Real-Time Estimation of Airflow Vector based on Lidar Observations for Preview Control

The control technique in a gust alleviation system by using the airborne Doppler Lidar technology is expected to enhance aviation safety to minimize the risks of turbulence-related accidents. Accurate measurement and estimation of the vertical wind velocity are very important in the successful implementation of a gust alleviation system by using the airborne Doppler Lidar technology. An estimation algorithm of airflow vector based on the airborne Lidars is proposed and investigated for preview control to prevent turbulence-induced aircraft accidents in flight. The use of the simple vector conversion method, which is an existing technique, assumes that the wind field between the Lidars is homogeneous. The assumption of a homogeneous field would be wrong when turbulence occurs due to large wind velocity fluctuation. The proposed algorithm stores the line-of-sight (LOS) wind data with each passing moment and uses recent and past LOS wind data in order to estimate the airflow vector. The recent and past LOS wind data are used to extrapolate the wind field between the airborne twin Lidars. Two numerical experiments – ideal vortex model and numerical weather prediction – were conducted to evaluate the estimation performance of the proposed method. The proposed method has much better performance than simple vector conversion in the two numerical experiments, and it can estimate accurate two-dimensional wind field distributions unlike simple vector conversion. The estimation performance and the computational cost of the proposed method can satisfy the performance demand for preview control.

Development and Test of a Fringe-Imaging Direct-Detection Doppler Wind Lidar for Aeronautics

DLR currently investigates the use of Doppler wind lidar as sensor within feedforward gust alleviation control loops on fast-flying fixed-wing aircraft. Such a scheme imposes strong requirements on the lidar system such as sub-m/s precision, high rate, high spatial resolution, close measurement ranges and sensitivity to mixed and pure molecular backscatter. We report on the development of a novel direct-detection Doppler wind lidar (DD-DWL) within these requirements. This DD-DWL is based on fringe-imaging of the Doppler-shifted backscatter of UV laser pulses in a field-widened Michelson interferometer using a fast linear photodetector. A prototype for airborne operation has been ground-tested in early 2018 against a commercial coherent DWL, demonstrating its ability of measuring close-range wind speeds with a precision of 0.5 m/s, independent of the actual wind speed.