## 3 Simulation Results and Discussion
### Performance Analysis
#### 3.2.2 GDOP Test  
The previous section verified that sufficient visibility of GNSS satellites at the geostationary orbit can be ensured. Thus, the precise analysis of GDOP, which refers to positioning accuracy via the geometric arrangement of satellites in the space environment, is critical. The simulation in this study analyzes GDOP only when the multi-GNSS is utilized so that the minimum four satellites are ensured. The mean GDOP simulation results of GNSS satellites for 24 h according to the longitude of the geostationary orbit is shown in Figure 7. The GDOP of the main lobe signal was only about 89.77 on average, where its GDOP intermittently exceeded the \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Figure**} & \multirow{2}{*}{**Type**} & \multicolumn{4}{c}{**Main Lobe Signal Only**} & \multicolumn{4}{c}{**(Main + Side) Lobe Signal**} \\ \cline{3-11}  & & **GPS** & **GLO** & **GAL** & **BDS** & **Multi** & **GPS** & **GLO** & **GAL** & **BDS** & **Multi** \\ \hline \multirow{3}{*}{All} & At least 4 (\%) & 0 & 0 & 0 & 23.33 & 100 & 100 & 0 & 100 & 100 & 100 \\  & Min (\#) & 1.63 & 0.42 & 2.13 & 1.67 & 5.96 & 6.15 & 2.60 & 7.06 & 6.02 & 21.95 \\  & Avg (\#) & 1.90 & 0.58 & 2.17 & 3.01 & 7.66 & 6.80 & 2.80 & 7.16 & 8.47 & 25.22 \\  & Max (\#) & 2.11 & 0.72 & 2.23 & 6.58 & 11.35 & 7.35 & 3.04 & 7.25 & 12.31 & 28.79 \\ \hline \multirow{3}{*}{Above} & At least 1 (\%) & 84.72 & 45.83 & 96.53 & 90.28 & 100 & 100 & 99.30 & 100 & 100 & 100 \\  & At least 4 (\%) & 5.56 & 0 & 21.53 & 5.56 & 97.92 & 95.14 & 29.86 & 99.30 & 91.67 & 100 \\  & Min (\#) & 0 & 0 & 0 & 0 & 2 & 2 & 0 & 3 & 2 & 14 \\  & Avg (\#) & 1.79 & 0.63 & 2.16 & 1.76 & 6.35 & 6.49 & 2.90 & 7.17 & 6.08 & 22.63 \\  & Max (\#) & 6 & 3 & 4 & 4 & 11 & 11 & 5 & 11 & 10 & 31 \\ \hline \multirow{3}{*}{Opposite} & At least 1 (\%) & 85.42 & 45.83 & 96.53 & 100 & 100 & 100 & 99.30 & 100 & 100 & 100 \\  & At least 4 (\%) & 4.86 & 0 & 21.53 & 97.22 & 100 & 96.53 & 29.86 & 99.31 & 100 & 100 \\  & Min (\#) & 0 & 0 & 0 & 3 & 5 & 2 & 0 & 3 & 6 & 18 \\  & Avg (\#) & 1.76 & 0.63 & 2.17 & 5.23 & 9.79 & 6.53 & 2.90 & 7.17 & 10.83 & 27.42 \\  & Max (\#) & 5 & 3 & 4 & 8 & 14 & 10 & 5 & 11 & 15 & 35 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: GNSS satellite visibility for 24 h in geostationary orbit about the longitude. Figure 6: GNSS satellite visibility for 24 h in geostationary orbit. threshold (i.e., 100) many times. Compared to the GDOP in the ground open space, which is approximately two to three, GDOP in the geostationary orbit is significantly larger. Thus, a performance degradation is expected due to the increase in position errors for point positioning. To overcome this problem efficiently, a method of using the side lobe signal is presented together. GDOP is 9.94 when the side lobe signal is used, and the signal could be observed at all times. The results also clearly show the effects of non-MEO BDS satellites. In Figure 7a, when using main lobe signal only, it can be seen that the GDOP had significantly low values corresponding to the visible area of the non-MEO BDS satellites compared to the area that is not. Next, the environment of the geostationary orbit over the Korean Peninsula compared with the other side was investigated thoroughly. As seen in the sky plot shown in Figure 8, it was verified that satellites were not observed around the nadir due to the signal blockage by the earth. The GNSS satellites were concentrated in the nadir direction when viewing from the receiver, and GNSS satellites are not observable due to the shadow effect of the earth in the nadir center direction. Thus, the GDOP was high. Since satellites are observed as a form of ring, a suitable beam pattern of the receiving antenna (e.g., antenna with donut shape beam pattern) should be used [26]. This analysis was the same for the above and opposite of Korea. However, if there was a difference, the GEO and IGSO satellites of the BDS were observable in the opposite of Korea, so that motion can be identified. Figure 8: _Cont_.