## 4 Ground Segment
### Gateway Antennas  
All satellites require gateways to connect to the core network and exchange data between users. Gateway stations (or ground stations) provide the interface between the satellites out in space, and the terrestrial networks for public switched telephone networks, cellular networks and data transmission networks.  
A gateway station consists of several different components that allow transmission and reception to and from the satellite, amplification of the signals, transformation and connection to the terrestrial network. The main part of a ground station is the antenna that sends and receives the satellite signals. Ground station antennas are typically parabolic dishes pointing to one single satellite each. Depending on the frequency, gateway antennasvary in size and complexity. For lower frequencies, they are generally in the order of 10 m diameter and decrease in size for higher frequencies. Generally speaking, the higher the frequency is, the smaller the antenna is, and the harder it is to point the antenna to the satellite. With GEO satellites, the task of pointing and maintaining the link to the satellite is simplified by the fact that the satellite is static in the sky; thus, the gateway does not need to track the satellite movements across the sky. NGSO gateways are more complex systems from a ground network perspective, as they need tracking antennas, handover between subsequent satellites on the orbital arc, and tracking of multiple satellites from the same site. Therefore, while traditional parabolic dishes are generally effective for gateways communicating with GEO satellites, they are limited when it comes to tracking fast-moving LEO satellites.  
The main problems associated with the traditional parabolic dish approach for a NGSO gateway station are the motorization of the antennas and the large footprint of the gateway station. The large footprint is due to the large number of separate reflectors needed (one LEO gateway station can track up to 15 satellites at the same time) and the need to guarantee enough distance between antennas to avoid line-of-site (LOS) issues between them, which is likely to happen during tracking, especially for low elevation angles over the horizon (as an example, see OneWeb Satellite Network Portal [SNP] in Figure 8). The antenna mechanical steering and the issues associated with it (need for frequent maintenance, reliability) is another limiting factor of traditional gateway stations.  
In this perspective, technologies are emerging aiming at applying the principle of flat multibeam antennas to gateway antennas as well. An electronic steerable gateway is one way to mitigate the previously mentioned issues. Motorization systems can be avoided completely, and the footprint can be reduced by using a single structure to track all the satellites in view. This requires an electronically steerable antenna technology that can dynamically establish a high number of simultaneous beams with a reduced ground infrastructure. The reduced footprint could also allow the installation of the gateway near to the backhaul control center nodes instead of far remote areas where land is available at a low price, thus further reducing the terrestrial network latency.  
Various architectures have been proposed for multibeam gateway antenna systems. To be able to achieve multi-beam behavior on a wide scan angle needed to track as many satellites as possible up to low elevation angle over the horizon, the gateway system is usually made up of a multifaceted structure combining different flat antenna panels distributed at different angles with respect to the ground, in the shape of a dome or similar. Some design solutions and recent commercial offerings are proposed in [192; 193; 194; 195; 196].  
Figure 8: OneWeb SNP, Sintra, Portugal.