| In recent years,blue-green lidar has been favored by domestic and foreign researchers because of its many advantages in optical wake detection.However,during the actual detection process,the backscattering intensity of the near-field water body is several orders of magnitude higher than that of the wake echo signal,which limits the extraction and identification of weak wake echo signals.Therefore,it is very necessary to study the backscatter suppression of the water body for the received lidar echo signal.Starting from numerical simulation and experimental research,this paper proposes an adaptive water backscatter suppression algorithm for the weak underwater wake signal of lidar.Firstly,the paper introduces the underwater light transmission theory and Wa-Li D model,and proposes and builds a backscattering model of the lidar near-field water body summed with the distance layer.Theoretical analysis of the relationship between the echo time-domain spread and the evaluation index of the backscattering ratio is introduced.The simulation analysis shows that the change of the axis spacing can control the distance of the near-field blind zone and realize the target detection in different distances.The SBR at an included angle of 10 mrad is18.2d B higher than 20 mrad.The small included angle suppresses the echo signal of the near-field water body and the effective detection area is larger.The experimental results show that the theoretical calculated spread and the measured error are 0.35 ns.Compared with the time integration model,the model in this paper is more consistent with the measured water echo curve,and the RMSE value is smaller.The average value of SBR obtained by reversely superimposing the measured wake data is increased by 8.66 d B.The model meets the high-precision requirements of the echo model of the water body.Secondly,the paper introduces the Mie scattering theory of single bubbles.By optimizing the number of iterations n,a faster calculation speed is achieved.The simulation shows that the increase of the backscattered light power and the bubble radius of a single bubble shows a trend of increased fluctuation.Furthermore,the estimated value of the optical backscattering coefficient of the clustered bubbles is obtained from the density and size of the bubble group.The back-scattering model of bubble group distance layer summation is established by approximate conditions.Simulation and experimental results show that the greater the thickness of the cluster bubble,the greater the pulse width expansion and the greater the peak shift.Thirdly,the principle of water backscatter suppression based on RLS algorithm is introduced.Since the high-order cumulants have the ability to suppress Gaussiannoise,an adaptive water backscatter suppression algorithm based on third-order cumulants and RLS is proposed.Improve the forgetting factor,and put forward the evaluation index for the processing result.Add different signal-to-noise ratios to the simulated wake data at a distance of 12 m.When ?min=0.95,?=0.3,L=2,the results of the algorithm in this paper have the minimum average absolute error percentage and root mean square error.The signal backscattering ratio is the largest,which is an average increase of 2.6d B compared with the SBR value of traditional algorithms.Finally,in order to verify the accuracy of the distance layer summation model and the superiority of the VFF-RLS-TOC algorithm,the designed lidar wake detection system was used to simulate the indoor bubble screen detection.The controllability experiment was carried out on the bubble group,and the attenuation length and multipath effect caused by the echo broadening effect were analyzed based on the measured data.The results show that,compared with the traditional algorithm,the SBR value obtained by the algorithm in this paper is improved by9.3d B on average,and the result obtained by the algorithm is more stable.The algorithm in this paper is more suitable for the detection of weak underwater wake of lidar. |
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