## 4 Experimental Results and Analysis
### Impacts of the Orbital Parameters on LEO-LEO Occultation Events
#### 4.1.4 Impact of the RAAN  
As shown in Figure 2b, for the polar orbit constellation, the number of 3-month occultation events varies with the Rx and Tx RAAN greatly in the range 0-5500, which is mainly related to the RAAN difference of the Rx and the Tx. When the RAAN difference of the Rx and Tx is 180 deg, the two orbits are co-planar and counter-rotating, which makes it easy to frequently form a limb-sounding (occultation) geometry and generate occultation events. Therefore, the number of 3-month occultation events reaches the maximum of about 5500 in this type of configuration. When the Rx RAAN and the Tx RAAN are equal, the Rx and Tx orbits are co-planar and running in the same direction, and the number of 3-month occultation events is almost 0.  
To further explore the impact of the RAAN on the spatial-temporal distribution of occultation events, Figures 5 and 6 show the global coverage and the number trend beside the GCF trend of the occultation events for nine representative constellations, i.e., Rxraan30-Txraan30, Rxraan30-Txraan90, Rxraan30-Txraan150, Rxraan30-Txraan180, Rxraan30-Txraan210, Rxraan30-Txraan240, Rxraan30-Txraan270, Rxraan30-Txraan300, and Rxraan30-Txraan330.  
As shown in Figure 5, fixing the Rx RAAN at 30 deg, when the Tx RAAN is 30 deg (Figure 5a), the global coverage fraction of 3-month occultation events is only 3%, and the occultation events are scattered across various latitude zones. With the increase in the Tx RAAN, the global coverage fraction first increases and then decreases. When the Tx RAAN is set to 210 deg (Figure 5e), the global coverage fraction reaches the maximum (97%), and when the Tx RAAN is set to 180 deg (Figure 5d) and 240 deg (Figure 5f), the global coverage fraction is about 91%. The RAAN difference of the Rx and Tx is very important to the occultation global coverage and distribution. When it is close to 180 deg, global coverage of occultation events can form. For a RAAN difference of 60 deg (Figure 5b,i) and 120 deg (Figure 5c,g), the occultation events are mainly distributed in high latitude zones, and the GCF is low. When the difference is 90 deg (Figure 5h), the occultation events are mainly distributed in the \(-\)70 degto \(-65^{\circ}\) latitude zone and the 65-70\({}^{\circ}\) latitude zone, and the global coverage fraction is only 7%.  
Figure 5: The global coverage of LEO-LEO occultation events for representative polar orbit constellations composed of a Rx satellite and a Tx satellite with particular orbital RAANs, illustrating the occultation numbers per 5\({}^{\circ}\)\(\times\) 5\({}^{\circ}\) latitude and longitude grid cell, while the GCFs were calculated using grid cell areas according to Equation (12).  
Figure 6: The accumulated time dependence of the LEO-LEO occultation event number (orange lines) and the global coverage fraction (GCF) (blue lines) for representative polar orbit constellations composed of a Rx satellite and a Tx satellite with particular orbital RAANs.  
Figure 6 illustrates the occultation number and the global coverage fraction trend with time for the constellations shown in Figure 5. One can see that the occultation number and global coverage fractions of these constellations increase almost linearly with time. When the RAAN difference of the Rx and Tx is 180\({}^{\circ}\) (e.g., Rxraan30-Txraan210), the rate of growth in the occultation number is at its maximum (about 60 events per day), and the global coverage fraction first grows quickly and then tends to slow. When the RAAN difference of the Rx and Tx is 0\({}^{\circ}\) (e.g., Rxraan30-Txraan30), the rate of growth in the occultation number is less than one event per day, and the 3-month global coverage fraction is only 3%. Moreover, with the increase in the RAAN difference of the Rx and Tx from 0\({}^{\circ}\) to 180\({}^{\circ}\), the number of occultation events increases gradually from about 1 event to 60 events per day.