## 3 Signal Quality Analysis
### SQA1: Signal Strength for Various Times and Satellite Elevation Angles  
SQA1 is used to confirm the performance of the general received signal strength and find the differences between the two systems. Figure 4 shows the GPS C/N\({}_{0}\) result for all GPS satellites during a day. The different colors stand for the different GPS satellites. The results are presented to local time in Taiwan (UTC + 8). The result for each satellite is shaped like a parabola due to signal strength becoming stronger when the elevation angle is high and weaker when the elevation angle is low. For GPS satellites, the maximum C/N\({}_{0}\) is above 50 dB-Hz. This good-quality signal can be received all day. Figure 5 shows the BDS C/N\({}_{0}\) results. The BDS satellites have a longer transmission range than that of the GPS satellites, so their signal strength is weaker. The location of the BDS GEO satellites does not change much. As a result, the signal strength of each GEO satellite is rather constant at a specific value. However, this value and the amplitude are different for different GEO satellites due to differences in their elevation angle. The IGSO C/N\({}_{0}\) shape is not a parabola but it also changes with the satellite elevation angle. The MEO satellites have the shortest distance. Their C/N\({}_{0}\) value is greater than those of the IGSO and GEO satellites. The maximum C/N\({}_{0}\) for the whole BDS constellation is below 50 dB-Hz. When there are no MEO satellites, the highest C/N\({}_{0}\) for non-MEO BDS is only 47 dB-Hz. Our previous work found that the signal received on the 22 nd of July 2014 was affected by local interference and that the signal power had an anomalous value [27]. The user receives the interference signal at a local time of around 18:30. All the GPS satellite C/N\({}_{0}\) values dropped at the same time. Figure 6 shows three GPS satellites at different elevation angles. C/N\({}_{0}\) drops when the interference signal is received. On the right-hand side of Figure 6, one GEO satellite, one IGSO satellite, and one MEO satellite are used to demonstrate how the interference signal affects the BDS signal. C/N\({}_{0}\) decreases for all BDS satellites at the same time. As a result, the interference signal also influences the BDS signal as the center frequency of BDS BII is close to that of GPS L1. Figure 4: GPS C/N\({}_{0}\) with respect to local time. Figure 5: BDS C/N\({}_{0}\) with respect to local time. (**a**) BDS GEO C/N\({}_{0}\); (**b**) BDS IGSO C/N\({}_{0}\); (**c**) BDS MEO C/N\({}_{0}\). The C/N\({}_{0}\) values for various satellite elevation angles were analyzed. Low-elevation-angle signals pass through a more complicated environment, and thus have more noise. As a result, the C/N\({}_{0}\) value increases with increasing elevation angle of the satellite. Different GNSS antennas have different gain patterns, and in order to compare the C/N\({}_{0}\) variations for GPS and BDS, the same GNSS antenna is used for GPS and BDS in the paper. Figure 7 shows the GPS satellite C/N\({}_{0}\) change with the satellite elevation angle. When the elevation angle was above 60\({}^{\circ}\), the C/N\({}_{0}\) value remains at its highest value. The blue circles and lines are the mean and standard deviation values for 10\({}^{\circ}\) bins. Figure 7 also shows that the distribution of C/N\({}_{0}\) values becomes wider with decreasing satellite elevation angle. Figure 6: Impact of interference on GPS and BDS. Figure 7: GPS elevation angle with respect to C/N\({}_{0}\). Figure 8 shows the three BDS constellation C/N\({}_{0}\) as a function of elevation. For GEO satellites (green), the results could not be obtained for all elevation angles due to them being almost fixed at a certain location. For users in Taiwan, the approximate elevation angles of GEO satellites PRN 1 to 5 are 55\({}^{\circ}\), 38\({}^{\circ}\), 60\({}^{\circ}\), 38\({}^{\circ}\), and 18\({}^{\circ}\), respectively. The BDS PRN 1 and 3 have the highest satellite elevation angles.