## 4 Experimental Results and Analysis  
### Impacts of the Orbital Parameters on LEO-LEO Occultation Events  
#### 4.1.1 Orbital Parameter Setup for the Two-Satellite Experimental Demonstration Mission  
Based on a pair of Rx and Tx satellites, the impacts of the orbital parameters on the number and spatial-temporal distribution of LEO-LEO occultation events were analyzed using the variable separation approach, in which the analyzed target parameter was set as a discrete variable while other parameters were fixed as certain values. Commonly, near-circular orbits are adopted for the LEO Rx. In this case, the impact of the orbital eccentricity on the occultation events is small for typical circular-orbit eccentricities of order 0.0001 in real cases (e.g., near 0.0005 in the case of the International space station). The eccentricity of both the Rx and Tx orbits was hence fixed at 0.0001 and this parameter not further analyzed. The true anomaly is a constantly changing variable with time, while the impact of the initial mean anomalies (M0) on the two-satellite constellation occultation events is very small; therefore, the M0 of the Rx and Tx orbits can be set as arbitrary values and here they were fixed at 210\({}^{\circ}\) and 30\({}^{\circ}\), respectively. The inclination, RAAN, and AP are three elements defining the orientation of the orbital plane in which the Rx and Tx orbits are embedded, while the semi major axis (a) defines the size of the orbits and strongly relates to the satellite's height and velocity. Hence, there are in principle four key orbital parameters for LEO-LEO occultation orbital and constellation design. Since our test experiments indicated that, for the near-circular orbits, the impact of AP on the number of LEO-LEO occultation events is very small, the AP of both the Rx and Tx orbits was fixed at 80\({}^{\circ}\) and this parameter not further analyzed. In the numerical study, three experimental groups named 'i', 'raan', and 'h', using lowercase letters to represent the research target parameters of inclination, RAAN, and height, respectively, were set up. The Rx and Tx orbital elements named 'I', 'RAAN', 'H', 'AP', and 'E' use uppercase letters instead, and the setups of the orbital parameters are shown in Table 1. The inclinations of Rx and Tx orbits were set to discrete values from 10\({}^{\circ}\) to 170\({}^{\circ}\) with a 10\({}^{\circ}\) step length for group 'i' and fixed values at 90\({}^{\circ}\)/90\({}^{\circ}\) for the other groups. The RAANs of Rx and Tx orbits were set to discrete values from 30\({}^{\circ}\) to 360\({}^{\circ}\) with a 30\({}^{\circ}\) step length for group 'raan' and fixed values at 180\({}^{\circ}\)/0\({}^{\circ}\) for the other groups. The heights of the Rx and Tx orbits were set to discrete values from 400 km to 1300 km with a 100 km step length for group 'h' and fixed values at 500 km/600 km for the other groups. \begin{table}
\begin{tabular}{c c c c} \hline
**Orbit Elements** & \multicolumn{3}{c}{**Experimental Groups**} \\ **(Rx/Tx)** & **i** & **raan** & **h** \\ \hline
**I (\({}^{\circ}\))** & 10:10:170/10:10:170 & 90/90 & 90/90 \\
**RAAN (\({}^{\circ}\))** & 180/0 & 30:30:360/30:30:360 & 180/0 \\
**H (km)** & 500/600 & 500/600 & 400:100:1300/400:100:1300 \\
**AP (\({}^{\circ}\))** & 80/80 & 80/80 & 80/80 \\
**M0 (\({}^{\circ}\))** & 210/30 & 210/30 & 210/30 \\
**E** & 0.0001/0.0001 & 0.0001/0.0001 & 0.0001/0.0001 \\ \hline \end{tabular}
\end{table} 
Table 1: The main orbital elements of the transmitting and receiving satellites for a two-satellite experimental constellation. 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.