## 5 Conclusions  
In this paper, an effective approach to monitor the displacement and deflection behavior of bridges that is called the integrated GPS/Galileo SFSE DD technique with baseline length constraint was introduced. The relative mathematical equations and models, including DD equations, combined modes, weight models, and constraint equations were derived. GNSS receivers monitored the Forth Road Bridge in Scotland to obtain bridge vibration information in order to assess its health condition. Two sets of GPS/Galileo monitoring data that were gathered from this bridge were used for the comparison of the performance of different processing strategies, such as single GPS, GPS with baseline length constraint, integrated GPS/Galileo, and integrated GPS/Galileo with baseline length constraint, respectively. The achieved ambiguity resolution success rates of the four strategies applied to the data set in the first experiment are 75.01%, 95.10%, 95.47%, and 99.12%. For the second experiment, the actual success rates are 93.24%, 97.57%, 98.42%, and 99.76%, which are all higher than the first one. The experimental results showed that integrated GPS/Galileo method can mainly increase the number of observable satellites and hence increasing the number of observations, and the baseline length constraint can improve the accuracy of the float ambiguity. Both of the methods can eventually improve the success rate of ambiguity resolution. In addition, the satellite visibility, PDOP, ADOP, and the baseline precision are also improved by using Galileo data and baseline length constraints. Therefore, integrated GPS/Galileo with a baseline length constraint had the best performance in the two experiments. This also illustrates that a single GPS is difficult to achieve all-weather real-time and high quality bridge deformation monitoring, but by busing Galileo data and baseline length constraints. These issues can be better resolved. In this paper, an FFT method was also used to analyze the bridge position time series that were obtained by GPS/Galileo with constraint. The FFT results are basically consistent with the result that was attained in a previous work. However, there are still some limitations that need further improvement. Firstly, if the receiver height difference is too large or the water vapor distribution between the rovers on the bridge deck and towers and the base stations is different, then the tropospheric delays cannot be effectively eliminated after double difference. Because of this, the results of experiment 2 are better than that of experiment 1. It is necessary to calculate the tropospheric delay residual while using the water vapor model of the  
Figure 10: FFT results of shm2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Station Name** & **XB (Longitudinal/Hz)** & **YB (Lateral/Hz)** & **ZB (Height/Hz)** \\ \hline Shm2 & 0.151 & 0.065 0.268 & 0.105 0.205 0.268 \\ Shm4 & 0.105 0.151 0.183 & 0.183 & 0.183 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Dominant Frequencies detected from the baseline time series. bridge deck, or estimate it as an unknown parameter during data processing. Secondly, the multipath error is an important factor that cannot be neglected, as it will periodically affect the monitoring results, especially those that are caused by the passing vehicles. We have studied static multipath mitigation in another article, but the corresponding methods are not applicable for dynamic multipath mitigation in the bridge environment. Further spectral analysis methods need to be developed to distinguish the bridge displacement contaminated by white noise and multipath error terms in the observation or coordinate domains. Finally, the Galileo system will be completed in 2020 and it will bring greater improvement in bridge monitoring. The full effect of using Galileo for bridge monitoring needs to be assessed. Conceptualization, Q.Z. and C.M. ; Methodology, Q.Z. and C.M. ; Software, Q.Z. and Y.X. ; Validation, Q.Z., C.M. and Y.X. ; Formal Analysis, Q.Z., C.M., Y.X. and X.M.