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Impact of abnormal electromagnetic propagation on UAS risk mitigation with radar

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Tuesday, 25 January 2011
Impact of abnormal electromagnetic propagation on UAS risk mitigation with radar
Randall Johnson, Univ. of North Dakota, Grand Forks, ND; and M. Askelson, J. Tilley, and C. Theisen

It is essential to determine the accurate ranges and ground locations of unmanned aircraft during operations, as well as any other airborne objects, in order to dispel the threat of possible collisions. The Ganged-Phased-Array-Radar Risk Mitigation System (GPAR-RMS) at the University of North Dakota has been designed to determine the location of airborne objects with a system of radars. The goal of the GPAR-RMS is to allow for the safe operation of unmanned aircraft in the National Airspace System (NAS).

Electromagnetic radiation emitted by a given radar bends as it propagates through the atmosphere because the index of refraction of the atmosphere is not constant. How radiation bends in various atmospheric environments is of particular interest to the developers of the GPAR-RMS since this system relies on knowledge of the path taken by electromagnetic energy. Since radar can be used to detect airborne objects by emitting radiation and detecting the reflected echoes, the ability to detect and accurately determine the location of these echoes is crucial. Thus, one must know the locations in space and time of the reflected radiation, and, therefore, must know how radiation bends through the atmosphere.

The foundations of an electromagnetic propagation climatology for eastern North Dakota are developed in order to determine the impact of abnormal propagation effects on systems used to operate unmanned aircraft in this region. This is accomplished by simulating atmospheric conditions in July 2009 and late September through October 2009 with the Weather Research and Forecasting (WRF) model. The data from these simulations are used to create index of refraction profiles of the atmosphere since the index of refraction governs the bending of electromagnetic waves. These profiles are incorporated into several ray tracing techniques that are designed to indicate the path of the center of an electromagnetic beam as it propagates through an atmosphere with the simulated conditions. These ray tracing techniques include a simple step-wise algorithm based on Snell's law of refraction in a Cartesian coordinate system, an integral ray tracing approach based on Snell's law in a radial coordinate system, and a step-wise constant beam curvature approach that is an adaptation of the classical 4/3 Earth Radius model. The ray tracing results are used to determine if any significant deviations from the standard electromagnetic wave trajectory are likely to occur.

The aim of this work is to utilize and perfect a ray tracing algorithm created from a synthesis of previous research in order to determine the impacts of abnormal electromagnetic propagation on systems used to operate unmanned aircraft in eastern North Dakota. An important component of this goal is to utilize the WRF model as a predictive tool of abnormal propagation conditions so that radar scanning strategies can be modified under such conditions to maximize safety during these operations.