitting residual or prediction error of clock offset. The clock offset accuracy can be evaluated through different methods such as the inner coincidence accuracy comparison method and the outer coincidence accuracy comparison method. However, it is very difficult to obtain the theoretically real clock offset. The optimal clock offset accuracy evaluation uses the theoretical real clock offset to evaluate the calculated clock offset accuracy. The accuracy evaluation of satellite clock offset is effective to verify the reliability of satellite clock offset data processing results. However, systematic errors of the measured SGL clock offsets and the calculated ISL clock offsets using different node satellite exist, which affect the accuracy of the time synchronization. These achievements have evaluated the performance of current system and given the application conclusions that the signal in space accuracy has reached up to 0.5 m. Moreover, Liu proposes a network adjustment model by the correction of inter-satellite clock offsets to detect and analyze the closed residuals, and has the results that the random noise of the inter-satellite clock corrections is reduced by 30% to 50%. Yan and Wang estimate the satellite antenna phase center offsets (PCOs), phase variations (PVs) and the differential code bias (DCB) of BeiDou-3 satellites, and has the results that PCOs and PVs is different with satellites, and DCB value is related to the receiver type. Some achievements have been published to estimate the affections of the systematic error. Compared with only SGL, the tracking coverage is extended by 40% by using ISL, the prediction error can be reduced from 3 ns to 1 ns, and the orbit result is also greatly improved. According to this method, when the satellite is visible by the Master Control Center (MCC), the clock offset is measured through the SGL L-band satellite-ground two-way time synchronization when the satellite is invisible by MCC, the clock offset is calculated by one node satellite (which is visible to both MCC and the invisible node satellite) of the SGL L-band two-way time synchronization and these two satellites of the ISL Ka-band two-way time synchronization, thus clock offsets of invisible satellite can be calculated through the “one-hop” reduction mode by one node visible satellite. This strategy was first published in 2018, and usually called as the “one-hop” reduction method. In contrast with GPS and GALILEO systems that update ephemeris and clock offset parameters mainly through the global uniform distribution of stations, BDS adopts the combined time synchronization strategy which determines satellite clock offset parameters by the satellite-ground link (SGL) for visible satellite and inter-satellite link (ISL) for invisible satellite. It can be seen that the accuracy of BDS time synchronization is significantly improved. In addition, inter-satellite station closure error and three-satellite closure error are reduced from 0.69 ns and 0.23 ns to about 0 ns. In comparison with the “one-hop” reduction method, the fitting residual and prediction error of the whole-network adjustment method reduces about 45.06% and 52.15%, respectively. Finally, experimental verification is completed based on BDS measured data. Secondly, the evaluation method combining internal and external symbols are proposed by the fitting residual, prediction error and clock offset closure error. The least square method is used to realize the whole-network adjustment of clock offset based on the observations of two sources, and to obtain optimal estimates of different clock offset reduction. Therefore, this paper firstly proposes a method of whole-network adjustment for clock offset based on the satellite-ground and inter-satellite two-way data. However, there exists a systematic deviation caused by the node satellite reduction, and there is still a large room for improvement in clock offset measurement and prediction. At present, BDS measures clock offsets of invisible satellite mainly through the “one-hop” reduction mode based on the satellite-ground clock offset of the node visible satellite and the inter-satellite clock offset between the two satellites. The inter-satellite link is an important technology to improve the accuracy of clock offset measurement and prediction for BeiDou Navigation Satellite System (BDS).
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