A Detection Method Of The Underwater Weak Target Signal Based On Duffing Oscillator

Underwater target detection and identification is the key focus area in the field of underwater acoustic signal processing,as well as an important part of modern sonar system and underwater acoustic countermeasure field.The continuous improvement of stealth technology and the complex and changeable ocean environment have caused the signal to noise ratio getting lower.Consequently,underwater target signal detection and identification in the condition of low signal to noise ratio(SNR)is becoming a difficult problem in modern underwater acoustic signal processing.By utilizing the advantage of typical nonlinear dynamic system in weak signal detection,studies are carried out in two aspects: one is the applicability research of the Duffing oscillator system in underwater target signal detection,which consists of researching the method of the system phase quantitative discrimination and studying the immunity of the system to the noise with different distribution characteristics.The other is,aiming at active and passive detection,to study the matching method of using the Duffing oscillator in detecting underwater weak target signal.Two key problems need to be studied in the application of Duffing oscillator to underwater target detection.Firstly,in order to successfully detect and track the signal of underwater target,it is necessary to find out a quantitative method to accurately,quickly and automatically discriminate phase state of the Duffing oscillator system,wihch solve the problems with the current quantitative discrimination method,such as large amounts of data processing,complex calculation steps and slow convergence speed.Based on the study on the nonlinear dynamic behavior of the Duffing oscillator system,according to the difference of Poincare mapping in different phase states,a parameter which can quantify the phase state of the system,i.e.,the Poincare mapping characteristic function(PMCF),has been constructed.A method for quantitative determination of system phase states based on PMCF has been proposed in this paper.Comparing with other quantifying methods,it can accurately,quickly and automatically identify the phase state of the system without human intervention.Secondly,it is also critical to analyze the immunity of the Duffing oscillator system to underwater noises with different distribution characteristics.In this paper,the sensitivity of Duffing oscillator system to noises with the characteristics of the Gauss distribution,the mixed Gauss distribution,the symmetric Alpha stable distribution and the K distribution has been studied.The numerical simulation results show that Duffing oscillator has strong immunity to the noises presented with the above four distribution characteristics.Passive detection of the line spectrum with unknown parameters by using Duffing oscillator has been studied.The line spectrum of ship-radiated noise is an important characteristic parameter of ship classification and navigation monitoring in the circumstances of sonar detecting ship targets passively.Usually,the frequency and quantity of line spectrum are unknown.Aiming at resolving the problems with the current Duffing oscillator system in detecting unknown frequency signal,such as complex structure,lengthy distinguishment,and the need for human participation,a single Duffing oscillator detection model with variable parameters is constructed,and a Duffing oscillator detection method with variable frequency scanning based on frequency slice wavelet transform(VFFSWT)has been proposed.By setting a parameter of the frequency sensitivity,the frequency of the system driving force is adjusted automatically,and combining with the PMCF method,components of the line spectrum are automatically detected.In this method,the anti-noise performance of the system can be improved by using the frequency slice wavelet transformation.In the processing of the measured data,the VFFSWT-Duffing method is used to detect data at different observation times in order to detect and trace the line spectrum and estimate frequency.Stable line spectrum is identified according to the frequency information on the time track.The processing results show that the method is feasible and effective.The application of Duffing oscillator in active underwater target detection has been studied.Focusing on the influence of target scatting and acoustic propagation characteristics on the characteristics of sonar echo,and considering the problems with underwater bottom and buried targets detection,such as strong reverberation interference,etc.,the weak signal detection methods for the continue wave pulse signal(CW)signal and linear frequency modulation signal(LFM)are studied.When the CW pulse signal is transmitted,the multiple scattering components are coherently superposed in the target echo,which resulting in the received CW echo signal presenting the phenomenon of amplitude fluctuating,fading and phase changing.Consequently,the detection performance of the Duffing oscillator is seriously impacted by these changing parameters.To solve this problem,a new method of the Duffing oscillator with variable coefficients and short-time branch complementation is proposed.In order to optimize the system performance and reduce the system failure rate,the variable coefficient PMCF method is proposed.The strategy of branch complementation solved the phase changing driving the system into the detection blind area.The short time and variable coefficient processing reduces false dismissal probability caused by amplitude fluctuation and fading of the received signal.This method is used to process the data collected in lakes,and presented a great success in single target and multi-target signal detection.When the transmitted signal is LFM signal,the target echo is a linear superposition of multiple LFM signals,which does not meet the single frequency detection condition of the Duffing oscillator.The frequency demodulation of the target echo signal is the main issue.Based on the theory of fractional Fourier transform(FRFT),in the optimal FRFT domain,infinite LFM signal exhibits spectral characteristics of impulse function,and its Fourier transform is a single frequency signal.The above process can achieve demodulation of LFM echo signal.Based on the highlight model,the signal form of the multi-highlight LFM echo signal demodulation is deduced.After demodulation,the number of single frequency components is the same as the number of highlights,and the frequency is related to the delay parameters of the highlight and the parameters of the transmitted signal.On this basis,a Duffing oscillator detection method based spectrum kurtosis in FRFT domain is proposed.By using this method,the existence interval of target echo is determined based on the spectral kurtosis value of optimal FRFT domain.The signal is demodulated and sent to the Duffing oscillator system.The single frequency components are searched automatically,which can realize the detection of LFM target echo signal and the estimation of the number of highligts.The validity and feasibility of the method is verified by the results of processing data collected in the lake trial and ocean trial.The above research results prove that the Duffing oscillator has good detection performance for underwater target signal with low signal to noise ratio.The research ideas and approaches of underwater acoustic signal processing are developed.