## 2 Space Segment
### Satellite Description and Classification
GEO satellites have however limited performance at high latitudes, where the terminals would be pointing at low elevation angles (typically below 20 degrees). This limitation has triggered the development of Highly Elliptical Orbits (HEO), including the Molinya and the Tundra orbits, characterized with a high eccentricity and inclined orbital planes, providing good visibility over northern regions, such as Russia and Canada. Similar orbits have been considered for southern coverage, specifically Australia. When the satellite is at the apogee, its relative motion to the ground will be significantly reduced, enabling an operation similar to that of a GEO satellite with terminals pointing at a more favorable high elevation angle. Other NGSO include very low, low, and medium Earth orbits (VLEO, LEO, MEO). These are generally circular orbits in inclined planes, although some developments also consider the equatorial plane, such as the first generation of O3b satellites. Inclined orbits are useful to extend the latitude range covered by the satellite. Indicative values for typical altitudes are provided in Table 3. In practice, LEO refers to systems ranging typically from 500 to 1200 km, while MEO generally refers to altitudes ranging from 5000 to 20,000 km. The onboard angular range increases greatly as the altitude reduces, requiring adequate antenna solutions for a proper sizing of the constellation. The visibility time also reduces drastically, indicating that fast steering technology is required for ground terminals connecting to VLEO and LEO satellites, typically imposing electrically steered solutions for both the space and ground segments. Finally, Table 3 also compares typical latency values for the different orbits discussed, considering only the propagation time between the satellite and a user on ground. This is the key parameter that has triggered several LEO constellation developments over recent years, as internet access services and real-time applications are typically not compatible with GEO systems latency, and terrestrial developments on 5G and beyond 5G are placing a particular focus on low-latency solutions. Other satellite system parameters that have a strong impact on antenna design include the onboard processing capabilities and payload design, which may dictate the number of beams to be produced by the antenna system for example. The adequate sizing of the power management is also critical, as the main parameter in the link budget is the power flux density (PFD), obtained as a combination of the antenna gain and the electronics amplification in transmit. A platform with limited power would require a larger antenna to provide a given PFD, leading to some accommodation issues and associated technological developments (e.g., deployable antennas). Conversely, a platform with higher DC power would require larger solar panels, resulting in accommodation issues, indicating that a good trade-off is needed at the system level. In addition, satellite payloads tend to dissipate a large amount of the available DC power. Thus, platforms with high power available also require adequate thermal control and power dissipation management, including active thermal control in some cases (e.g., active antennas). Finally, another key satellite subsystem having a strong impact on antenna technology is the attitude control. While large satellites generally implement attitude control, with pointing accuracy in the order of \(\pm 0.1^{\circ}\)