## 2 Space Segment
### Satellite Antennas Technologies
The evolution of GEO satcom payloads, from broadcasting missions in C- and Ku-bands to broadband multiple-spot beam missions in Ka-band, has triggered the development of more advanced feed systems, still relying on reflector-based antenna configurations. The first high throughput satellite embarked a single-feed-per-beam (SFB) antenna system with separate transmit and receive antennas, resulting in a large number of apertures [78]. The development of more compact and integrated feed systems, with dual-band and dual-polarization functionalities, also including a tracking port, reduced the number of apertures from eight down to four or even three [79; 80]. Further developments considered more advanced feed arrays with overlapping clusters in a multiple-feed-per-beam (MFB) configuration to further reduce the number of apertures to only two [81; 82; 83]. This generally comes at the expense of slightly degraded performance due to the sub-optimal cluster excitation. A solution combining polarization-sensitive sub-reflectors and polarizing main reflectors was proposed to obtain the performance of an SFB configuration with only two apertures [84]. An alternative solution considered the use of a dichroic sub-reflector to produce complete multiple beam coverage using a single large aperture [85]. This field of research is still active, as the renewal of existing broadcasting satellites provides an opportunity to embark secondary broadband payloads, and the accommodation of the reflector antennas is always the main limiting factor. The key antenna system parameters highlighting the evolution of broadband satellite solutions are summarized in Table 4. These developments in K/Ka-bands have also benefited from lower frequencies, as multiple-spot beam antennas have been considered at C-band in replacement of more conventional shaped-beam broadcasting antennas [86]. Besides more conventional satellite payloads, there are several developments aiming at introducing higher flexibility through the use of reconfigurable phased array antennas, made possible due to major advances in the field of RFICs. While LEO and MEO solutions, such as Starlink and O3b's mPower, are mostly direct radiating arrays, GEO solutions still rely on reflector-based imaging configurations to achieve higher gain values. The solutions currently under development, including the OneSat program of Airbus Defense and Space [87] and the INSPIRE program of Thales Alenia Space [88], aim at providing fully reconfigurable software defined payloads based on single-reflector imaging antenna \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Frequency Band** &
\begin{tabular}{c} **No. of Main** \\ **Reflectors** \\ \end{tabular} & **Sub-Reflector** & **Configuration** & **Feed Systems** \\ \hline
[78] & K/Ka-band & g (1) & – & SFB & \
\begin{tabular}{c} Single-band \\ dual-CP \\ Dual-band \\ dual-CP \\ Single-band \\ dual-CP \\ Single-band \\ dual-CP \\ up to 25 feeds per beam \\ Single-band \\ dual-CP \\ 7 feeds per beam \\ Dual-CP \\ dual-CP \\ 4 feeds per beam \\ \end{tabular} & K/Ka-band & 4 & – & SFB &
\begin{tabular}{c} Dual-band \\ dual-CP \\ Dual-CP \\ Single-band \\ dual-CP \\ Dual-CP \\ Dual-CP \\ dual-CP \\ Single-band \\ dual-LP \\ 7 feeds per beam \\ Dual-CP \\ 4 feeds per beam \\ \end{tabular} \\
[84] & K/Ka-band & 2 & Gridded & SFB &
\begin{tabular}{c} Dual-band \\ dual-LP \\ Single-band \\ dual-CP \\ 7 feeds per beam \\ Dual-CP \\ dual-CP \\ 4 feeds per beam \\ \end{tabular} \\
[85] & K/Ka-band & 1 & Dichroic & MFB &
\begin{tabular}{c} Dual-band \\ dual-CP \\ 7 feeds per beam \\ Dual-band \\ dual-CP \\ 4 feeds per beam \\ \end{tabular} \\
[83] (3) & K/Ka-band & 1 & – & MFB &
\begin{tabular}{c} Dual-band \\ dual-CP \\ 4 feeds per beam \\ 4 feeds per beam \\ \end{tabular} \\ \hline \hline \end{tabular}  
* (4) Eight user link antennas plus two dedicated tracking antennas [78]. (2) Antenna solution described in Section III.A in [83]. (3) Antenna solution described in Section III.B in [83].  
\end{table}