**8. Conclusions and way forward**

During the last decades, space has been populated by a wide variety of satellite systems. From monolithic GEO satellites to current constellation approaches, space missions have experienced a considerable evolution to provide new services to the users. The traditional television broadcast and phone calls lead to more resourcedemanding services based on current Internet applications. Constellations of LEO satellites have emerged as the necessary infrastructure to support these new services in space.

These constellations were originally conceived as independent satellites orbiting in an ad-hoc architecture. Nevertheless, they started to be considered as satellite networks when interactions between satellites were needed to satisfy the novel delay and throughput demands. The configuration of these interconnected satellites promoted the proliferation of different architectures to leverage on satellite altitudes or to mitigate satellite dynamics (e.g. MLSN, LEO Satellite Networks, etc.) All of them inspired satellite architectures that are currently working in space, such as

#### **Figure 7.** *NTN architecture proposed in the 3GPP. Figure from [18].*

the Tracking Data Relay Satellite System (TDRSS) or the Iridium constellation [4]. In this way, the New Space trend promoted the apparition of novel flexible and distributed architectures to keep evolving the space for future user demands, like the mega-constellations or the IoSat paradigm.

The next step of this space evolution seems to be associated with the ground network revolution experienced with the 5G. The possibility to extend the current infrastructure with seamless connectivity puts satellites as potential systems for this purpose. The existence of different entities that are working on standardize the integration of satellites with ground networks, conforming the NTN concept, is an example of how satellites will become more and more a reality in our infrastructure. **Figure 7** presents a conceptual view of the NTN architecture composed of satellites, High Altitude Platforms (HAP), and Low Altitude Platforms (LAP). Different missions have started to experiment with satellite capabilities to provide IoT connection around the Earth glove. Their success demonstrates the potential of these new systems (for ground networks) and helps to believe in a promising future with heterogeneous networks composed of space, air, and ground infrastructure. Only time will tell whether this paradigm would become a reality.
