###### Abstract
Consequently, an accurate path-loss estimation should be inserted in the link-budget calculations of unmanned aerial vehicles in system-level  
Figure 1: UAS-connectivity scenarios. simulations. Finally, the application of rigorous path-loss models is crucial, since path-loss miscalculations may compromise the study of the wireless link, affect the service-range estimation, lead to outage, and extract misleading interference results. In the frame of the A2G channel, there has been extensive research in the past years, in which various channel propagation models were developed based on measurement campaigns, simulations, studies, and surveys ([1; 2; 10; 11], and the references therein). In contrast to previous research, this article highlights all the recent trends in propagation-channel modeling for UAVs, emphasizing state-of-the art research explicitly for the A2S-and-A2A communication channel. The additional contributions of this survey are as follows:  
* A comprehensive survey is delivered in which available state-of-the-art channel-propagation models are studied. The presented models are retrieved from available experimental campaigns or from theoretical studies and simulations regarding the A2S-and-A2A wireless channel. * Specific cases studies are provided that deliver system-level simulations in order to evaluate and assess the performance of selected path-loss models based on their fundamental parameters. Further, an attempt is undertaken to extend current models' attributes and gain valuable information on the channel behavior in conjunction with the UAS flight characteristics. * A modified time-series-attenuation synthesizer is introduced, which evaluates the induced excess loss of precipitation in the Ku and Ka bands. Valuable simulated results are provided in terms of rain attenuation and exceedance probability. The specific stochastic model can be applied in both A2S and A2A links. * This article suggests future research directions and areas of interest and describes the potential challenges in the modeling of the UAS-propagation channel that stem from the inherent nature of the wireless link, or from the structure of the UAV itself. Finally, antenna diversity and interference-related issues are also introduced, along with potential new measurement campaigns. The methodology of this contribution and the performance of the survey were based on a systematic literature review using journals' and conference proceedings' databases (e.g., IEEE, Elsevier, MDPI, etc.) with specific keywords related to path-loss prediction, air-to-air, and air-to-space channel models for unmanned aerial systems. When screening and filtering the outcomes of the literature search, air-to-ground models were excluded and are not presented in this study, since there are many surveys on this specific subject. The search was conducted from 1973 to 2023 to ensure a comprehensive review of the literature in this area, with a greater emphasis placed on the most recent studies. Another aim was to present models that can be employed for system-level simulations of UAS in various geometries and scenarios. We focused on A2S and A2A channel models in experimental campaigns, theoretical studies and simulations, as well as rain-attenuation time-series synthesizers. Figure 2 illustrates the article's layout and a brief summary of the organization of its main content. More specifically, Section 1 provides an overview and the applications of the UAS, as well as the main motivation of this survey. The available experimental campaigns and theoretical studies regarding the A2S communication link are described analytically in Section 2. Additionally, a rain-attenuation time-series synthesizer that can be applied in both the A2S and the A2A link is presented. State-of-the art channel models for the A2A wireless link are introduced in Section 3, based either on measurement campaigns or on theoretical studies. System-level simulations were conducted, producing interesting results and conclusions regarding the channel behavior.