## 2 Space Segment
### Satellite Antennas Technologies  
Providing an exhaustive list of satellite antenna technologies is obviously impossible in a paper format, and excellent books are already available on this topic [66, 67]. The objective of this section is instead to provide a review of key technologies for the applications discussed in this paper and highlight some interesting trends in research. Antennas are generally the most visible sub-systems onboard satellites together with solar panels. Accommodating the antennas to achieve the desired performance while keeping the stowed volume in line with launcher restrictions is often a challenging task, and the type of antennas that may be embarked is often dictated by the platform, or conversely, a mission having specific antenna performance requirements may impose a certain class of platform, either generic or custom-made. We start this review by addressing first antennas onboard of small satellites. As discussed in Section 2.1, the range of platforms referred to as small satellites is quite broad. small satellites, such as CubeSats and PocketQubes, typically use simple low gain antennas. Commonly encountered solutions include monopoles and dipoles, as well as turnstile antennas operating at relatively low frequencies, e.g., VHF and UHF. These filar antennas are easy to stow in a small volume and can deploy once in orbit using simple mechanisms, providing an antenna size substantially larger than the platform itself. Several products based on filar antennas are available with a generic CubeSat mechanical interface. An exhaustive review of VHF antenna technologies is provided in [68] with particular focus on satellite-based maritime applications. Most of the technologies discussed are applicable to low frequency payloads, in some cases up to L- and S-bands. Interesting solutions under development include a deployable trifilar helix antenna providing a high stowage efficiency [69] and a miniaturized axial mode quadrifilar helix antenna [70]. Fully metallic-folded patch designs with a compact footprint are also reported for microsatellites [71] as well as cross-dipole antennas over an Artificial Magnetic Conductor (AMC) providing a low profile design [72]. These antennas are well suited for communication links requiring low data rates. A breakthrough S-band antenna design providing both beam-steering and polarization agility that advantageously exploits the hosting platform as efficient radiator by resorting to Characteristic Modes Theory (CMT) is presented in [73]. A metasurface superstrate antenna designed with the aid of CMT suitable to be mounted on a single face of a 1U CubeSat platform and operating in the whole Earth Exploration Satellite Services (EESS) frequency band (2025-2290 MHz) adopted for telemetry/payload downlink as well as telecommand uplink is illustrated in [74]. Some solutions are also reported to provide higher directivity from CubeSats typically using higher frequencies, such as X-, Ku-, and even Ka-band. Reflectarrays have attracted some attention as a possible candidate technology, taking advantage of the low stowage volume achievable with flat panels. The first in-flight demonstration of a reflectarray was NASA's ISARA antenna onboard a 3U CubeSat [75]. This antenna had the particularity of integrating a solar array on the opposite side of the panels to provide enhanced power harvesting capabilities. GomSpace's GomX-5, a 12U technology demonstration CubeSat developed with the support of ESA and expected to be launched in 2022, will embark a multi-panel X-band reflectarray [76]. Kepler Communications is developing 3U CubeSats embarking Ku-band array antennas, with the transmit antenna having an aperture size of 10 \(\times\) 20 cm and the receiver antenna occupying an area of 10 \(\times\) 10 cm [77]. For larger satcom platforms, reflector antennas have been the historically preferred solution. Solid reflector technology provides the best trade-off between cost, performance and reliability.