Research On Key Technology Of Underwater Target Echo Test

With the development of underwater acoustic technology,underwater target noise level is much lower than before,some close to or even below the level of ocean ambient noise,which are difficult to be detected by passive sonar.However,active sonar is a very effective way to detect such quiet targets.Underwater target echo test has a great significance to the development of testing technology of underwater acoustic stealth performance and underwater acoustic stealth design capability.The performance of target stealth and detection should be validated by the strength of the target reflected echo.The result of echo test is often effected by target scattering characteristics especially in shallow water,which results in unstable test results.It is important to establish robust test method of target strength.It is possible to make the research of target characteristics by modeling and solving the target echo based on real data.In this paper,sonar echo test problems includingmodeling and model parameters solving and target strength measurement are researched.The main work is listed as follows:The problem of frequency selection of target strength test is studied in terms of incident sound as well as the effect from underwater acoustic channel.Target strength is subject to steady-state condition.The transient target strength and broadband target strength are defended corresponding to pulsed and broadband test signal respectively.Simulations demonstrate that the weighted mean of bandwidth target frequency response can be achieved with small fluctuation by using broadband test signal.The conclusions that broadband's echo signal is more stable than narrow-band's is obtained by the numerical simulation.For the problem of modeling and model parameters solving for sonar targets,it was researched in terms of impulse response model and coupling vibration model.Wiener filter and Convex Optimization are applied to get the parameters of the impulse model.Numerical simulations have been operated to test the two methods.Adaptive algorithm is applicable to random excitation of target response.The coupling vibration model of sonar target is proposed by formula derivation and analysis.It's found that coupled vibration analysis model can be solved by the ARX(Auto-Regressive with eXtra inputs)model.Comparing with the impulse response model,this method has the advantages of shorter required order,and noise component isolation in echo.Ocean ambient noise is one important factor for echo fluctuation.The target strength measurement method based on active time reversal mirror is proposed in order to improvethe performance in the presence of anti-jamming.This method with the property of time reversal mirror focusing power can improve the SNR of echo.The effectiveness of this method is verified by numerical simulations.A multi-point synchronous response system and target strength testing software based on time reversal mirror are developed.The target strength trial for solid spheres,spherical shells,a cylindrical shell and some different size plates are operated.Variety methods for reducing target strength fluctuations are validated.Target strength test method based on active time reversal mirror has been further tested,validity and anti-noise performance of this algorithm is demonstrated.In order to further improve the stability of target strength measurements,the fluctuation of reference signal is reduced by taking the average ofmultiple transponders.The test results of using broadband signal instead of the traditional narrow-band signal are compared with the theoretical value.Further study indicates that the transmission fluctuation gets smaller by using broadband signals.Sea trials have been operated for the double layer cylindrical shell target strength test based on time reversal mirror and multiple response method.The sea trial results demonstrate that the uncertainty of target strength measurement is reduced by using the proposed methods.