## 4 Galileo Augmenting GPS SFSE Precise Positioning Experiment
### Experiment Analysis  
#### 4.2.1 Experiment 1  
Figure 5 shows position precision of successful epochs with different strategies for baseline shm1-shm4 and Figure 6 presents the curves of the corresponding NOS, ADOP, and Bsrt. Once the integer ambiguity is fixed incorrectly, the baseline deviation will be up to decimeter level or even worse and is usually not accepted, so the baseline precisions are analyzed only based on the results of successful epochs. The mean NOS, PDOP, ADOP, success rates (both Bsrt and Asrt), and precision of different strategies are presented in Table 3. As we can see, the mean NOS of GPS is 8.8, and for GPS/Galileo, it is about three more. The mean PDOP of GPS and GPS/Galileo is 1.93 and 1.50, respectively. From single GPS to C-GPS/Galileo, the mean ADOP are successively 0.456, 0.389, 0.252, and 0.243. It is obvious that GPS/Galileo and C- GPS/Galileo get better performance in ADOP. For the indicator of Asrt, they are successively 75.01%, 95.10%, 95.47%, and 99.12%. Accordingly, the Asrt of GPS is the worst, that of C-GPS and GPS/Galileo is better and close, and that of C-GPS/Galileo performs best. It is important to note that, for all strategies, Bsrt is much higher than Asrt, especially for single GPS. In terms of precision, both horizontal and vertical precision for the four strategies are very close, and those of the GPS/Galileo combination is only slightly better. For the single GPS, it is difficult to obtain a high success rate result for baseline resolution, because the number of visible satellites is insufficient--the baseline only has 8.8 common satellites that can only form 7.8 double-differenced equations. Moreover, the height difference between the two ends of the baseline exceeds 100 m, which means that the double-differenced technology may not perfectly eliminate the tropospheric delays. In this condition, the mean Asrt of the single GPS is poor and many interruptions that represent failure epochs appear, as seen in Figure 6a. In view of this, three methods are used to enhance the reliability of positioning and improve the success rate of ambiguity resolution. C-GPS does not increase the number of satellites, thus the PDOP does not change either. Its core idea is to improve the accuracy of the float solution and reduce the search space of the ambiguity by adding the strong constraint of the baseline length, so the ADOP and the Asrt of ambiguity resolution are improved when compared to the case of single GPS. The integrated GPS/Galileo method mainly increases the number of satellites to increase the number of observations, thereby improving the accuracy of the ambiguity floating point solution, and finally improving the ambiguity resolution success rate. Using this method, the mean ADOP decreases rapidly and the Asrt increases sharply when compared to single GPS. It seems that GPS/Galileo is better than C-GPS, because the ADOP of GPS/Galileo is much better than that of C-GPS. However, it is worth noting that not all-epoch equations have baseline constraints added when C-GPS is adopted. Only if the position deviation of the station exceeds the threshold, the solution will be recalculated with the baseline length constraint, which leads to a smaller ADOP of C-GPS than GPS/Galileo, but their Asrt is similar. It can also be found that C-GPS is more helpful in improving the ambiguity resolution near 16 pm, but GPS/Galileo act around 5 am. This indicates that the combination of the two methods--C-GPS/Galileo may bring about a better baseline resolution effect. From the statistical results, the success rate of the C-GPS/Galileo method is 99.12%, which is the best among the four methods. This indicates that, with the constraints and redundant observations added, the effect of the tropospheric residuals gradually decreases. In addition, since the station shm4 is located at the top of the tower and it is rarely affected by the vibration of the bridge, the horizontal and vertical precision of the baseline solution in the four cases are all about 1 cm.