## 4 Ground Segment
### User Terminal Antennas  
Historically, a GEO system User Terminal (UT) is made of a parabolic antenna plus an antenna control unit mounted on a fixed structure on top of a building [179]. As the antenna is looking at a fixed point in the sky and may require achieving low elevations with respect to the zenith, a parabolic antenna is well suited for the task, guaranteeing a steering capability for pointing, a good performance at any steering angle (the well-known key-hole limitation at the zenith can be effectively mitigated for GEO terminals) and a relatively low cost and high reliability. When mobility came along however, a traditional parabolic antenna was not the best option for all markets anymore, as it is bulky, heavy, fits into an unappealing dome and not fulfilling the requirements of a terminal that needs to be mounted on a possibly small moving, or flying, vehicle. The need for compact low-profile antennas for mobile terminals contributes to the development of more compact (and complex) geometries for steerable reflector antennas [180], which have the capability to fit into a smaller, more compact volume and to maintain contact with a GEO satellite while the vehicle is moving. These antenna designs are usually extremely expensive, given their complexity, and therefore have a quite limited market, mainly limited to high-end satellite communications (trains, big vessels, commercial airplanes, etc.). When referring to NGSO UTs, antennas need to continuously track moving satellites in the sky. The continuous tracking adds a significant mechanical stress to the reflector antenna motors with respect to a traditional GSO user terminal. LEO tracking antennas must also move rapidly, as a typical LEO satellite can stay in the visibility span of a user terminal (up to 120 degrees typically) for as little as 10 s. This makes traditional reflector antennas not particularly suited for LEO applications. A shift in the paradigm of the UT came along with the introduction of flat panel User Terminals, integrating flat panel antennas in their enclosure. Flat panel antennas have the potential to be more integrated into mobility platforms, but this is not all: being smaller, flat, fast tracking, less expensive and immune to mechanical stress, opens the door to markets that have not been touched by satellite communication before. These characteristics are rather important for a GSO system UT, but are utterly fundamental for an NGSO system, making the flat panel UT the Holy Grail for the success of low orbit satellite systems. The challenges in developing a flat panel antenna for Satcom applications are numerous and span from the engineering aspect to the marketing and regulatory [181]. The most challenging design goals for the flat-panel antenna are the trade-off between performance, power consumption, bandwidth, aperture efficiency, reliability, and manufacturability. Performance at low elevations (due to steering loss resulting from the projected aperture) is also a major limiting factor, especially for GSO systems (NGSO UTs have satellites approximately overhead for most of the operational time). Most flat panel systems also suffer regulatory issues, as many existing regulatory requirements for Satcom user terminals are historically based on the parabolic-type antenna technology with invariant radiation patterns over antenna steering and more stringent side lobe level requirements. For the flat-panel type antenna, however, the radiation patterns are changing with beam steering, and it requires substantially more design efforts for a flat panel antenna to comply with a typical radiation mask. The first solutions for flat antennas to arrive on the UT market were mostly hybrid solutions at low frequencies (L-band, X-band) for the GEO market, combining electronic steering with mechanical pointing. With the advent of NGSO and higher frequencies, the challenges have increased due to required miniaturization, increased operational bandwidth and the need for faster 2D tracking.