## 2 Space Segment
### Satellite Constellations  
A satellite constellation is normally intended as a plurality of similar satellites working together as a system [61]. Unlike a single satellite, a constellation can provide global or near-global coverage, as it can be designed such that from everywhere on Earth (or most of the inhabited surface) at least one satellite is visible at any time. In constellations, satellites are typically placed in sets of complementary orbital planes and connect to a distributed ground stations network on Earth. Depending on the design, they may also use inter-satellite link, in optic or RF [62]. It is possible to classify satellite constellations in different ways, the first is by orbital altitude, for example low Earth orbit (LEO) constellations (e.g., OneWeb, London, UK, Starlink), medium Earth orbit (MEO) constellations (e.g., OSB) or even geostationary orbit (GEO) constellations (e.g., Inmarsat GX, Viasat-3), this usually comprises of a limited number of satellites, typically three or four. Another way of classifying satellite constellations is by constellation geometry, which is based around satellite positioning and orbit type. This, together with intended service and the limitations of the link budget, determines coverage, which can be global, regional, or targeted. There are a large number of possible useful orbits for satellite constellations, but circular orbits are a popular choice in communication constellations, as all the satellites are at a constant altitude requiring a constant strength signal to communicate and also minimizing the effects of precession [63]. At MEO and LEO, the common geometry types are mainly two: \"Walker star\" or polar constellation [64] and the \"Walker delta\" or rosette constellation [65]. A polar orbit is a circular orbit with orbital planes inclined at nearly 90\({}^{\circ}\) with respect to Earth's equator. The orbit is fixed in space, and the Earth rotates underneath. Therefore, a single satellite in a polar orbit provides, in principle, coverage to the entire globe, although there are long periods during which the satellite is out of view from a single observation point on Earth. This limitation, in a polar constellation, is overcome exactly by using multiple satellite equally spaced on the polar orbital planes, providing continuous coverage of the Earth surface by handing over the active communication link from one satellite to the following one on the same orbital plane. In this way, a polar orbit constellation in LEO is naturally providing global coverage of the Earth surface. An example of a polar constellation is provided in Figure 2.  
Some LEO and MEO constellations use a rosette design: they are characterized by what are called \"inclined orbits\" (with inclination substantially smaller than 90\({}^{\circ}\)). An inclined orbit constellation provides its best coverage in the areas where the Earth population is concentrated (at latitudes below 45\({}^{\circ}\)) but cannot provide a global coverage by itself. A third way of classifying satellite constellations is by frequency bands used for services, from L- and C- up to Ka- and V-band. The operational frequency band has an impact on the design of the payloads and the link characteristics, and it is usually closely connected with the service that the Satellite intends to provide. Constellations have been extensively used in the past for navigation (e.g., GPS, Galileo, GLONASS), voice telephony (e.g., Iridium), or Earth Observation (e.g., PlanetLabs), which operate typically in the range of the low frequencies, up to L-band and S-band. In most recent years, multiple projects have surfaced aiming at providing broadband internet connection via satellite on a global or near global scale using large scale constellations in LEO and MEO.