On the use of low-cost unmanned aircraft platforms as part of a system to determine short-term electromagnetic propagation characteristics in the surface and boundary layers

As part of work underway at the University of North Dakota (UND) to develop sense-and-avoid technologies for Unmanned Aircraft Systems (UAS) operations, we have been investigating the utility of low cost unmanned aircraft platforms equipped with atmosphere state variable instrument payloads for near-surface and boundary layer observations. Such observations, when assimilated into a high-resolution modeling system such as the Weather Research and Forecasting (WRF) model, have the potential to characterize the near-surface and boundary layer electromagnetic (EM) propagation environment at not only the time of the unmanned aircraft flight but also within the near term (0-3 hrs), given typical propagation speeds of mesoscale atmospheric phenomena.

Characterization of the EM environment is important for several reasons. First, the presence of surface-based or elevated (e.g., mixed layer) ducting layers can potentially interfere with air-ground communications for commercial and general aviation. Second, the presence of ducting layers can lead to misrepresentation of (or completely obscure) aircraft positions by tracking radars, given that the ducting layers significantly impact the propagation of radar beams. Both air-ground communications and tracking radar technologies are presently heavily utilized in the management of airspace capacity. Poor communications between the control tower and aircraft, as well as potentially ambiguous aircraft radar positions, necessitate wider separation (in both space and time) between incoming and outgoing aircraft given the reduced time for both traffic controllers and pilots to react. As such, prediction of EM ducting regimes several hours in advance would give controllers more time to implement strategies designed to manage periods of reduced capacity at the airport.

Further, current work at UND is focused on improved sense-and-avoid technologies for unmanned aircraft systems, as a means of providing a pathway towards enhanced airspace access for such unmanned systems. This work utilizes both technologies, in particular ADS-B transponders (communications) and ganged phased array radars, for tracking. Thus, we are also investigating any and all strategies to improve diagnosis and prediction of unfavorable electromagnetic propagation environments, leading to the tests described below.

During August 2010, we conducted test flights of a low-cost Telemaster unmanned aircraft, equipped with Global Positioning System receivers as well as instrumentation to measure state variables (pressure, temperature, humidity), within restricted military airspace in east central North Dakota. The data from these test flights, involving several vertical levels, has been assembled into spatial profiles and input into codes designed to compute refractivity in the boundary layer and free atmosphere. The resulting refractivity profiles provide information on the EM propagation environment at flight time, and can also be assimilated into short range (3 hr), very high resolution (500 m horizontal grid, 85 vertical levels including 30 in the PBL) WRF simulations, which then provide forecasts of the EM propagation environment, including the presence of ducting layers.

In this presentation we describe the Telemaster aircraft and payload design, the experimental design of the test flights, data analysis/reduction methodologies including specifics on construction of the refractivity profiles, as well as our data assimilation and modeling methodologies. We also discuss the observed refractivity profiles, their implications for EM propagation, and the utility of assimilating these profiles into high-resolution, short-range WRF simulations for predicting EM propagation conditions within a 3-hour window of the flight time. Such information should be useful to those considering the use of unmanned aircraft flights as an aid towards managing airspace capacity via incorporation of improved knowledge of impeded air-ground communications and radar-based aircraft tracking.