Air-ground channels for UAS: Summary of measurements and models for L- and C-bands

Unmanned aircraft systems (UAS) are being used in an ever-expanding variety of applications. For their safe integration into the worldwide airspace, reliable control and non-payload communication (CNPC) systems must be developed. The performance of any CNPC system depends critically upon the characteristics of the air-ground (AG) channel. Hence the authors, supported by NASA Glenn Research Center, gathered data from an air-ground measurement campaign explicitly aimed at characterizing this channel in two bands recently allocated for UAS, the L- and C-bands. The measurements were conducted in 2013, and produced the largest set of AG channel data ever gathered to date. This data, for seven ground site (GS) environments, was subsequently processed to develop models for the AG channel's characteristics. In this paper we summarize the measurement campaign, measurement results, and completed models for propagation path loss, small-scale fading, airframe shadowing, wideband multipath characteristics, and correlations among the channel effects on two antennas and in the two bands. Path loss in the line-of-sight (LOS) AG channels generally follows that of free-space, but with significant two-ray attenuation peaks (exceeding 18 dB) recorded for relatively smooth earth surfaces. Small-scale fading was found to be well-modeled as Ricean, with K-factors on the order of 12 dB for L-band, 28 dB for C-band. Airframe shadowing was also found to be potentially severe, yielding attenuations of larger than 30 dB for both bands. As expected in the LOS conditions, channels were highly correlated for signals in the same band, but essentially uncorrelated between the two bands. Wideband models contain from one to seven intermittent multipath components, with power levels ~ 20 dB down from that of the LOS component; these yield root-mean square delay spreads up to ~ 2 microseconds. These results have been incorporated into the Radio Technical Commission for Aeronautics (RCTA)'s Special Committee (SC)-228 work on UAS, and should be of use to future designers and developers of UAS communication systems.