Radio Propagation Measurement in 38-GHz Band from Lower Stratosphere Actualizing HAPS Communication System

High-altitude platform stations (HAPSs) using broad-band millimeter waves promise to enable the timely provisioning of the required high-speed large-capacity access in various locations including mountainous areas, air, and sea. Since no radio propagation measurements from the stratosphere to the ground have been conducted in the 38 GHz band, it is necessary to clarify the characteristics of radio propagation to realize the HAPS broadband system. In this paper, we report the world's first measurement of 38 GHz radio propagation from the lower stratosphere using a high-altitude aircraft, which was conducted in an environment closely similar to real HAPS operations. We present an evaluation of radio propagation characteristics under various elevation angles and weather conditions, as well as the influence of aircraft turning on radio propagation. Experimental results show that it is possible to apply Recommendations of the International Telecommunication Union Radiocommunication Sector (ITU- R) on attenuation values due to weather conditions for terrestrial radio links and satellite communication links to the stratospheric radio propagation environment under the influence of aircraft turning.