## 4 Results
### Regional Deployment Optimization
The performance results of the deployment for each scenario are shown in Table 5.  
It is shown that the increase in the number of GSs in the same deployment creates a concentration of GSs in same area and even increases the SPDOP value. The mean value of SPDOP decreases (enhancement in SPDOP distribution value) when the number of GSs increases. The SPDOP can reach 11.53 for the optimal deployment of 15 GSs, while the maximum value of the SPDOP is less than 7.6 for the deployment of 25 GSs. In the work of [PERSON] et al. [18], the mean SPDOP was determined for selected MEO BDS-2 satellites (C11, C12, C14) using two schemes with 30 GSs regionally and globally distributed. Table 6 shows the results of the mean SPDOP values and the corresponding standard deviations. Compared to the SPDOP values of the MEO satellites, the mean SPDOP of the LEO constellation is better with global or regional GS distribution. In scheme 2, the SPDOP is around 208 with high dispersion (Std of 189.43), where 30 GSs are selected with global distribution, while it is around 1.86 with low dispersion (Std of 1.06) for LEO satellites. Therefore, precise orbit determination should be better for LEO satellites than MEO satellites. * SPDOP with Ground Stations and Intersatellite Links  
ISLs are used to enhance the accuracy of a satellite's orbit. In this study, ISLs are evaluated for LEO SPDOP enhancements. LEO constellations are composed of a huge number of satellites compared to MEO (GNSS) constellations, which means the number of ISLs at each time is greater; therefore, the SPDOP value will be enhanced. To evaluate the SPDOP of LEO satellites and due to the large number of satellites (210), the first satellite in each plane is selected (for example, LEO0301 denotes the first satellite (01) in the third (03) plane), in order to evaluate the SPDOP for 14 days of observation (DOY 1 to DOY 14 of the year of 2023). Figure 13 shows the SPDOP value of 15 LEO satellites with only GSs versus the SPDOP with GSs and ISLs (case of optimal 15 GSs). It should be noted that there is no weighting of observations; so, all the measurements are considered with equal weight. SPDOP is improved using ISLs with visible GSs. It should be noted that the data visualization (of the mean SPDOP with 20 GSs and 25 GSs with ISLs) is not straightforward due to the similarity. For this reason, only the case of 15 GSs is shown. For the 15 LEO satellites considered in this evaluation of 14 days of observation, the mean SPDOP improved from 2.058 to 0.439, from 1.403 to 0.422, and from 1.33 to 0.409 for 15, 20 and 25 GSs, respectively. Using ISLs, SPDOP is enhanced and tends to reduce the dispersion between the three deployment scenarios of 15, 20 and 25 GSs. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scheme** & **Mean SPDOP** & **Std SPDOP** & **Observation** \\ \hline Scheme 1 & 207.97 & 189.43 & 30 GSs (focus on Eastern hemisphere) \\ Scheme 2 & 149.43 & 126.17 & 30 GSs (globally distributed) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Statistics of SPDOP of MEO satellites [18]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scenario** & **Mean SPDOP** & **Max SPDOP** & **Std of SPDOP** \\ \hline
15 GSs & 2.0544 & 11.5266 & 0.7973 \\
20 GSs & 1.3990 & 9.0000 & 0.4701 \\
25 GSs & 1.3301 & 7.5696 & 0.4687 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: SPDOP statistics for optimal deployment. The results of the SPDOP--mean, maximum and minimum values--for each deployment with 14 days of observation are shown in Table 7. The SPDOP value is dependent on the number of ISLs in each epoch. The LEO constellation, with a large number of ISLs, has an advantage in enhancing the precision of orbit determination when SPDOP is enhanced due to the large number of ISLs observed at each epoch. As is shown in Table 8, the number of ISLs at each epoch (1 min) is at least 37, while it can reach a maximum of 69. In 14 days of observation, the average number of ISLs observed at each epoch (1 min) is around 51.43 for each LEO satellite.