Initial results for airframe shadowing in L- and C-band air-ground channels

In order to ensure the reliability of unmanned aircraft system (UAS) control and non-payload communication (CNPC) systems, comprehensive characterization of the air-ground (AG) radio propagation channel is vital. As part of the UAS integration into the National Airspace System (NAS), NASA's Glenn Research Center has sponsored an AG channel measurement and modeling campaign in the two UAS CNPC bands (L-band centered at 968 MHz and C-band at 5060 MHz). Measurements have been made in multiple ground site environments with a customized dual-band single-input/multiple-output (SIMO) channel sounder. In this paper we report on experimental data collected in Cleveland, OH, in which airframe shadowing occurred. Airframe shadowing (or blockage, obstruction) is a channel impairment that has not been well characterized. When shadowing occurs, typically during flight maneuvers such as banking turns, the line-of-sight (LOS) signal may be blocked by the aircraft wing or fuselage. This blockage can cause significant attenuation, in some cases enough to sever the communication link. In this paper we provide a brief motivation for our study, describe the measurements, then quantitatively characterize initial airframe shadowing results in the two UAS bands. We provide statistics on shadowing depth and duration. Small scale fading behavior and spatial correlation of multiple aircraft antennas during shadowing events are also compared with those in LOS conditions.