Air-ground channel characterization for unmanned aircraft systems: The over-freshwater setting

Wireless communication systems must operate in the presence of potentially dispersive and rapidly time-varying channels, and the unmanned aircraft system (UAS) control and non-payload communication (CNPC) system is no exception. To ensure high reliability, the UAS air-ground (AG) channel must be quantitatively characterized. In this paper we report on measurements of the AG channel in an over-freshwater setting, for a project sponsored by NASA Glenn Research Center. The measurements on which we report were conducted with a ground site near the coast of Lake Erie, with flights over the lake. The flight test measurements collected simultaneous channel impulse responses in two UAS bands, and employed two receivers for each band to enable assessment of antenna diversity as well as inter-band channel characteristics. We provide a short introduction and description of the measurements, then describe results for propagation path loss, delay spread, and correlations across antennas for the over-freshwater AG channel, for two example flight paths. Path loss generally follows the curved-earth two-ray model, with a weak surface reflection; multipath components from buildings near the coastline are also present. Maximum root-mean square delay spreads range from 70 to 160 ns, and inter-band correlations are well represented as zero-mean Gaussian with standard deviation 0.3. We also comment on the connection with recent results for the over-sea setting, and the AG channel models we are developing.