Research On Methods Of Active Sonar Target Detection In Shallow Water Condition

To improve the passive sonar system’s ability of target detection and parameterestimation, the shallow water environment was considered in this paper, against theline-spectrum signal, design and detection method of high resolution sonar signal, linespectrum signal high-resolution frequency estimation method and multipath time-delayestimation method for single frequency acoustic signal were analyzed researched theoreticallyand experimentally, respectively. The key contributions are as follows:1. Based on the analysis of common sonar signal and traditional comb spectrum signal, anew method of comb spectrum signal design was proposed. The Doppler spectrum aliasingwas reduced effectively by adding a ratio of equal sequence in frequency as an anti-speedfuzzy frequency interval. According to the spectrum characteristics of comb spectrum signals,a set of comb spectrum signals with different Doppler tolerance and frequency range werelaunched. Consequently, the efficiency of frequency resources and frame rate of the systemwere improved. Simulation results demonstrated the better time delay and frequencyresolution, as well as the improved speed measuring precision and reverberation suppressionability of the system. Focusing on the high peak to average power ratio (PAPR) of the combspectrum signals, a quick phase optimization algorithm was presented. The PAPR of thesignal was reduced effectively on the basis of dramatically decreased computation.2. A detection method of comb spectrum signal based on the matching search infrequency domain was proposed. The Doppler tolerance of comb spectrum signal is small.Considering the huge computation of traditional matching/copies correlation method andmulti-peak characteristics of comb spectrum signal, a detection algorithm based on frequencydomain matching search was proposed. The frequency distribution difference between signaland noise was used. Doppler echo signal detection and velocity estimation were achieved bysearching the matched point and accumulation in the frequency domain. Furthermore, theamount of computation was reduced effectively. In view of the signal detection problems inlow SNR, the signal-to-noise ratio of the received signal was greatly improved through theinstantaneous correlation integral method based on the characteristics of the comb spectrumsignal. Simulation results showed that in the noise and reverberation conditions, the proposedalgorithm had better performance of signal detection and velocity estimation. Theperformance could be further improved by combining the frequency domain search methodand instantaneous correlation integral method. 3. Focused on the frequency extracting in low signal-to-noise ratio, a high-accuracyfrequency estimation algorithm based on the double recursive adaptive filtering was proposed.The extracted line spectrum frequency was an important parameter to estimate the targetfeatures. The traditional frequency estimation methods, such as short time Fourier transformmethod, quasi-orthogonal sampling frequency estimation method, zero frequency estimationmethod and time-frequency analysis method, were either limited by calculation or sensitive tonoise. The algorithm of the frequency estimator which was based on second-order adaptivenotch filter (ANF) was simple. The signal filtering, amplitude and phase estimation could alsobe achieved. The basic principle and performance were described in detail. Based on theanalysis of the relationship of the output instantaneous frequency variance and centerfrequency of the notch filter versus SNR were deduced, a frequency estimation method usingthe dual-recursive adaptive filtering was presented. Multiple adaptive notch filters werecomposed as a recursive loop. A better filtering result was gained by updating the filterparameters based on the analysis of the output instantaneous frequency variance. Simulationanalysis showed that this method had good noise suppression capability and could achievesignal detection and frequency estimation in low SNR.4. To estimate the multipath timedelay, a time delay estimation algorithm based onhomomorphic filtering was proposed. Focusing on the narrowband signal multipathtime-delay estimation, the principle of obtaining multipath time-delay through cepstrumalgorithm was deduced. Concerning the problem of its sensitivity to noise, a homomorphicfiltering delay estimation algorithm based on spectral subtraction in logarithm domain wasproposed. Taking advantage of the active system and the priori information of signal, signalwas eliminated in the logarithmic domain. The subtracted signal was filtered to eliminate theresidual signal component and noise. Then the signal was recovered to the time domain to getthe multipath time-delay information. Simulation results showed that the multipath time-delayestimation method based on homomorphic filtering had high estimation accuracy. Comparedto the direct complex cepstrum method, the proposed method effectively improved theanti-noise ability and adapted better to the Doppler frequency shift of the signal.