## 4 Results and Discussion
### Performance of Ranging and Positioning
With the elevation of QZS-1 reduced to almost 10 degrees, L1C of the QZSS-only can still be used for autonomous positioning at 8:34, as shown in Figure 16. It can be seen from the skylplot that the QZSS satellites are generally concentrated in the southeast direction of the observation site. Influenced by the geometric configuration of the constellation, the positioning error is gigantic in the east (E) direction. Soon after, GNSS Receiver using L1C signal lost its positioning ability because barely the remaining 3 satellites were visible in the QZSS-only constellation. GNSS Receiver using the GPS III + QZSS L1C joint positioning method restored the positioning capability at 11:02, although the PDOP value became worse than at 8:34, as shown in Figure 17. Since the satellites are generally concentrated at a higher elevation, the positioning results show a large deviation in the up (U) direction. The signal collected in Sanya when five satellites are simultaneously visible is used to compare the positioning performance of GPS III + QZSS and QZSS-only. By use of the same correlator spacing (0.1 chips), the three-dimensional positioning error of TMBOC(6,1,4/33) and BOC(1,1) is given in Figures 18 and 19, respectively using the constellations of GPS III + QZSS or QZSS-only. The positioning error is averaged by 2000 positioning samples. Whether using joint positioning or QZSS-only positioning, the positioning error of TMBOC(6,1,4/33) is generally superior to BOC(1,1) in three dimensions. At 15 s, the positioning performance of the signal is affected by the environment which may include multipath and signal distortion. It can be seen that TMBOC(6,1,4/33) shows better resistance at this time. Regardless of whether TMBOC(6,1,4/33) or BOC(1,1) is used, the positioning error of joint positioning is little than that of QZSS-only in three dimensions. This is the result of joint positioning optimizing the constellation composition while reducing the PDOP from 11.22 to 6.45. Since a priori position information cannot be obtained by GNSS Receiver, the position coordinates must be zeroed at the initial of regression, which results in a large positioning error in the initial stage. The joint positioning error using 0.15 chips and 0.2 chips correlator spacing is shown in Figures 20 and 21. The enormous ranging difference from Figure 13 of TMBOC(6,1,4/33) at 0.2 chips is reflected in the three-dimensional positioning error. In this case, the positioning error of TMBOC(6,1,4/33) deteriorates drastically compared to that of BOC(1,1), referring to Figure 20, and its resistance to interference is worse than BOC(1,1) at about 15 s.  
The 0.175 chips spacing is selected here for clear contrast with other correlator spacing. Although the positioning error of TMBOC(6,1,4/33) also deteriorated at 0.175 chips, the positioning error at 0.4 chips was significantly restored, as shown in Figure 22. As shown in Figure 23, the positioning error increases with the correlator spacing for BOC (1,1). These phenomena agree with the variation of the ranging difference. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Period (UTC)** & **Visible** & **Invisible** \\ \hline
8:34:28–8:35:28 & QZS-1, QZS-2, QZS-3, QZS-4 & Vespucci \\
11:02:33–11:03:33 & QZS-2, QZS-3, QZS-4, Vespucci & QZS-1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Satellites visibility during two acquisition periods on January 25 2019. Figure 16: Positioning results of data collected in Xi’an at 8:34:28 (PDOP = 9.604). (**a**) Skyplot; (**b**) Variation in East (E), North (N) and Up (U); (**c**) Positioning result in the 3D map. Figure 17: Positioning results of data collected in Xi’an at 11:02:33 (PDOP = 13.848). (**a**) Skyplot; (**b**) Variation in East (E), North (N) and Up (U); (**c**) Positioning result in the 3D map. Figure 19: Position error from QZSS-only (PDOP = 11.22, correlator spacing = 0.1 chips). Figure 20: Position error from GPS III + QZSS (correlator spacing = 0.15 chips). Figure 23: Position error from L1 CD when using different correlator spacing.