Air-ground channel characterization for unmanned aircraft systems: The mountainous environment

Unmanned aircraft systems (UAS) technology has advanced quickly, and the civil use of UAS is expected to expand greatly in the near future. Government, industry, academia, and standards bodies are all doing research for the safe integration of UAS into the worldwide airspace (in the USA, this is designated the National Airspace System (NAS)). For safe operation, requirements for control and non-payload communication (CNPC) systems for UAS are being developed for two frequency bands (960-977 MHz and 5030-5091 MHz) that have been recently allocated by the ITU. Since the air-ground (AG) channel can have a significant effect upon signal reliability, the comprehensive characterization of the AG radio propagation channel plays an essential role in the reliability and safety of CNPC links. Based upon a customized dual-band SIMO wideband channel sounder, NASA Glenn Research Center has conducted an AG channel measurement campaign for various ground site local environments, e.g., over sea, over freshwater, suburban, near-urban, desert, hilly and mountainous terrain. This paper addresses the quantitative models for one of these AG channels: the mountainous terrain. We briefly introduce the current status of our AG channel study and the NASA project, then describe our testing system and the measurement site in the Rocky Mountains. This paper provides example results for propagation path loss, dispersion as quantified by the root-mean square delay spread, and small scale fading Ricean K-factors. We also describe correlations among channels seen by multiple antennas, useful in assessing aircraft antenna diversity.