Research On Key Technologies Of Wideband Electronic Reconnaissance And Its Application

In the modern battlefield,electromagnetic environment is becoming more and more complex.In electronic warfare,both sides wish their own radars can effectively detect the radar and target of the opponent;at the same time,has the ability to lower the probability of being reconnoitred and intercepted so that to occupy the initiative of the war and gain more chances of survival.Therefore,low probability of intercept(LPI)radar signal has been widely used in modern radar system.In terms of time and frequency domain,LPI radar signal has the characteristics of large time-bandwidth product,which can greatly reduce the peak transmitting power of LPI radar signal.So low transmitting power raises very high requirements on the receiving bandwidth and processing sensitivity of reconnaissance receiver.The main function of electronic reconnaissance receiver is to intercept the signals of the radiation source and extract the useful information so that we can take corresponding countermeasures.LPI radar signal interception problem is the probability problem that the reconnaissance receiver overlaps with LPI radar signal in multi-dimensional space such as signal energy domain,frequency domain and space.However,LPI radar signal has wide bandwidth,even several GHz to tens of GHz.For such a wide bandwidth,in the Nyquist sampling theory nowadays,analog-to-digital converter(ADC)is a limiting factor for many practical engineering applications,for example,in the case of very wide RF bandwidth,Nyquist sampling can not be used for information processing of radar signal.This is because the digital bandwidth(sampling rate)and analog bandwidth(the ability to directly digitize high RF bands)of ADC sampling rate are limited so that the LPI radar signal detection and acquisition cannot be operated directly.As a consequence,researchers have proposed many other sampling structures,such as the non-uniform sampling and a series of new sampling structure based on compression perception(CS).This thesis mainly investigates the detection method and parameter estimation algorithm of LPI signals intercepted by Nyquist folding receiver(NYFR)and studies the implementation of a certain receiver which has been used in a space-borne platform.This thesis makes the following contributions:In Chapter 2,the characteristics of LPI radar is analyzed,and the principle of LPI radar and its common LPI radar signals are introduced,then the detection and reception strategy of LPI radar signal according to its characteristics of large time-bandwidth product are studied.Firstly,the principle of LPI radar is briefly introduced(section 2.2).Secondly,the signal waveforms commonly used in modern LPI radar are introduced,and the performance of the corresponding waveforms is analyzed in detail(section 2.3).Finally,the strategy of LPI radar interception receiver is analyzed according to the characteristics of LPI radar signal with large time-bandwidth product,and the requirements and implementation technology of LPI radar interception receiver are comprehensively studied,and NYFR is taken as the focus of theoretical research in this thesis(section 2.4).In Chapter 3,the basic principles of NYFR and the detection technology of LPI radar signals captured by NYFR are briefly introduced.First,the classical structure of NYFR and the form of its output signal are briefly introduced(section 3.2).Secondly,in view of the inherent defects of NYFR prototype reception,other improved NYFR structures are studied and an improved dual-channel NYFR reception structure is proposed(section 3.3).Then,this thesis studies the LPI radar signal form of time domain after NYFR,has proved NYFR treatment doesnot affect the LPI radar pulse position in the time domain,and on this basis,the thesis proposes two NYFR output signal pulse detection algorithm: NYFR output pulse detection algorithm based on iterative autocorrelation and NYFR output pulse detection algorithm based on short-time Fourier transform(STFT)and the performance of the algorithm are analyzed in detail(section 3.4).In Chapter 4,two parameter estimation methods of LFM signals intercepted by NYFR are proposed,which are based on random sampling consistency(RANSAC)and particle swarm optimization(PSO).For the ultra-wideband of frequency coded(FSK)signals,an improved two-channel NYFR structure and corresponding Nyquist zone estimation algorithm are proposed.Firstly,an information recovery and parameter estimation algorithm of LFM signal based on RANSAC algorithm after NYFR is proposed,and the estimation accuracy of FM slope is improved by using Fractional Fourier Transform(Fr FT)(section 4.2).Then,inspired by RANSAC algorithm,a new parameter estimation algorithm based on standard PSO algorithm is proposed,and a fast Fr FT optimization algorithm(section 4.3)is proposed in the process of Fr FT estimation of FM slope.Finally,in view of the ultra wideband situation,this thesis proposes an improved dual-channel NYFR structure,and an information recovery algorithm when the signal spectrum crosses multiple Nyquist zone,the simulation results demonstrate that the algorithm can well adapt to the information recovery of ultra-wideband signal,and the simulation results show that the single Nyquist zone recovery effect outperforms multiple Nyquist zones(section 4.4).As there are still many problems to be solved in the implementation process of NYFR,real engineering application is far from practical.Therefore,in chapter 5,based on the actual project engineering,the real-time detection and digital parallel processing of broadband radar signals are studied,and the real-time processing platform mainly is FPGA.Firstly,the digital orthogonal down-conversion technology based on polyphase filter is studied,and a parallel digital down-conversion structure without mixer is introduced in the actual project(section 5.2).Then,the real-time detection technology of wideband radar signal based on time domain is studied,and a real-time detection method of rising and falling pulse edges based on mn method and DOB filter is proposed,the detection threshold setting method is given and its performance is analyzed in detail(section 5.3).Next,the method of real time parameter estimation of wideband radar signal is studied.Facing the very complex pulse modulation form,a center frequency estimation method of multiframe FFT pipeline frequency measurement is proposed,and its implementation steps and matters needing attention are described in detail(section 5.4).Then,the system simulation of the time-domain and frequency-domain parameter estimation method proposed above is carried out to verify the feasibility of the algorithm,and the algorithm is verified with the measured data,which further proves its effectiveness and reliability in engineering implementation(section 5.5).