## 1 Introduction
The Position Dilution of Precision (PDOP) simulation showed that the mean PDOP values of the integrated GPS/Galileo system are better than that of only GPS [14]. Another simulation result revealed that, given the positioning accuracy, the horizontal positioning availability of GALILEO is comparable to that of GPS, but its vertical positioning availability performs better [15]. The performance of GPS/Galileo was evaluated through simulation in terms of global availability, accuracy, and reliability [16]. The performance of a GPS/Galileo network-based real-time kinematic was evaluated by assessing the success rates of correct integer ambiguity resolution with simulation software [17]. With the launch of Galileo in-orbit validation (IOV) satellites, more work was conducted to evaluate the performance of Galileo and GPS/Galileo using real data. The performance of the Galileo IOV satellites was evaluated with respect to carrier-to-noise density ratio (C/N0), pseudo-range multipath, Galileo broadcast satellite orbit and single point positioning (SPP) accuracy in Galileo-only mode, as well as in integrated GPS/Galileo mode [18]. Several strategies that are based on the processing of dual-frequency GPS, multi-frequency Galileo, and tightly combined GPS/Galileo signals were tested and analyzed for their applicability for long-range precise instantaneous positioning [19]. It is shown that the current accuracy level of daily baseline solutions can be improved by using the additional Galileo constellation for high precision long baseline [20]. The ambiguity resolution success rate of GPS/Galileo for a 250 km baseline that is based on the ambiguity variance matrix was predicted and it was concluded that ambiguity resolution will likely become faster (less than ten minutes) in the case of integrated GPS/Galileo when based on triple frequency data of both systems [21]. The performance of several precise point positioning (PPP) models, combining dual-frequency GPS/Galileo observations in the un-differenced and between-satellite single-difference (BSSD) modes, was compared. It is shown that the traditional un-differenced GPS/Galileo PPP model, the GPS decoupled clock model, and the semi-decoupled clock GPS/Galileo PPP model improve the convergence time by about 25% in comparison with the un-differenced GPS-only model [22]. The contribution of Galileo satellite data to GNSS solutions for short and long baselines was also evaluated. When compared to GPS, GLONASS, and all solutions, the Galileo results vary greatly. When the Galileo observations are combined with other observations, the precision goes down [12]. The real time positioning results using signals from Galileo satellites only were presented. 14 Galileo satellites were used and the number of simultaneously observed Galileo satellites varied from five to seven. The tests results showed that it is possible to infer the current availability, continuity, convergence time, and accuracy of the RTK measurements [23]. Besides, there is some research that focuses on the applications of GPS/Galileo differential inter-system biases (DISBs) [24; 25; 26] and GPS/Galileo/BDS/QZSS RTK [27; 28]. Some literature only analyzed the Galileo positioning performance based on simulation results, while other work examined the GPS/Galileo or GPS/Galileo/BDS/QZSS data sets that were gathered from stationary stations. These results could not comprehensively reflect the current features of the Galileo constellation and its enhancement effect on GPS for dynamic situation, such as for bridge monitoring. Since dual frequency GNSS receivers are very expensive (around at least $10,000 per unit) for many engineering applications, the SFSE double-differenced (DD) baseline resolution became the most frequently used method for achieving high-accuracy positioning [29]. Accordingly, the performance of the SFSE carrier phase DD baseline resolution for the integrated GPS/Galileo needs more attention, especially for the deformation monitoring of large structures.