Research On Signal Detection Technology Of Weak Nuclear Electromagnetic Pulse In Wideband Noise

The development trend of nuclear explosion detection technology demands stronger recognition capability in further distance.The traditional land-based short-range detection methods can not meet these requirements,and it is urgent to research new signal detection and extraction methods with higher sensitivity and stronger recognition ability in complex background noise.This thesis applies modern signal processing technology and transient weak signal detection technology to detect and estimate weak nuclear electromagnetic pulse(NEMP)signals from wideband noise,to promote the development of mediumrange and long-range nuclear explosion monitoring technologies.The main research contents and innovative achievements of this paper are as follows:(1)Study of characteristics of weak NEMP signals,including NEMP signal in remote area and trans-ionospheric NEMP signal.(2)Theoretical research of detection algorithm of coupled Duffing chaotic oscillators.Based on nonlinear dynamic Duffing equation,a detection model of strongly coupled Duffing oscillators was constructed to detect the weak land-based NEMP signals in remote area.The biggest characteristic of this model is that the damping force and the restoring force are coupled simultaneously.We proposed a theoretical explanation of "general in-well out-of-synchronization" and discussed the detection principle of this Duffing model based on this theory.Using the tools of signal-to-noise ratio improvement(SNRI)and waveform similarity,we assessed the effects of the parameters in this model,such as the amplitude of driving force,the period of driving force,the coupling coefficient,the calculation step and the damping coefficient,et al.We studied the detection effects of this model on remote NEMP signals in detail.By unifying the coupling strength of the damping force and the restoring force,the stability of the system is enhanced.The noise reduction capability of the system is strengthened by the phase mirroring technique.Comparing with weakly coupled Duffing oscillators,it can work in any phase state,and can detect pulse signals with various waveforms.Moreover,this method is not sensitive to the initial values and has advantage of performing parallel computing.The detection threshold of this model of-15 dB for pulse signals,which is superior to other conventional methods of not less than-10 dB.(3)Application of detection algorithm of strongly coupled Duffing oscillators.The strongly coupled Duffing oscillator equation was solved by RTRK4 algorithm with maximum stability domain and minimum truncation error.Using Simulink and Verilog simulation tools,the realization process of real-time detection system in FPGA was analyzed,and the problems of staged sharing pipeline design,multi-segment parallel calculation,distortion removal and waveform recovery were solved.The FPGA sampling rate is 50 MHz,which is 4.5 times faster than the 11.1 MHz sampling rate of the current continuous wave weak signal detection algorithm.The system was used to detect the simulated LEMP and NEMP signals on the experimental platform,which proved the effectiveness of the system and the feasibility of the method.(4)Application research of fractional Fourier transform(FRFT)detection algorithm in trans-ionospheric NEMP.For the first time,the FRFT algorithm was applied to the trans-ionospheric NEMP weak signal detection.Experimental results show that the minimum detectable SNR can reach-21 dB in the condition of detection probability being 98%.Compared with the bandpass filter trigger algorithm used by the "Test 3" satellite,the minimum detectable SNR is reduced by about 10 dB,and compared with the STFT algorithm used by FORTE satellites,the minimum detectable SNR is reduced by 3~5 dB.The exploration of this method provides ideas for further research.The research of this thesis can promote the development of long-distance nuclear explosion detection technology.Applying the “negative SNR” nuclear explosion effect signal detection and extraction technique to engineering practice,it is possible to achieve leapfrogging development on key technical specifications of long-distance nuclear explosion detection.