## 2 Space Segment
### Satellite Description and Classification  
The size and mass of satellites have progressed together with the capabilities of launchers. The average \"wet mass\", i.e., including propellant, of a satellite has steadily increased from modest beginnings up to about 10 tons in the late 1990s, on par with the capabilities of launchers to geostationary satellite orbit (GSO) [55]. From then on, the development of constellations in Non-Geostationary Satellite Orbit (NGSO), also including Global Navigation Satellite System (GNSS) constellations, and the emerging trend of CubeSats for commercial use, and more generally small satellites, has resulted in a notable reduction of the average mass per satellite. Currently, the majority of satellites launched into space are small satellites [56], referring to satellites with a wet mass typically below 500 kg. This called for a more detailed differentiation between satellite systems, generally following the classifications reported in Table 2, also including examples of commercial satellite systems in respective categories. The list is obviously nonexhaustive, as there are many on-going developments expected to turn into commercial programs in the near future. Some companies, such as GomSpace and Endurosat, provide generic small satellite platforms. The category of femto-satellites is mostly considered these days for educational purposes and laboratory developments, as were CubeSats two decades ago, and may turn in the near future into commercial developments as well. An example of these developments is the SunCube FemtoSat, with a unit size of only 3 \(\times\) 3 \(\times\) 3 cm, proposed by the Arizona State University [57]. This is also the case of some PicoSat developments, such as the ThinSat program by Virginia Space, with dimensions corresponding to 1/7U [58]. On the other end of the spectrum, there are a number of satellite developments that are slightly larger than a MiniSat. This includes for example the first generation of O3b satellites (SES) already in orbit and the Telesat Lightspeed constellation under development, both around 700 kg per satellite. A key parameter in the design of satellites and associated systems is the orbit. This has a significant impact on the antenna design, in particular its directivity and beam steering specifications. Key parameters of typical satellite Earth orbits are listed and compared in Table 3. We distinguished previously between GSO and NGSO. The GSO, also referred to as geostationary Earth orbit (GEO), is particularly convenient for broadcasting applications, as satellites in that orbit have a motion that makes them appear static to a user on the ground. This unique feature is obtained when the orbit of a satellite is in the equatorial plane with \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Classification**} & \multirow{2}{*}{**Mass**} & **CubeSats and** & \multirow{2}{*}{**Industrial Developments and Products**} \\  & & **PocketQubes \({}^{*}\)** & \\ \hline FemtoSat & \textless{}0.1 kg & & \\ PicoSat & 0.1 to 1 kg & 0.25U/1 to 3 p & SpaceBEE (Swarm Technologies), \\ NanoSat & 1 to 10 kg & 1 to 6 U & Docure (Planet), LEMUR (Spire) \\ MicroSat & 10 to 100 kg & 8 to 27 U & 12U (EnduroSat), up to 27U (HEX20), \\  & & & VesselSat (LuxSpace) \\ MiniSat & 100 to 500 kg & Starlink (SpaceX), OneWeb \\ \hline \hline \end{tabular} \({}^{*}\)assuming a typical mass of less than 1.33 kg (3 lbs) per U and 250 g per p.  
\end{table} 
Table 2: Classification of small satellites [59]. an altitude of 35,786 km above the reference geoid. This enables fixed terminals, as often used for example in Direct-to-Home (DTH) satellite broadcasting applications as well as satellite-one-the-pause (SOTP). In the case of satellite-on-the-move (SOTM) applications, the beam steering capabilities are mostly defined by the moving platform (e.g., car, bus) with typically low steering speed requirements. A global coverage is achievable with only three GEO satellites, as implemented for instance with the ViaSat-3 satellite constellation [60].