## 4 Ground Segment
### User Terminal Antennas
In recent times, the most popular flat-panel antenna solutions for broadband satellite communication are phased arrays using either analog, digital or hybrid beamforming techniques. These antennas are also commercially known as Electronically Scanning Antennas (ESA). Analog beamforming is a relatively affordable (USD \(\sim\)1.5 per element) and low-power solution, but the antenna performance usually struggles with broad bands due to the frequency dependence of the integrated phase-shifters, generating distortions in beamforming away from the designed center frequency. Conversely, digital beamforming is more flexible and can be performed over wide bandwidths due to its intrinsic use of true time delay, which ensures frequency-independent behavior [5]. The digital beamforming processor can be extremely power hungry and challenging from both cost and performance point of views, especially for high frequency bands such as Ka. Hybrid Beamforming combines aspects of analog and digital beamforming, achieving a lower power consumption but still maintaining some of the flexibility given by the digitalization. Passive beamforming solutions are also being developed with the aim of achieving a better trade-off between performance and power consumption, which is considered of high importance in some Satcom market applications. The passive beamforming arrays usually have significantly lower DC power consumption than active arrays. In the range of passive beamforming, metamaterials and metasurfaces are currently used to design flat panel antennas. Metamaterials are artificial structures with electromagnetic properties that cannot be obtained in nature and can be used in an antenna to steer the beam without the use of complex Beam Forming Networks (BFNs), by locally tuning the reflective index with discrete low cost/low power active elements such as diodes [182; 183]. The use of metamaterials however poses some challenges, mainly linked to the resonant nature of the design: they normally exhibit low bandwidth, high losses and a relatively small steering range. Liquid-crystal (LC)-based passive beamformers have been designed for Ku-/Ka-band UTs [184]. This design is based on the principle of phase delay in a planar transmission line. It is possible to introduce a phase delay to a signal on the transmission line by controlling, with the application of a DC voltage bias, the alignment of the LC molecules in a LC substrate, causing a change in the local dielectric constant. While this design presents improvements in operational bandwidth with respect to a traditional metamaterial-based design, it is subjected to the intrinsic slow response of LCs and may result in slow beam steering and switching, especially at low temperatures, as LC response time is temperature dependent. Another solution successfully used on the market is represented by Variably Inclined Continuous Transverse Stub (VICTS) antennas [185], which is a hybrid mechanical/electronic design combining stacked radiating surfaces with rotating motors. Different RF design of the disks and different rotation methods can be used [186; 187], achieving different degrees of compactness and RF performance, but generically VICTS antenna design are characterized by a wide scan angle, reduced steering losses and low power consumption. This technology is usually less low profile and heavier than an ESA and is subjected to the drawbacks of integrated moving parts (motor reliability, usage, etc.). An example of other hybrid mechanical-electronic designs are presented in [188]. Lastly, microwave lenses can also be used to design steerable antennas. An example of antenna design including lens is presented in [189]. The base design is still an active phased array, but the lens work as an optical beamformer reducing the complexity of electronic BFNs, and therefore actively reducing the overall number (and therefore cost) of electronic components and the power consumption of the antenna.