## 5 Discussion
### Regional Deployment Optimization  
In view of the fact that precise orbit information is required for LEO constellation navigation, the global deployment of GSs offers a good opportunity for highly accurate orbit determination. When LEO satellites are visible to at least three GSs at a given time, the period without GS network observation tends toward 0. Therefore, at each epoch time, all satellite orbits can be determined with high precision.  
In regional deployment and according to the LEO satellite's revisit time (around 2 h), the period of GS coverage is around 10 min. In addition, the average observed period of each satellite increases in proportion to the width of the regional area. However, the previous results for global GS deployment suggest that even if all land is considered for deployment, having a high level of heavy observing coverage is limited by the available land on Earth. As more than 71% of the globe is oceans and seas, further study and analysis are required to determine the possibility of deployment in Antarctica and isolated island areas. This study is based on the supposition that any part of Earth--whether land, ocean or island--can be considered for GS deployment. Hence, the scenario of the regional distribution of GSs persists as a solution in cases where global distribution fails due to different causes.  
The SPDOP value of LEO satellites was evaluated based on a comparison between LEO, MEO, IGSO and GEO satellites. Selected satellites of the BDS constellation were evaluated using SPDOP values in the work of [PERSON] et al. [29]. Table 10 shows the mean SPDOP value for GSs only and for GSs and ISLs (in one-way ranging). The selected satellites in the mentioned study were in the GEO, IGSO and MEO constellations. It should be noted that the values in the table are the mean value and mean Std of SPDOP, according to [PERSON] et al. [29].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Satellite’s Orbit** & **GEO** & **IGSO** & **MEO** & **LEO** \\ \hline Mean SPDOP with GSs only & 475.72 & 354.27 & 185.10 & 2.06 \\ Std of SPDOP & 2.57 & 136.27 & 148.65 & 0.79 \\ Mean SPDOP with GSs + ISLs & 12.00 & 2.12 & 2.67 & 0.44 \\ Std of SPDOP & 0.75 & 0.91 & 1.34 & 0.09 \\ Reference & & [29] & & / \\ \hline \hline \end{tabular}
\end{table}
Table 10: SPDOP value comparison without and with ISLs.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Single-Heavy** & **Double-Heavy** & **Triple-Heavy** & **Quadruple-Heavy** \\
**Observing Coverage** & **Observing Coverage** & **Observing Coverage** & **Observing Coverage** \\
**1-HC** & **2-HC** & **3-HC** & **4-HC** \\ \hline Value & 100\% & 100\% & 97.37\% & 92.01\% \\ \hline \hline \end{tabular}
\end{table}
Table 9: Observation rate for the GS deployment of 3-heavy observing coverage (HC).  
It can be seen that SPDOP presents a low value for the LEO satellite compared to other satellites. The mean SPDOP value of the LEO is no more than 2.06 with 15 regionally distributed GSs (without ISLs in one-way ranging), while it exceeds 185, 354 and 475 for the MEO, IGSO and GEO satellites, respectively. In addition to excellent SPDOP distribution, the dispersion values of these satellites are obvious compared to LEO due to the large change in geometry. The strong impact of ISLs (used in one-way ranging) is obvious in their enhancement of SPDOP values.  
In the observation based on ISLs and GSs, the mean SPDOP is improved from 475.72 to 12.00, from 354.27 to 2.12, and from 185.10 to 148.65 for the GEO, IGSO and MEO satellites, respectively (according to [29]). For LEO satellites, and using ISLs in addition to GSs in observation, the SPDOP value improves significantly, reaching 0.44 due to the large number of ISL observations available in each epoch time.  
Compared to MEO and high Earth orbit (GEO and IGSO), the LEO satellite constellation presents an excellent SPDOP, which indicates a high precision of orbit determination.