## 6 Results and Discussion  
The performance of time and frequency transfer determined by a CP technique with a RVCE approach (i.e., a RVCE scheme) was compared with the performance of the traditional approach that treats the three GNSSs equally (i.e., the raw scheme). In data processing, the dual-frequency observations (L1/L2 for GPS, B1/B2 for BDS and E1/E5a for Galileo) are used to establish their corresponding ionosphere-free combinations for eliminating the first-order effects of the ionosphere. The combinations for pseudo-range and carrier phase measurement are also called P3 and L3 observation in the area of time and transfer [36; 37; 38]. The cycle slips of carrier phase are detected by the geometry-free (GF) and Melbourne-Wubbena combination [39], which are performed according to the signal frequency of each satellite system. The satellites with elevations less than 7\({}^{\circ}\) are eliminated. Multi-system precise satellite orbit and clock products are obtained from the Multi-GNSS Experiment (MGEX) Center for Orbit Determination in Europe (CODE), in which system time refers to the GPS system scale [40]. The hydrostatic component of the tropospheric delay is corrected using a Saastamoinen model, and the residual non-hydrostatic component is estimated as a random-walk. It is noteworthy that the parameter of interest with respect to the receiver clock offset is estimated by combining pseudo-range and carrier phase observations following a white noise stochastic process. In brief, the receiver clock offsets at adjacent epochs are uncorrelated, and are utilized based on the characteristics of a free-running atomic clock. The details of the observation models and data processing strategies are provided in Table 3. With respect to the RVCE, we used the ratios between pairs of the three estimated variance components as the threshold to control the number of iterations in the experiment. If the ratio value was within the range of 0.9 to 1.1, the iteration was stopped. The unique receiver clock offset parameter was then estimated, and the results of the time transfer were further derived using Equation (4). \begin{table}
\begin{tabular}{c c} \hline \hline
**Time Link Name** & **Geodetic Distance (km)** \\ \hline ONS1-BRUX & 883.7 \\ NTS1-KAT1 & 5704.2 \\ NTS1-CEDU & 7294.5 \\ BRUX-NTS1 & 7537.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Time links formed and their geodetic distances. \begin{table}
\begin{tabular}{c c} \hline \hline
**Items** & **Models and Strategies** \\ \hline Observations & Undifferentiated carrier phase and code observation \\ Signal selection & GPS:L1/L2; BeiDou: B1/B2; Galileo: E1/E5a \\ Satellite orbit and clock & Using the precise satellite products from CODE [40] \\ Satellite antenna phase center & Corrected using MGEX value \\ Initial weight between code and phase & 0.0001:1 \\ Ionosphere & Eliminated by ionosphere-free combination [36; 37; 38] \\ Tropospheric delay & Initial model + random-walk process \\ Tropospheric mapping function & Neill mapping function (NMF) [41] \\ Elevation cutoff & 7\({}^{\circ}\) \\ Sampling rate & 30 s \\ Observation weight & Elevation dependent weight \\ Estimator & LS in sequential mode \\ Receiver clock offset & Estimated with white noise \\ ISB & Estimated with epoch-wise variable method \\ Phase wind-up effect & Model corrected \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Observation models and multi-system RINEX data processing strategies. Figure 3 preliminarily presents a typical example of receiver clock time series at two stations with and without the RVCE approach. It can be noted the receiver clock value has unobvious discrepancy for two approaches apart from a few epochs. Considering that the time link is the primary analysis unit in practical work of time and frequency transfer, we therefore pay more attention to it in following text.