**3. The era of low-altitude satellite constellations**

Gaffney et al. analyzed the feasibility of the new non-geosynchronous or nongeostationary satellite systems proposed on the US Federal Communications Commission (FCC) by different private companies [20]. These innovative proposals enhanced the communications end-to-end latency using MEO, and LEO satellites. Those orbit regions were not originally proposed because of the continuous relative movement—unlike the geosynchronous case—that satellite experience. Satellites in this configuration suffer from a small field of view and temporal contact with

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

ground devices, which leads to service disruptions. Therefore, satellite constellations have risen as a promising architecture to deal with this challenge, by performing user handovers between adjacent satellites. An in-depth study of the benefits of applying these constellations was presented in the dissertation [21]. Kashitani remarks that satellite constellations at LEO region can serve a larger number of customers with a relatively reduced deployment cost as compared to their MEO homonyms.

These performance expectations encouraged the development of numerous LEO satellite constellations by private companies. Globalstar emerged as a private company that could provide satellite phone and low-speed data communications with a dedicated satellite constellation [5]. The first satellites were launched in 1998 to start composing a constellation that would be finished in 2000. The Globalstar constellation was composed of satellites that worked as bent pipes or repeaters between two users located in the same spot. Although this design enables remote users to communicate, the system was also limited by having a common satellite to interconnect the end devices. If this common satellite was not available, the communication was not feasible.

The Orbcomm company developed a specific constellation to deal with this situation offering discontinuous coverage between end-users. Unlike the previous case, this constellation was designed to provide low-speed data communications, and it was not capable to offer telephone services. To provide this discontinuous coverage, satellites in this constellation were able to store incoming messages to download them later over another region. This store-and-forward mechanism was crucial to achieve this desired performance and became a key technology to develop the concept of Disruption Tolerant Networks (presented in the following section).

Alternatively to the previous constellations, Iridium Communications company decided to deploy its constellation to provide voice and data coverage to custom satellite phones [4]. This new constellation, known as Iridium, extended the original concept by interconnecting satellites with radio interfaces to relay data down to the ground; i.e. the development of Inter-Satellite Links (ISL). By defining a custom and specific constellation, a route between two end-users and composed of satellites could be defined. This revolutionized the concept of satellite constellation because they could transfer data among the satellites with a reduced delay. The original Iridium constellation was extended with a new generation of 66 satellites, called Iridium NEXT, in 2017. This extension aimed to enhance the satellite capacity from 2.4 kbps to 1.5 Mbps, using high-throughput techniques.

The LEO satellite constellations proposed for broadband telecommunications revolutionized the concept by evolving from a repeater-based approach for voice and data, passing through a store-and-forward solution for data, and reaching an interconnected architecture for voice and data. This last approach presented a constellation as a set of satellites that compose a network. This new satellite network concept paved the way for new interconnected architectures.
