## 3 Simulation Results and Discussion
### Performance Analysis  
#### 3.2.1 Visibility Test of GNSS Satellites  
The number of visible satellites for each constellation averaged for 24 h observed at a GEO satellite located at the different longitude is shown in Figure 4. In Figure 4a the results show that if each of the GNSS system is utilized individually, the number of satellites necessary for point positioning does not satisfy the minimum number of required satellites, and the GNSS satellite visibility problem may occur. On the other hand, if all GNSS satellites are integrated and inter-operationally used at the receiver, the number of visible satellites becomes more than six, which verifies that the visibility required for point positioning is ensured. In addition, when the side lobe signal is utilized, as shown in Figure 4b, it can be seen that satellite visibility increases significantly. In this case, even if the single system satisfies frequently the minimum number of visible satellites, the multi GNSS system can increase navigation accuracy using satellite visibility advantage. As verified in Figure 4, the number of visible satellites in the GPS, GLONASS, and Galileo does not change significantly according to the longitude and does not satisfy minimum visibility in all areas. This is because GPS, GLONASS, and GALILEO are in the MEO satellite group whose orbital inclination angle is constant, whereas BDS has five GEO satellites and three IGSO satellites arranged in the IGSO, so that mean visibility is increased at a specific longitudinal section. Figure 5 shows the visibility of BDS satellites according to longitude divided by MEO, GEO, IGSO, and all, over 24 h. Since BDS includes non-MEO satellites, the difference in satellite visibility is evident about longitude. For MEO satellites, the satellite visibility remains constant. On the other hand, IGSO and GEO satellites differ by a maximum of more than four satellites for the main lobe. As a result, the Multi-GNSS GDOP trend is similar to the BDS GDOP, which is discussed later. The figure of using side-lobe signals indicates that the visible area and the number of the GEO and IGSO of the BDS have expanded. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Configuration** \\ \hline Simulation time & 0–86,400 sec (0–24 h), epoch interval: 600 s (10 min) \\ Orbit & GEO (Korean Peninsula (128\({}^{\circ}\)E), the opposite side of \\  & Korea (308\({}^{\circ}\)E)) \\ Multi-GNSS (number of satellites) & GPS (32), GLONASS (24), GALILEO (30), BDS (35) \\ Beam-width & Main lobe (23.5\({}^{\circ}\)), first side lobe (27\({}^{\circ}\)–39\({}^{\circ}\)) \\  & \(C/N_{0}\) threshold: 20 dB-Hz \\ Receiver setting & DLL bandwidth: 1 Hz \\  & Correlator spacing: 0.1 chip \\ Performance & Satellite visibility, GDOP, \(C/N_{0}\), UERE, Navigation Error \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameter setting. Next, the environments for GNSS satellite signal reception at the geostationary orbit over the Korean Peninsula (about 128\({}^{\circ}\)E) and the other side of Korea (about 308\({}^{\circ}\)E) were investigated. Figure 5(a),b show the results of a simulation of GNSS satellite visibility for 24 h at the geostationary orbit over 128\({}^{\circ}\)E longitude. Because the geostationary orbit over the Korean Peninsula is not a region that can receive non-MEO BDS satellite signals, the visibility of GNSS satellites due to BDS is not improved. Figure 4: GNSS satellite visibility for 24 h in Geostationary orbit with longitude. Figure 5: BDS satellite visibility for 24 h in Geostationary orbit with longitude. Figure 6: _Cont._  
On the other hand, the area on the other side of Korea can utilize non-MEO BDS satellite signals, resulting in a significant increase in BDS satellite visibility, as shown in Figure 6c,d. In this area, it is possible to obtain stand-alone navigation using even only BDS itself, and on average more than three satellites are observable using only main-lobe signals.