Influence Of Aero-optics Effects On Lidar Ranging Accuracy And Correcting Method

Complex flow field around high-speed aircraft will cause the fluctuation of theatmospheric density which leads to the nonuniform distribution of index ofrefraction, and then affect the light propagation. Lidar can detect multipleinformation of the target, with high ranging accuracy and resolution, butaero-optical effects will severely impact the performance of laser radar systems.Therefore, It is very valuable to study the influence of this flow field on theparameters of lidar system, and hence to find the correcting method。This thesis investigated the research actuality of aero-optics and lidar rangingaccuracy. Currently, the study on aero-optics is mainly focus on improving theprecision of numerical simulation, establishing the linking equations of flightparameters and optical distortion, and adaptive optics correction. The research aboutaero-optical effects on the ranging accuracy of laser radar systems is rarely reported.Moreover, the research of aero-opticas has always been focused on the area near theoptical window, but the research of lidar system in which the target has aero-opticseffects on its surface is rarely mentioned. This thesis is aimed at studying theaero-optics effect in lidar system from this point of view.Firstly, statistical theory of aero-optics and atmospheric optics are compared.Based on the linking equations of atmospheric state parameters and flight conditionsand the corresponding relationship between flight conditions and optical properties,the study on flow field is introduced into the investigation on aero-optical effects.And then, the linking equation of Strehl ratio and root mean square of optical pathdifference (OPD) is deduced, which provides a theoretical foundation for computingthe echo energy decay of laser radar systems.Furthermore, based on the corresponding relationship between index ofrefraction and flight parameters, the deflection of light rays through shock layer arequantitatively analyzed and how the deflecting angle changes with flight parametersand aircraft shapes is obtained. The maximal value of the deflecting angle can reach1.5mrad. The additional focus due to the curved surface of shock layer is alsoquantitatively analyzed. The changing curve of the additional focus with flightparameters and aircraft shapes is plotted. By combining the deflection effect of lightrays and the additional focus effect of shock layer, the echo energy of laser radarsystems is computed. The results show that the echo energy could be reduced70%due to the existence of shock layer, and the lidat system is severely affected.Finally, the echo impulse response function of laser radar systems and its relationship with the target shape is deduced. With the experiment results of OPD,the effect of the turbulence layer around a target on the echo waveform of pulseranging laser radar is analyzed. A kind of ray tracing method is used to calculate thewavefront aberration and Strehl ratio due to the aerodynamic flow field. The energydecay of laser radar echo that results from the turbulence scattering is furtheranalyzed. With the consideration of both wavefront aberration and energy decay, theinfluence of aerodynamic flow field on the ranging accuracy of laser radar isdiscussed. The results show that, with the influence of turbulence flow, the rangingerror will be36mm for a target with length of12m. Ranging error is30%of thescale of target, which indicate that scattering effect of turbulence has significantinfluence on the ranging accuracy of Ladar syetem. Moreover, peak dection methodhas the minimum ranging error.