## 3 The Three-Segment Optimization Architecture in LEO
The main take-away idea from Figure 2 is that certain optimization methods can be used to solve different problems in different segments, as a method's applicability is determined by problem formulation as well as the problem's nature. Although space-segment optimization may deal with additional problems such as controller placement for LEO-based SDN (a controller in terms of SDN is an application that manages flow control for improved network management and application performance), its primary design problem is **constellation optimization**. Constellation optimization is one of, if not the most, critical aspect of LEO space-segment design, as the constellation parameters are directly related with critical operating parameters of all end-user applications, such as communication and navigation on autonomous vehicles. Some examples are: the orbital altitudes directly affect the latency of LEO satellite networks; the orbital plane positioning of satellites directly determine the coverage areas, which, at its turns, is related to the feasibility of user applications. In addition, the optimization of such parameters will have to deal with and successfully satisfy any regulatory criterion set by entities, such as International Telecommunication Union (ITU), Federal Communications Commission (FCC), or any other local or international regulatory entity. Related to the regulatory aspects, physical-layer parameters (e.g., frequency allocations, used modulations, maximum transmitted powers, etc.) must be taken into account in order to keep unintentional interference with the rest of systems (radio astronomy, already functioning GNSS systems etc.) to a minimum degree [43; 44]. Additionally, it should not be a surprise that some of the optimization parameters regarding different architectural segments (as well as some metrics) relate with each other in varying proportions, as seen from Table 2.  
In contrast to the space segment, the user segment deals with optimization problems that occur for particular application interests, and, as a result, it includes a very large number of optimization-related problems that are generally case-specific. Technically, any application that uses both LEO satellite systems and can be optimized is included in the 'user-segment' terminology. A simple example would be a communication network with swarm drones that uses LEO satellites, which have optimization aspects ranging from data transmission to network topology to the actual goal of the swarm application, such as obstacle avoidance or drone tracking. Another straightforward example is the satellite-selection problem in GNSS; this refers to finding the optimal number of satellites to track if there are more available satellites than necessary. As GNSS information can used in a variety applications, it is a broad enough problem to be provided as an example in Table 2. Additionally, the satellite-selection optimization problem is also an optimization problem  
Figure 2: Block Diagram of the Optimization Aspects with respect to the Three-Segment Architecture of LEO-Based Networks. that has to happen at the user-segment side, as it heavily depends on the user/vehicle's location and continuous motion. Of course, one can only select among visible satellites in cases where coverage is not an issue, and this is also related to the space-segment design. Therefore, the satellite-selection optimization problem is a good example of how problems from different LEO segments relate with one another. Unlike the other segments, the ground segment is quite straightforward to design, as it is generally the segment that monitors the network for control-related purposes. Due to this aspect, the ground segment has only one significant optimization problem: GS planning. This problem deals with the sky coverage of the GS and focuses on geographic and cost-related aspects. There also exists optimization problems that require being addressed in all three segments (not present in Table 2) for clarity purposes and because they fall outside the scope of this paper).