## 7 Ground-Segment Optimization Aspects
### Applicable Optimization Methods for Ground Segment
#### 7.2.2 Long Short-Term Memory (LSTM)
The specific parameters the software uses to simulate the satellite channel models are described in [168], which, in turn, are modified versions of Third-Generation Partnership Project (3 GPP)/ITU-R channel models. Table 2.0 DOP metrics comparison for Oneweb and Starlink LEO constellations with two different elevation masks: (**a**) 10\({}^{\circ}\) elevation mask and (**b**) 50\({}^{\circ}\) elevation mask. Table 2.1 Example of Coverage Maps for Two Selected LEO Constellations: Oneweb (**top**) and Starlink (**bottom**), as number of satellites in view per Earth point for two different elevation masks: 10\({}^{\circ}\) in the left-side plots and 50\({}^{\circ}\) in the right-side plots. From Figure 11, we can observe that the received \(C/N_{0}\) from OneWeb satellites is about 5 dB lower with respect to the Starlink constellation. Oneweb typical \(C/N_{0}\) is about 45 dB-Hz, while Starlink is 50 dB-Hz. The \(C/N_{0}\) levels are mainly influenced by: (i) the carrier frequency used by each constellation and (ii) the altitude of the orbit. Even though both constellations use a similar carrier frequency, in Ku-band (e.g., 12 GHz), their orbital altitude and orbital configuration is different. While Oneweb satellites are to be at altitudes of about 1200 km, Starlink satellites are to be distributed in orbits closer to the Earth, between 300 km and 600 km, thus offering a better link budget than those placed at higher altitudes. This is the main reason for the difference between \(C/N_{0}\) levels; nevertheless, the \(C/N_{0}\) distributions, as seen from the histograms in Figure 11, are very similar, and they resemble an exponential distribution. Such information can be incorporated into an optimization problem, for example, by imposing the constraint of a minimum desirable \(C/N_{0}\) at any Earth point or in a target region. As we have seen in Figures 9-11, we have many different metrics that can be used from the user perspective to optimize those satellites contributing more and in a better way to the final PNT solution. ### Applicable Optimization Methods for User Segment

Besides the overlapping methods that we have already explained in Sections 6 and 7, Table 2 identifies the main applicable optimization methods for our selected problems in the user segment as ML, RL, Multi-Agent Reinforcement Learning (MARL), and KF, as well as the more traditional approaches, such as search methods based on modeling and routing algorithms (Dijkstra, A*, etc.). In this subsection, we will provide an overview of these methods with the exception of the ML-based approaches (including RL and MARL), which are too complex for us to provide a brief mathematical overview in this paper. Although we will include these methods in our comparison discussions, for details regarding those methods and how they can be used with regards to the user segment in LEO networks, we encourage readers to refer to the cited studies [30; 32; 41; 42]. Before we start with the optimization methods, it is worth briefly talking about some traditional methods. Although not exactly methods of optimization, some traditional methods are still used to address some of the problems we included in Table 2, at the very least, as a reference for more complex methods. Such a method is the 'Elevation Method', which is a simple and straightforward method of satellite selection in the user segment. The method sorts the satellites using their elevation angles and keeps the \(n\) satellites with the largest values. If there are more satellites above the mask than there are tracking channels, the lowest elevation satellites are excluded. While such a method of removing the lowest

Table 2.2 Example of \(C/N_{0}\) distributions across the Earth points for LEO constellations Oneweb (blue) and Starlink (orange). satellites can increase the DOP-based performance metrics, it lacks other important factors, such as satellite health or weighting factors. Simply looking at the elevation angle discards the possibility of using these additional metrics.