###### Abstract  
Unmanned aerial systems (UAS) have recently gained popularity, and they are envisioned as an integral parts of the current and future wireless and mobile-radio networks. Despite the exhaustive research on air-to-ground channels, there are insufficient studies, experimental campaigns and general channel models related to air-to-space (A2S) and air-to-air (A2A) wireless links. This paper presents a comprehensive review of the available channel models and path-loss prediction for A2S and A2A communications. Specific case studies attempting to extend current models' parameters and provide important knowledge of the channel behavior in combination with UAV flight characteristics are also provided. A time-series rain-attenuation synthesizer is also presented that describes quite accurately the impact of the troposphere at frequencies above 10 GHz. This specific model can be also applied to both A2S and A2A wireless links. Finally, scientific challenges and gaps that can be used for future research on the upcoming 6G networks are highlighted. air-to-air (A2A); air-to-space (A2S); path-loss prediction; channel modeling; propagation study; unmanned aerial systems (UAS) [1  
the real-time video feed from a thermographic camera) or other measuring equipment [4]. These established communication links are examined as parts of flying ad hoc networks (FANETs), which envisage the networking and cooperation of multiple UAVs [5]. The introduced channels are the key elements in the design of proper UAV datalinks and protocols for heterogeneous networks and system-level simulations. The frequencies allocated for the aforementioned communication links, including CNPC and payload links, are in the license-free spectrum at the segments of 2400-2483.5 MHz, 5470-5600 MHz, 5650-5725 MHz, and 5725-5875 MHz, in which specific restrictions apply that concern the effective isotropic radiated power (eirp). Furthermore, specific regulations apply in which, according to the International Telecommunications Union (ITU) and the International Civil Aviation Organization (ICAO), a specific protected spectrum is allocated in the L-band (960-977 MHz) and the C-band (5030-5091 MHz) for the CNPC link [6]. Furthermore, the Ku-band-frequency segments of 10.95-12.75 GHz (downlink), and 14.0-14.47 GHz (uplink), as well as the Ka-band segments of 19.70-20.20 GHz (downlink), and 29.5-30 GHz (uplink), are also authorized for BLOS and CNPC links of satellite aeronautical communications [5]. In addition, ITU has specific spectrums allocated at 13.25-13.40 GHz, 15.4-15.7 GHz, 22.5-22.55 GHz, and 23.55-23.60 GHz, for frequency sharing between UAS, for BLOS and CNPC communication links, as well as other existing or planned services in these bands [7]. Finally, the total required bandwidth for operational line-of-sight (LOS) terrestrial links is 34 MHz, whereas for BLOS satellite systems, a bandwidth of 56 MHz is recommended [4]. In this context, to ensure robustness and reliability for the operation of UAVs, it is essential that appropriate and versatile channel-propagation models are studied and developed for each connectivity scenario. The cornerstone of successful rudimentary network planning and establishment is to estimate accurately the propagation loss of a wireless link, thus ensuring robust, secure, and convenient communication between the terminals [8]. Path loss is an important large-scale channel characteristic that, together with shadow fading, defines the variation in signal strength over the Tx-Rx distance. Therefore, it is essential that accurate models are employed and evaluated to provide theoretical guidance and assist not only in coverage estimation, but also in frequency allocation and interference assessment [9]. Path-loss models are simpler and more easily applicable than geometrical stochastic models, for example. This study focuses not only on providing existing path-loss models, but also on their use in different scenarios and, in many cases, the extension of their applicability.