## 4 Experimental Results and Analysis
### Impact of the RAAN Precession on the LEO-LEO Occultation Events
#### 4.2.2 Impacts of the Height on the Near-Polar Orbit Constellation Occultation Events  
Although the Tx and Rx orbital planes are not fully co-planar, it is still easy for the near-polar orbit (NPO) constellation with the same or very similar Rx and Tx inclinations to form the LEO-LEO occultation geometry and achieve global coverage, with a similar observation efficiency as a fully polar orbit constellation. Therefore, it is valuable to further investigate the NPO constellations. The impacts of the height on the constellation Rxi98-Txi98, which is composed of two sun-synchronous orbits, and the constellation Rxi98-Txi82, which is composed of a sun-synchronous orbit and a supplementary orbit, were simulated using the variable separation approach. The main orbit parameters are shown in Table 2.  
Figure 8 illustrates the number of 3-month occultation events of the constellations Rxi98-Txi98 and Rxi98-Txi82 as a function of orbital height. As shown in Figure 8a, the variation in the occultation number with the height of the Rxi98-Txi98 constellation is quite similar to that of the Rxi90-Txi90 constellation shown in Figure 2. The lower the orbit, the higher the number of occultation events. However, as shown in Figure 8b, the 3-month occultation number of the Rxi98-Txi82 constellation is smaller than those of the Rxi90-Txi90 and Rxi98-Txi98 constellations due to the effect of the opposite-direction RAAN precession of the Rx and Tx satellites. Figure 7: Rx and Tx RAANs and their absolute differences for Rxi100 (red dashed line), Txi100 (red dotted line), 1 Rxi100-Txi1001 (red bold line), Rxi80 (magenta dashed line), 1 Rxi80-Txi1001 (magenta bold line), Rxi140 (blue dashed line), Txi40 (blue dotted line), and 1 Rxi140-Txi401 (blue bold line). In the case of Rxi98-Txi82, the impact of height is obvious, especially when the Rx and Tx orbital heights are equal as the number of occultation events drops sharply. To explore how the number of occultation events varies with height, Figures 9 and 10 show the global coverage and the number trend beside the GCF trend of the occultation events for the Rxi98-Txi82 constellations with different height combinations. As shown in Figures 9 and 10, when the Rx and Tx heights are equal, their velocities are the same, and the occultation events occur in the same latitude zone during each orbital period. Further, since the RAAN precession is in the opposite direction, the Rx and Tx satellites do not see each other for a long time. So, the occultation number is very small and the events are concentrated near the equator zone (a-c). As shown in Figure 9d-f, fixing the Rx height at 400 km, when the Tx height increases, the occultation number and the global coverage fraction increase. As shown in Figure 9g-i, fixing the Rx height at 1300 km, when the Tx height increases, the occultation number and the global coverage fraction also increase. The occultation number and the global coverage fraction of the Rxh1300-Txh1200 constellation reach a maximum of approximately 2400 and 61%, respectively. However, compared with the Rxi90-Txi90 and Rxi98-Txi98 constellations, the occultation number and the global coverage fraction of Rxi98-Txi82 are both smaller. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Orbit Elements**} & \multicolumn{2}{c}{**Experimental Groups**} \\ \cline{2-3}  & **Rxi98-Txi98** & **Rxi98-Txi82** \\ \hline
**I** (°) & 98/98 & 98/82 \\
**RAAN** (°) & 180/0 & 180/0 \\
**H** (**km**) & 400:100:1300/400:100:1300 & 400:100:1300/400:100:1300 \\
**AP** (°) & 80/80 & 80/80 \\
**M0** (°) & 210/30 & 210/30 \\
**E** & 0.0001/0.0001 & 0.0001/0.0001 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The main orbital elements of the transmitting and receiving satellites for the near-polar orbit (NPO) constellations. The chosen set of values for some elements is denoted in the format of StartValue:Step:EndValue; simulations were performed using an ensemble of configurations stepping through all value settings for these elements.