## 3 LEO-LEO Occultation Simulation and Assessment Methods  
### LEO-LEO Occultation Geometry Simulation Model  
As shown in Figure 1, during a LEO-LEO occultation active period, the transmitting signal ray path from the Tx first passes through the atmosphere and is captured by the Rx; then, with the relative motion of the Tx and Rx, the atmosphere is vertically scanned and remotely observed. A LEO-LEO occultation event occurs when the connecting line between the Tx and Rx and its tangent point with the atmospheric layers satisfy a certain geometric relationship. The mathematical criteria are: 1 the tangent point is between the Tx and the Rx; 2 the height of the tangent point is between the bottom and the top of the atmosphere; and 3 the azimuth angle of the Tx relative to the Rx's running direction is in a particular range. The unperturbed, two-body orbits are always conic sections; therefore, the Earth orbits are commonly described as an ellipse by Keplerian elements, i.e., eccentricity (e), semimajor axis (a), inclination (i), longitude of the ascending node (\(\Omega\)), argument of periapsis (AP, \(\omega\)), and true anomaly (v). However, real Earth orbits have perturbations due to the gravitational pull of bodies other than the Earth, the nonsphericity of the Earth, atmospheric drag, relativistic effects, radiation pressure, electromag  
Figure 1: Occultation geometry between one LEO transmitting satellite (Tx) and one LEO receiving satellite (Rx), schematically illustrating the signal ray path passing through the atmosphere and the tangent point of a rising occultation event. The solid-line squares and dotted-line squares represent the occultation event starting time (near-surface) and ending time (top-of-atmosphere) positions of the LEO-LEO satellite pair, respectively. Therefore, in the simulation, Rx and Tx satellite positions and velocities were calculated from two-line element (TLE) sets through the simplified general perturbations-4/simplified deep space perturbations-4 (SGP4/SDP4) model propagator, which takes the general and deep space orbital perturbations into account [53]. Among these perturbations, the right ascension of the ascending node (RAAN) precession is a key factor impacting the long-term LEO-LEO occultation event numbers and distributions, which can be described as Equation (1), and the real RAAN at epoch \(t\) can be estimated by Equation (2) [54]:  
\[\dot{\Omega}=-1.5J_{2}n\Big{(}R_{E}/(a-a\cdot e^{2})\Big{)}^{2}\cos i \tag{1}\]  
\[\Omega=\Omega_{0}-\Big{(}\dot{\Omega}_{E}-\dot{\Omega}\Big{)}(t-t_{0}) \tag{2}\]  
In these equations, the constant \(J_{2}=1.08262668\times 10^{-3}\) is a zonal coefficient according to the Joint Earth Gravity Model 3 (JGM-3) geopotential model [54], \(n\) is the angular velocity, \(a\) denotes the semi major axes, \(e\) is the eccentricity, \(R_{E}=6378\) km is the Earth's equatorial radius, and \(\Omega_{0}\), \(\dot{\Omega}_{E}\), and \(\dot{\Omega}\) are the RAAN at the reference time \(t_{0}\), the angular velocity of the Earth's rotation, and the drift angular velocity of the RAAN, respectively. From Equation (1), one can find that when the orbital inclination is \(90^{\circ}\), the RAAN precession angular velocity is 0, the prograde orbit RAAN precession angular velocity is greater than 0, and the retrograde orbit RAAN precession angular velocity is less than 0; that is, the RAAN precession of the prograde and retrograde orbits proceeds in different directions. Additionally, the greater the difference between the orbit inclination and \(90^{\circ}\), the greater the precession speed, while the higher the orbit altitude, the lower the precession speed. All the possible ray paths connecting the Tx and Rx were considered by setting the sampling rate to 3 s, which we found to be adequate to safely capture all events while at the same time affordable in terms of computing time for the large number of simulation cases.