Estimating Angle of Attack and Sideslip Under High Dynamics on Small UAVs

John Perry, Ahmed Mohamed, Baron Johnson, Rick Lind

Abstract:	High dynamic flight is an underutilized realm of flight which has the potential to drastically expand the flight envelope and mission capability of small unmanned aerial vehicles (UAVs). The introduction of high angle of attack (alpha) and angle of sideslip (beta) flight modes to a UAV remote sensing platform would expand the mission capability of the craft by allowing for high dynamic maneuvers that orient the plane toward the observation target while maintaining trajectory control of the aircraft. Such capability would provide some compromise between the aerodynamic control of aircraft and the desire to point the sensor payload toward a desired target. Flight control systems for small autonomous UAVs rely on real-time navigation parameters as inputs. Typically, an INS/GPS is used to solve for the six degrees of freedom motion model in the local level frame. Often, the navigation parameters are augmented using a Pitot tube to measure the air speed and tri-axial magnetometers to aid attitude estimation. On the assumption of low angle of attack and sideslip, these parameters provide adequate input to the autonomous flight control system. In order to extend the autopilot to high dynamic flight modes, however, the navigation parameters must be further augmented by measures of the angle of attack (alpha) and sideslip (beta). These additional parameters are introduced when the longitudinal axis of the air frame is non-tangential to the trajectory of the craft relative to the air mass. This paper develops and analyzes the error associated with estimating the angles of attack and sideslip using the primary navigation parameters. The alpha and beta angles are then modeled as a zero-th order Gauss-Markov process to be augmented into the main navigation filter. The effect of the wind is also examined, as alpha and beta angles are defined relative to the motion of the air mass. A rudimentary dynamics model for estimating a constant horizontal wind is also produced. Using the developed model, the errors associated with estimating alpha and beta angles with low-cost navigation sensors commonly used for small UAV applications is analyzed. The performance of the sensors are evaluated using data collected from a benchmarking test that emulated the expected dynamics of high dynamic flight on a small UAV. The results show that with these readily available sensors, the alpha and beta angles can be estimated to within 4.5° at a confidence level of 95% with the assumption that wind is properly accounted for. Although this does not meet the desired performance goal of 2° at a 95% confidence interval, the available level of precision is feasible for implementation.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	1165 - 1173
Cite this article:	Perry, John, Mohamed, Ahmed, Johnson, Baron, Lind, Rick, "Estimating Angle of Attack and Sideslip Under High Dynamics on Small UAVs," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 1165-1173.
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