## 2 Experimental Setup and Raw Observations Analysis
It can be shown that the number of satellites is greater than 10, which can meet the positioning needs, but there is still a certain gap compared with the geodetic receiver. For GPS and GLONASS, the number of satellites observed by the smartphones and geodetic receivers is similar; for BDS, the number of BDS satellites that can be observed by geodetic receivers is about 15, which reflects the fact that the BeiDou Navigation Satellite System has good coverage over our country's territory. As the number of GALILEO satellites captured by each smartphone is small, this article selects GPS, GLONASS, and BDS system satellites to analyze the continuity and the C/N0 of the GNSS raw observations. Analyzing Figure 3a, it can be seen that the continuity and signal strength of the Xiaomi 8 B with forced tracking is significantly better than those of Xiaomi 8 A, and the satellite signal capture capability is stronger. Taking the G21 satellite as an example, it can be found that all smartphones have a signal loss-of-lock phenomenon, but the moments of losing lock are different, and s  
Figure 2: The number of visible satellites in each system observed by the experimental equipment. (**a**) The number of GPS observed by the experimental equipment; (**b**) The number of GLONASS observed by the experimental equipment; (**c**) The number of BDS observed by the experimental equipment; (**d**) The number of GALILEO observed by the experimental equipment. reacquired at certain times. Xiaomi 8 B and OPPO smartphones have a stronger ability to track BDS satellite signals than Xiaomi 8 A and HUAWEI smartphones, but the quality of observations collected by Xiaomi 8 B is better. Figure 3: _Cont._  
To further analyze the availability of GNSS raw observations of smartphones, this article compares the pseudorange and carrier observations. Due to the pseudorange and carrier of smartphones use different clocks [13], we deal with them as follows: Firstly, a reference satellite is selected separately in each system, then the difference between reference satellite and others is calculated to obtain the combined observations to eliminate the influence of the equipment clock error; secondly, calculating the difference between the epochs of the obtained combined observations to obtain its rate of change; finally, taking the results of iRTK as the standard, the root mean square error (RMSE) of each smartphone is calculated. Figure 4 shows the rate of change of the pseudorange and carrier combinative observation processed by each experimental device. Due to space limitations, each satellite system selects only one for display (the Galileo satellite observations are not be considered because the amount of it is small). It can be found from Figure 4 that the change rate of the combined pseudorange and carrier observations of each satellite is consistent with the results of the geodetic receiver in terms of numerical value and change law, while in stability, there is still a certain gap compared with geodetic receivers. Comparing the satellites of different systems in Table 2, it is found that most RMSE of pseudorange of the GPS and BDS are less than 15 m/s, and the RMSE of carrier are less than 10 m/s. The data accuracy is significantly better than that of the GLONASS. Strikingly, the experiments found that some satellites' carrier combinative observations have gross errors or systematic errors for which the cause is not clear, and the influence of these errors is greater than 1000 m/s. In addition, the RMSE of Xiaomi 8 A and Xiaomi 8 B is significantly smaller than that of HUAWEI and OPPO smartphones. Among them, Xiaomi 8 B has the best data accuracy, which is consistent with the conclusion that Xiaomi 8 B can effectively improve the quality of the GNSS raw observations of the smartphone after enabling the mandatory tracking model. Based on the above analysis, this article uses Xiaomi 8 B smartphones for subsequent positioning experiments. Figure 3: Continuity and the C/N0 of the GNSS raw observations from smartphones and receivers.