Antenna and frequency diversity in the unmanned aircraft systems bands for the over-sea setting

The use of unmanned aircraft systems (UAS) is expected to grow rapidly in the next decade, and because of this, there are many UAS research, development, testing, and standardization efforts underway. A key concern in all this work is safety, and this has direct implications for the control and non-payload communication (CNPC) systems that must be used to operate UAS in the national airspace. Another key concern is sufficient (and protected) spectrum for UAS, and at present there are two bands in the United States, and potentially internationally: the L-band (960-977 MHz) and C-band (5030-5091 MHz). In any wireless system, the wireless channel can severely impair performance because of dispersion and time variation. Thus in order to design highly reliable CNPC systems, a thorough quantitative knowledge of the air-ground (AG) channel is required. Historically, AG channel research addressed simple cases and short duration, narrowband signals. Yet for UAS that may operate in more complex settings (e.g., low elevation angles, near ground clutter, etc.) and use wider bandwidth CNPC signals, more accurate representations of the AG channel are required. This paper addresses this topic via the quantification of channel characteristics for one of the simplest AG channels: the over-sea setting. We report on measurements conducted as part of a project sponsored by NASA. These measurements in the over-sea environment collected simultaneous wideband channel characteristics (impulse responses) in both L-band and C-band. This was done with a ground based transmitter, and two separate antennas in each band on the aircraft. Thus these measurements allow us to assess both spatial and frequency diversity for both straight and curved flight trajectories. We briefly describe the project and the measurements, and provide a short overview of our recently-published results for propagation path loss, delay spread, and correlations across antennas. We also quantify small-scale fading effects and the diversity attainable across both frequencies and antennas. We conclude with a short description of the statistical models being developed for the various AG channel settings.