Abstract
The combination of multiple Global Navigation Satellite Systems (GNSSs) may improve the performance of time and frequency transfers by increasing the number of available satellites and improving the time dilution of precision. However, the receiver clock estimation is easily affected by the inappropriate weight of multi-GNSSs due to the different characteristics of individual GNSS signals as well as the outliers from observations. Thus, we utilised a robust Helmert variance component estimation (RVCE) approach to determine the appropriate weights of different GNSS observations, and to control for the influence of outliers in these observation in multi-GNSS time and frequency transfer. In order to validate the effectiveness of this approach, four time links were employed. Compared to traditional solutions, the mean improvement of smoothed residuals is 3.43% using the RVCE approach. With respect to the frequency stability of the time links, the RVCE solution outperforms the traditional solution, particularly in the short-term, and the mean improvement is markedly high at 14.89%.
Highlights
Since the Global Positioning System (GPS) was first utilized for precise time transfer [1], it has increasingly been applied in time frequency dissemination
When combining the three code-division multiple access (CDMA) Global Navigation Satellite Systems (GNSSs) for time and frequency transfer, the observation model can be formed according to Equation (3), and the receiver clock offset is our parameter of interest
The cycle slips of carrier phase are detected by the geometry-free (GF) and Melbourne-Wübbena combination [39], which are performed according to the signal frequency of each satellite system
Summary
Since the Global Positioning System (GPS) was first utilized for precise time transfer [1], it has increasingly been applied in time frequency dissemination. With the development of precise multi-GNSS satellite orbit and clock products, all satellites within the network use the same time scale for different GNSS products [15,16,17] This provides the opportunity to combine the three code-division multiple access (CDMA) satellite constellations (i.e., GPS, Galileo and BDS) for precise time and frequency transfer. A robust Helmert variance component estimation (RVCE) was introduced when using combined multi-GNSS for precise time and frequency transfer estimates based on a CP technique. This was employed to allocate the adaptive weight for different GNSSs and to suppress the influence of outliers based on their adjustment residuals.
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