**4. Interconnecting satellites to compose satellite networks**

This ISL concept revolutionized the perception of satellite constellations, which started to be conceived as networks composed of satellites. Werner discusses the challenges of deploying these networks and concludes that satellite mobility is a key factor in the stability of the links that compose the network [7]. In particular, the nature of an ISL is determined by the relative motion between two satellites. Therefore, an ISL may be feasible and active during a lapse of time depending on the orbits.

These temporal links are also known as satellite contacts and drive the topology representation of a satellite network. In particular, a topology is represented by a set of nodes that are interconnected by edges. Werner proposed the representation of a satellite network topology with the concept of a snapshot. A snapshot of a network is the topology representation with the connections established between the satellites that remain stable during a lapse of time. The creation or destruction of an ISL results in the generation of another snapshot. The overall generated snapshots compose a sequence that represents the evolution of the topology over time.

**Figure 2** presents an example with three snapshots, identified by *sk* (*s*0, *s*<sup>1</sup> and *s*2), that remain stables between the lapse of times *ti* (snapshot *s*<sup>0</sup> remains stable between *t*<sup>0</sup> and *t*1). This evolution is characterized by the movement of the nodes, which in this case corresponds to the orbit trajectory. Therefore, the number of snapshots and its stability time depends directly on satellite orbits and their communications means. Just as a brief reminder, this motion is determined by a set of parameters that allow estimating the complete trajectory of a satellite. Furthermore, this trajectory follows a periodic pattern, if no orbit disturbances are considered. Consequently, the sequence of snapshots is periodic and predictable.

The transition of these snapshots may represent an evolution of a satellite network in which parts of it are isolated during a lapse of time. These isolated fragments of the network may be sporadically connected with other satellites over time, depending on the nature of satellite contacts. This intermittent connectivity encourages to define an environment in which network partitions are frequent, and satellites must leverage opportunistic and sporadic satellite contacts to communicate. The understanding of this temporal nature of satellite contacts becomes crucial in the definition of end-to-end routes in this scenario.

Delay and Disruption Tolerant Networks (DTN) approaches envision the establishment of routes in this disruptive environment [9]. A proposal to solve this disruption is a store-and-forward approach, that is a way to store messages from a satellite to lately propagate them to another satellite was conceived to leverage the opportunistic and sporadic satellite contacts. Nevertheless, the definition mechanisms to identify end-to-end routes in this disruptive environment were largely discussed.

Among the different proposed techniques, a classification was conducted in [22] based on the generation of replicas of the messages. In this regard, a protocol that replicates messages is known as a replication-based protocol, which is characterized by delivering the data to the destination according to a probability, based on the number of replicas generated. Alternatively, a forwarding-based protocol estimates future satellite contacts to define routes over time, requiring a larger computational effort. Authors in [22] conclude that it must exist a balance between future knowledge of the network evolution, and the computational capacity.

**Figure 2.** *Representation of different snapshots (sk) over time (ti) associated with five satellites. Figure from [17].*

*From Monolithic Satellites to the Internet of Satellites Paradigm: When Space, Air… DOI: http://dx.doi.org/10.5772/intechopen.97200*

The Iridium constellation aimed to compensate this topology evolution and mitigate network disruption by conceiving a custom constellation architecture that would later be known as LEO Satellite Networks. Ekici et al. started to work with a satellite constellation configuration that mitigates this mobility impact on the communications performance [6].

The constellation is designed with an orchestrated and fixed architecture in which satellites are specifically located on purpose. This constellation builds a mesh architecture in which each satellite has four satellite-to-satellite interfaces to communicate with its neighbors. The resulting topology of the network is characterized by nodes located in a grid with a set of rows and columns. Despite this design to mitigate the disruption, satellites are always in motion. However, the movement of the satellites in this constellation results in a continuous shift of the satellites in the column axis of the mesh.

This coordinated movement ensures that from the local view of a satellite the connections with its neighbors remain unaltered. This condition is satisfied in the most populated latitudes because when the satellites pass over the polar region the formation cannot be respected. Moreover, an abstract line represents a seam in this mesh that separates the direction of the satellites. On one side of this seam, the satellites move from the South to the North, while on the other side they travel in the opposite direction. Traditionally, communications through this seam were forbidden.

**Figure 3** presents this satellite constellation with its corresponding mesh topology.

This constellation is founded over two classes of ISL, defined according to the vicinity of the neighbor. The intra-plane ISL allows a satellite to communicate with its two neighbors that are located in the same orbit plane. Meanwhile, the interplane ISL allows a satellite to communicate with its two neighbors located in adjacent planes. This differentiation was conducted because the nature of both ISL types differs: intra-plane ISL are always stable and feasible, while inter-plane ISL may be disconnected in the polar region. The goal to relay data from ground users was satisfied by defining the concept of a virtual node. This kind of node is associated with a logical location that corresponds to a square of an entire grid that covers the entire Earth surface. Each satellite is then associated with a logical location when it passes over this surface square, being responsible to serve the users allocated in this area. Due to the satellite movement, the satellite changes over time their logical

#### **Figure 3.**

*Representation of (a) the constellation design that represents a LEO satellite network, and (b) its resulting map to a mesh topology. Figure from [17].*

location when they bypass the corresponding square. Furthermore, the logical location is also mapped in the mesh topology by vertical and horizontal coordinates that correspond to the column and row numbers.

The LEO Satellite Network concept was extended in future researches by integrating other satellites in further orbit regions. The combination of multiple satellite systems stood out as a potential architecture to offer new capabilities or improve the capacity achieved by their own.

Multi-Layered Satellite Networks (MLSN) are a system-of-systems architecture [23] compounded of distinct satellite constellations deployed at different altitudes, which corresponds to the layers in this system. This architecture was proposed in [8] to enhance the traffic capacity and the stability of a satellite network. The proposal leverages the visibility of the satellites located at higher altitudes that can orchestrate a group of satellites deployed in lower altitudes.

A hierarchical structure in which successively upper layers always gather lower layers is designed under the previous premise. Despite the original proposal did not specifically define the type and the number of layers, the LEO, MEO, and GEO were typically the three main layers associated with this network. In this configuration, GEO satellites would manage a group of MEO satellites, which at the same time each one would gather an ensemble of LEO satellites. Satellites located in the same layer can communicate among them using Intra-Orbital Links (IOL), while they are also able to interact with satellites in adjacent layers using ISL.

**Figure 4** illustrates this multi-layered architecture with the three main altitudes. This hierarchical architecture enhances the stability of the network thanks to the large visibility of upper-layer satellites. Despite the connections between the satellites still changes over time, the topology changes correspond to fluctuations of the low-layer satellites that belong to the group of an upper-layer satellite. This feature mitigates the influence of satellite mobility on network dynamism. Nevertheless, the architecture design of the low-layer satellite system may still provoke irregular changes in the topology. [24] discussed this behavior and suggested the use of a LEO satellite network—which ensures the mesh formation—as a lowest-layer

**Figure 4.** *Illustration of a MLSN with three layer. Figure from [17].*

*From Monolithic Satellites to the Internet of Satellites Paradigm: When Space, Air… DOI: http://dx.doi.org/10.5772/intechopen.97200*

satellite system. This satellite constellation simplifies the computation of end-to-end routes among the layers, and in the same layer. The integration of a LEO satellite network into the MLSN demonstrates the potential of this heterogeneous architecture, that may accept including multiple distinct satellite systems.

The architectures of these satellite networks were presented considering always the traditional concept of satellite missions, in which a constellation is properly defined. Nevertheless, the apparition of new trends in the space related to the New Space movement has motivated novel concepts of satellite networks with nonconventional strategies.
