Investigation of tropospheric attenuations for UAS beyond-line-of-sight communications, and the ITU-R tropospheric attenuation model

The growth in use of unmanned aircraft systems (UAS) will require highly reliable control and non-payload communication (CNPC) links to ensure safety. In cases where UAS are beyond the range of a communication ground station (GS), an alternative communication path must be employed. The primary candidate for such beyond-line-of-sight (BLOS) links is an air-satellite (AS) link. Wireless communication between a satellite and GS or satellite and aircraft can suffer severe fading due to tropospheric effects, and hence these effects must be quantified as accurately as possible for reliable CNPC link design. Although such tropospheric attenuation modeling is relatively mature, these attenuations have not been accurately modeled as a function of aircraft altitude, nor has attenuation variability been thoroughly studied. In this paper, tropospheric attenuation modeling for AS links is described and analyzed. After providing a brief discussion on background and related literature, we include a short description of the characteristics of tropospheric attenuation and its causes: hydrometeors and gases, and tropospheric inhomogeneities that cause scintillation. We discuss each attenuation component individually, and describe the most popular time series attenuation model from the ITU-R Recommendation P.1853. This model is based upon both atmospheric physics and several decades of experimental data from locations worldwide. We also describe a simulation implementation of this model, and provide example attenuation results and statistics for three temperate-tropical climate locations: Cleveland, OH, Columbia, SC, and Miami, FL. We employ frequencies that are being considered for UAS BLOS links in the Ku band (12 GHz) and the Ka band (30 GHz). The impact of tropospheric attenuation on BLOS UAS communication performance is described, including a discussion of attenuation variability and link margins.