## 4 Experimental Results and Analysis
### Impact of the RAAN Precession on the LEO-LEO Occultation Events  
#### 4.2.1 RAAN Precession Impact Analysis  
If the RAAN precession velocities of the Rx and Tx are different, the RAAN difference of the Rx and Tx will change with time, and the RAAN difference is a sensitive parameter for the LEO-LEO occultation event number and distribution. As shown in Figure 4, for the constellations with supplementary Rx and Tx inclinations, the number and global coverage fraction increase nonlinearly with time, and the occultation number grows very slowly during certain periods.  
To explore the reason for the nonlinear trend of the occultation number and coverage fraction curves, Figure 7 illustrates the RAAN precession phenomenon of the Rai100, Txi100, Rai80, Rai140, and Txi40 satellite orbits within the three months. The thick solid lines show the absolute RAAN differences of the Rx and Tx inclinations of the Rai100-Txi100, Rai80-Txi100, and Rai140-Txi40 constellations. One can see that when the Rx and Tx orbital inclinations are equal to each other (e.g., Rai100-Txi100), their RAAN precession velocities are similar and the absolute RAAN difference remains around 180\({}^{\circ}\). When the Rx and Tx orbital inclinations are supplementary (e.g., Rai80-Txi100 and Rai140-Txi40), their RAAN precession velocities are in opposite directions, and the absolute RAAN differences change periodically between 0\({}^{\circ}\) and 180\({}^{\circ}\), which is consistent with the RAAN precession law described by Formula (1).  
When the absolute RAAN differences of the Rx and Tx are close to 180\({}^{\circ}\), it is easy to form an occultation geometry and the total number of occultation events increases rapidly. When the absolute RAAN differences of the Rx and Tx are close to 0\({}^{\circ}\), it is difficult to form an occultation geometry and the increase in the total number of occultation events stops. Comparing the relevant curves in Figures 4 and 7, the corresponding time periods are consistent with the abovementioned change trends of the occultation numbers and RAAN differences. This is also the reason why the NPO and MIO constellations with supplementary Rx and Tx inclinations have reduced occultation numbers.  
However, for the NEO constellations, when the Rx and Tx inclinations are supplementary, the two orbits are counter-rotating and easily form the occultation geometry. The opposite direction of the RAAN precession accelerates the relative motion of the Rx and the Tx and helps to form the occultation geometry. Therefore, the NEO constellations with supplementary Rx and Tx inclinations have a large number of occultation events. For the NEO constellations with the same Rx and Tx inclinations, the Rx and Tx RAAN precession velocities are similar, and the RAAN difference remains around 180\({}^{\circ}\) for a long time. This means that the Tx and Rx satellites are running in the same direction. In this setting, it is difficult to form an occultation geometric relationship.  
For the MIO constellation, when the Rx and Tx orbital inclinations are supplementary, the absolute RAAN differences change periodically between 0\({}^{\circ}\) and 180\({}^{\circ}\). The periods of the co-planar setting are short, so the occultation number is small. When the Rx and Tx orbital inclinations are equal, although the RAAN differences remain at 180\({}^{\circ}\), the dihedral angle between the Rx and Tx orbital planes is large, which makes it difficult for the Rx and Tx satellites to see each other.