# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### Space Optical Satellite Communications
The laser beam in the case of uplink immediately passes through the ground atmosphere; as a result, it is significantly affected at the start, and to approach the spacecraft, the warped beam travels a great distance in a vacuum. In contrast, well-concentrated optical beams in the downlink travel over a long distance in clear space and are only distorted when they reach the Earth's atmosphere (typically 30 km). As a result, the atmospheric influence is stronger in the uplink than in the downlink. In inter-satellite data links, beam pointing is a major challenge, i.e., PAT systems, between two moving platforms [167]. To come up with new ideas in conventional satellite communications technology, a variety of microsatellites outfitted with lasers for connectivity are being launched for greater-precision Earth findings, telehealth, armed services, data routing, transportation, investigations, literacy, communication, and business purposes. A lunar laser communications demonstration (LLCD) with a 622 Mbps FSOC link between the Earth and Moon was successfully established in 2013 [168]. By the end of 2023, NASA plans to operate a space probe with an optical communication relay system for geostationary positions. Many nations are scheduling and operating satellites equipped with optics to build broadband space info-communication which can be used for a variety of applications on land, sea, and in space. The ESA launched five LEO Sentinel satellites to form FSO networks and with a geostationary orbit, i.e., with a functional optics information relay spacecraft. Japan has already started an optics information  
Figure 27: Different installation scenarios of satellite FSOC networks: (**a**) RF and FSOC link; (**b**) direct satellite FSOC link; and (**c**) global satellite FSOC link. -relay spacecraft scheme. Many nations have begun HTS launching programs, using the Ka-band and launching services with a capacity of 100 Gbps. To satisfy the growing need for greater capacity connectivity, the information response time at the gateway, i.e., Earth to satellite feeder connections, must be improved. In 2014, the main goal of the SOTA program [168; 169] was the development of an incredibly photographic satellite (usually 50 kg) that was successfully built at a cheap cost and delivered into LEO. For the first time, images captured by this satellite were transmitted to a base station through an optical link using a laser beam of 1.5 micron. Several countries accessed this satellite's picture information and conducted elementary quantum key distribution experimentations while it was in orbit as a worldwide optic spacecraft test platform until 2016. This SOTA program relied heavily on AO with extremely precise PAT and mass stabilizing systems [170]. Many organizations are involved in space science and are working on determining the exact orbits of debris (rocket parts, decommissioned satellites, and fragments), long-distance communication ranges in deep space (beyond 380,000 km), inter-satellite optical communications, global positioning, weather data collection, ground mapping, and spacecraft guiding using optical beams [25; 169]. Th main aim of the National Institute of Information and Communications Technology, which is one of numerous space science organizations, was to demonstrate a 100 Gbps optical satellite equipped with high-speed communication and an advanced laser instrument by 2022 [168]. The references [24; 25; 26; 27; 166; 167; 168; 169] cover a wide spectrum of optical transmissions to/from/in deep space for various space-related applications. In order to support the exploration of Mars, there must be a platform that can support communications between the Earth station and the landed devices such as the Mars Rover of Perseverance and the Curiosity robot. As can be seen in Figure 28, the distance between Earth and Mars is about 54.6 \(\times\) 106 km. Thus, sending a signal over this distance is the major problem.