## 3 Results
### Four-GNSS
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**System** & **GPS** & **Galileo** & **GLONASS** & \multicolumn{2}{c}{**-BDS-3**} & \\ \hline Orbit Type & MEO & MEO & MEO & GEO & IGSO & MEO \\ Satellite Number & 32 & 24 (36) & 24 & 3 & 3 & 24 \\ Angle of Inclination & 55\({}^{\circ}\) & 56\({}^{\circ}\) & 64.8\({}^{\circ}\) & 0\({}^{\circ}\) & 55\({}^{\circ}\) & 55\({}^{\circ}\) \\ Altitude (km) & 20,200 & 23,222 & 19,100 & 35,786 & 35,786 & 21,528 \\ Center of Longitude & - & - & - & 80\({}^{\circ}\)E/110.5\({}^{\circ}\)E/140\({}^{\circ}\)E & 110.5\({}^{\circ}\)E & - \\ \(\kappa(\times 10^{-4})\) & 2.6730 & 1.7046 & 2.0026 & - & - & 1.8574 \\  & & & & & & \\ \end{tabular}
\end{table} 
Table 1: Constellation parameters of four-GNSS and \(\kappa\) of the observation probabilities of MEO satellites. \begin{table}
\begin{tabular}{c c c c} \hline \(i_{orb}\) (\({}^{\circ}\)) & **Area** & **Ave. 1 (\({}^{\circ}\))** & **Imp. Rate 2 (\%)** \\ \hline
55 & & 31.02 & - \\
60 & 0\({}^{\circ}\leq L\leq 180^{\circ}\)E & 33.43 & 7.77 \\
65 & 75\({}^{\circ}\leq|B|\leq 90^{\circ}\) & 35.75 & 15.25 \\
70 & 37.95 & 22.34 \\
75 & & 40.06 & 29.14 \\ \hline \end{tabular} \({}^{1}\) The \({}^{\circ}\)Ave.\({}^{\circ}\) represents the average. \({}^{2}\) The \({}^{\circ}\)Imp. Rate\({}^{\circ}\) represents the improvement rate, which is defined as \(\frac{\text{Ave.}(i_{orb})-\text{Ave.}(i_{\text{spec}})}{\text{Ave.}(i_{\text{spec }})}\times 100\%\), where \(i_{orb}=60^{\circ},65^{\circ},70^{\circ},75^{\circ}\).  
\end{table} 
Table 2: Statistical results of the average elevation angles of IGSO satellites with different inclinations and their improvement rate compared those with a 55\({}^{\circ}\) IGSO inclinations. The first satellite of the Galileo 2 nd generation is anticipated to launch before the end of 2024, and the full Galileo constellation will consist of 36 satellites ([PERSON] et al., 2016; [PERSON] et al., 2017). Figure 3 shows the satellite visibility and GDOP of the 1 st generation Galileo (Galileo 1 st) and the full constellation (Galileo 2 nd). With the number of total satellites increasing, the visible satellites of Galileo 2 nd range from 11 to 14 (average: 12.80), which is about four more than that of the 1 st generation (average: 8.53). Ranging from 2.02 to 2.78 (average: 2.26), the improvement rate of the Galileo 2 nd GDOP is 18.4% on average. In general, the GDOP of the Galileo 2 nd is smaller in mid-low latitude regions (range from 2.08 to 2.41) than it is at 75\({}^{\circ}\leq|B|\leq\) 90\({}^{\circ}\) latitude regions (range from 2.44 to 2.72). \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**System**} & \multicolumn{4}{c}{**Ave.**} \\ \cline{2-5}  & **Satellite** & **Imp. Rate (\%)** & **GDOP** & **Imp. Rate (\%)** \\ \hline BDS-3-simulation & 10.49 & 0.6 & 2.52 & 2.1 \\ Galileo 2 nd & 12.80 & 50 & 2.26 & 18.4 \\
30-GLONASS & 10.59 & 28.2 & 2.45 & 10.1 \\ four-GNSS-NG & 44.86 & 17.2 & 1.19 & 7.8 \\ \hline \end{tabular}
\end{table} 
Table 3: Statistical results of satellite visibility and GDOP for BDS-3-simulation, Galileo 2 nd, 30-GLONASS, and the four-GNSS-NG, and its improvement rate compared to BDS-3-nominal, Galileo 1 st, 24-GLONASS, and four-GNSS, respectively. Figure 2: Satellite visibility and GDOP of BDS-3: (**a**) visible satellites of BDS-3-nominal; (**b**) visible satellites of BDS-3-simulation; (**c**) GDOP of BDS-3-nominal; (**d**) GDOP of BDS-3-simulation. To improve performance over Russia, GLONASS plans to add IGSO or high Earth orbit (HEO) satellites; however, orbit parameters are still under discussion [11,12]. In this study, we simulate six IGSO (GLONASS-I) satellite-enhancement constellations. Satellites equally operate in three orbits with a center longitude of 60\({}^{\circ}\)E, 90\({}^{\circ}\)E, and 120\({}^{\circ}\)E, respectively, to ensure the availability of augmentation over the whole of Russia. The inclination of the orbital planes is 64.8\({}^{\circ}\), which is the same as that of the GLONASS MEO satellites.