## 3 Measurement Campaign  
QZSS satellites have an elliptical geosynchronous orbit which allows them to spend a significant portion of their orbital period visible to East Asia. They provide ever-present high elevation navigation signals for places such as Japan, Korea, Australia or eastern China. As of May 2019, Vespucci is in medium earth orbit and its surrounding cycle is about 12 h, making it not visible at any time in the collection site. In southern China, Vespucci can be observed at a higher elevation.  
The skylplot for a user in Xi'an (34\({}^{\circ}\)8\({}^{\prime}\) N, 108\({}^{\circ}\)59\({}^{\prime}\) E), China is shown in Figure 6. In Xi'an, the elevations of QZS-1, QZS-2, QZS-4 are sometimes less than 10 degrees or even invisible. The elevation of each satellite is greater than 10 degrees for some duration, which is approximately 18 h. Meanwhile, QZS-3 is always at about 46 degrees. Vespucci has a total of 8 h of elevation above 10 degrees for two periods. The trend of the elevation of Vespucci is always opposite to that of a satellite in QZSS. It should be noted that the SDR cannot use the constellation with less than four visible satellites for positioning measurement.  
The signal collection site is located on the 1 st-floor roof of the National Time Service Center (NTSC) of the Chinese Academy of Sciences. The position where the omnidirectional antenna was placed is shown in Figure 7. Although only four satellites are visible in Xi'an, they can be used to verify the optimization of joint positioning on GPS III + QZSS constellations.  
Since Sanya is in southern China, the elevation of satellites of QZSS and Vespucci of GPS III are several degrees higher than those of Xi'an. The skylplot for a user in Sanya (18\({}^{\circ}\)18\({}^{\prime}\) N, 109\({}^{\circ}\)15\({}^{\prime}\) E), China observing GPS III and QZSS orbits is shown in Figure 8. At Sanya, the satellites of QZSS is always visible and always significantly above 10 degrees elevation. Vespucci of GPS III is greater than 0 degrees about 12 h. As shown in Figure 8, most of the dwell time, QZSS is at high elevation that greater than 45 degrees (-15 h). But the trend of the elevation of Vespucci is always opposite to that of  
Figure 6: Skylplot of GPS III and QZSS satellites with L1C observed at Xi’an (34\({}^{\circ}\)8\({}^{\prime}\) N, 108\({}^{\circ}\)59\({}^{\prime}\) E), China, on 25 January 2019, with 0\({}^{\circ}\) elevation cutoff.  
Figure 7: Signal collection site at NTSC roof.  
a satellite in QZSS, which is the same as in the Xi'an constellation. The signal collected in Sanya also using the test equipment. If the constellation contains satellites with an elevation below 10 degrees, the positioning results will get tremendous deterioration [38]. So satellites collected below 10 degrees should be avoided.  
The location at the hotel building by the sea was used for the signal collection. To receive GPS III and QZSS satellites signals, an omnidirectional antenna was placed on the third-floor balcony, as shown in Figure 9. We analyze the signal collected in Sanya at UTC 12:08:10, 13 April 2019. The skyplot of the satellites visible in the course of the signal collection is shown in Figure 10. As seen in the skyplot, GPS III and QZSS satellites are at high elevation and their direct signals could be visible at the balcony location.  
Figure 8: Skyplot of GPS III and QZSS satellites with LIC observed at Sanya (18\({}^{\circ}\)18\({}^{\prime}\) N, 109\({}^{\circ}\)15\({}^{\prime}\) E), China, on 13 April 2019, with 0\({}^{\circ}\) elevation cutoff.  
Figure 9: Signal collection site at the hotel balcony.