## 3 NTNs for IIoT Applications
### Overview of Airborne Platforms for IIoT  
Beyond satellites, NTNs indicate networks or segments of networks with high- and low-altitude platforms (HAPs/LAPs) or airborne vehicles acting as aerial transceivers that operate at altitudes ranging between 8 and 50 km above ground level [40; 41; 42]. The term HAP defines both aircrafts flying in a roughly circular tight path in the stratosphere layer and quasi-stationary, solar-powered, non-pollutant, and environmentally friendly airships [43]. Besides the above, LAPs fly at lower altitudes, in the troposphere, and intend to accomplish diverse missions. Among them, unmanned aerial vehicles (UAVs) [44] constitute a type of small fueled aircraft employed for short time periods and allow for a rapid relay-based deployment of a multi-hop communication backbone [45]. Indicative types of UAVs are drones, remotely piloted vehicles (RPVs), pilotless aircrafts, and robot planes, whose size may significantly differ. Since the links of terrestrial systems are often blocked, aerial platforms have great potential to attain a higher chance of LoS communication with the ground users and thus enhance the coverage and connectivity. It is also noted that flying ad-hoc networks (FANETs) among aerial platforms are also envisioned [46], owing to possible connectivity and coverage restrictions of terrestrial and/or satellite networks. Prospective 5G, B5G, and IIoT systems are expected to include UAVs as autonomous communicating nodes or aerial relays for attaining highly reliable connections between sensors and data collection points at high elevation angles and across urban, suburban, and rural terrains [47]. Both HAPs and LAPs can be rapidly deployed and moved on-demand and can also carry a range of sensors, including geospatial sensor technologies gathering massive amounts of valuable data [41]. To retain the high level of stability required for critical IIoT applications and prevent displacements in any direction (e.g., roll, pitch, and yaw effects) due to wind and pressure variations, flight control tilt sensors along with accelerometers and gyroscopes can be exploited, combined with ultrasonic sensors for obstacle avoidance. These aerial platforms may be supplied with electro-optical sensors and radars, achieving adequate resolutions for data acquisition purposes. To enhance the surveillance and monitoring applications, low- or high- resolution red-green-blue (RGB) cameras are required, whereas normalized difference vegetation index (NDVI) cameras are recommended for unrivalled precision farming. Moreover, UAV-based light imaging, detection, and ranging (LIDAR) is suggested for efficient mapping and localization [48]. Besides this, the hyperspectral depth and thermal sensors facilitate the creation of aerial thermal imaging for analysis and reporting. Moreover, 3 GPP suggested the deployment of aerial platforms for Long-Term Evolution (LTE) standard [49], whereas the notion of the nomadic relay was proposed by the IEEE 802.16s Relay Task Group [50]. In 2016, the Radio Technical Commission for Aeronautics (RTCA) aspired to introduce UAVs into the national airspace system [51]. The National Aeronautics and Space Administration (NASA) and Federal Aviation Administration (FAA) have also cooperatively considered the integration of UAVs into the airspace system of United States [52], while in Europe, Single European Sky ATM Research (SESAR) has intended to meliorate air traffic management (ATM) in order to overcome challenges arising from large UAV swarms and the different types of UAVs [53]. In this regard, a UAV swarm network architecture with multiple layers for IoT scenarios was presented in [54], and a low-latency routing algorithm (LIRA) was proposed that relies on the position and the connectivity of the UAVs. From an industry perspective, Google has recently initiated Project Loon, which aims at leveraging balloons in high altitudes as a means of provision of broadband services in remote locations [55].