Research On LPI Signal Analysis And Processing For Radar Reconnaissance & Passive Location

In modern warfare, the conflict between the electronic counter measures and anti electronic counter measures is becoming fiercer day by day. Gathering intelligence about hostile radars and locating their positions are a prerequisite to the precision strike and is also the master key to the initiative of war. One of the core issues is to analyze and process the low probability of intercept (LPI) radar pulse compression signals, etc. the large time-bandwidth product signals LFM and polyphase coded signals. We deeply study some key technologies/subjects in this dissertation, including the modulation rec-ognition, the modulation parameter estimation and the high-precision measurement of Doppler frequency rate for the LPI pulse compression signals. The main contributions of this dissertation are given as follows:In Chapter 2, the intra-pulse modulation parameter estimation problem is studied. We focus on solving the problem of intra-pulse modulation parameter estimation for multi-component linear frequency modulation (LFM) signals. (1) From the viewpoint of the output signal-to-noise-ratio gain, the anti-noise performance of the quadratic phase function (QPF) is analyzed and a closed-form expression for the output SNR is derived to evaluate the performance of the QPF method for single LFM signal. The constraint to guarantee a reasonably estimation accuracy is then obtained. Thereafter the high SNR condition required by the first order perturbation analysis is given. The selection of the signal processing length and the search step for the frequency rate is also given for prac-tical applications. (2) To deal with the problem of spurious peaks and interferences pre-sent in the QPF of multi-component LFM signals, a modified algorithm, named IQPF, is proposed. The method can suppress interferences and spurious peaks caused by the multi-component signals effectively. In what follows, the analytical expressions of the output SNR gain and the estimation accuracy are derived. The simulation results verify the theoretical results.In Chapter 3, the detection and intra-pulse modulation parameter estimation prob-lem are investigated. We are concentrating on solving the blind encoding parameter es-timation problem of the polyphase coded signals, which will be used by the following chapter for the purpose of the modulation recognition. (1) For the analysis of the char-acteristics of the polyphase coded signal's QPF, an IQPF-based detector is proposed. The performance of the detector is evaluated by the receiver operating characteristic curve and the detection rate merits at a constant false-alarm rate. (2) The ambiguity function (AF) of the polyphase coded pulse signal is derived. A conclusion is drawn which states that, the intercepts, on the Doppler frequency and the time delay axes, of the ridge nearest to the main ridge contain information on the subcode rate and the time duration of the pulse signal. In addition, an AF based method is also proposed to obtain the blind estimation of the duration and the subcode rate. (3) We proposed a partial AF based algorithm for estimating the subcode rate and the modulation period of polyphase coded continuous wave signals. The algorithm is high efficiency and easy to be imple-mented. The theoretical mean square error of the subcode rate estimates is derived using the first order perturbation analysis.In Chapter 4, the modulation recognition of the polyphase coded pulse signal is studied. We focus on classifying the polyphase coded signals with the same modulation type but different code sequences. (1) To solve the error identification problem that may occur caused by using only the subcode rate and the code length to construct the refer-ence signals, a new reference signal generation method is proposed. The positive and reverse sequences are considered. The influence of the carrier frequency estimation er-ror on the performance of the correlation classifier is investigated by the numerical simulation. (2) The existing modulation recognition algorithms have a drawback that their performance is sensitive to the carrier frequency estimation error. Thus, a new cross-correlation spectrum based modulation recognition method, which doesn't need estimating the carrier frequency, is proposed. The algorithm can be implemented by us-ing the fast Fourier transform (FFT). Thus it has low computational complexity.In Chapter 5, the high accuracy Doppler frequency rate estimation is studied for the single observer passive location application. We concentrate on the Doppler frequency rate estimation for the pulse compression signals by using the coherent pulse trains. (1) The influence of the relative motion between the observer platform and the object emit-ter on the characteristics of the observer-received signal is analyzed. The received signal model is described. (2) The Cramér-Rao lower bound (CRLB) of the Doppler frequency rate estimates for the LFM coherent pulse train signal is deduced. A new fractional Fou-rier transform (FrFT) based algorithm is proposed to estimate the Doppler frequency rate for LFM coherent pulse train signals. The theoretical analysis shows that the vari-ance of the estimates is approaching the CRLB at high SNRs. (3) A new Doppler fre-quency rate estimation algorithm is proposed which is based on the phase transition compensation. Different from the other algorithms that based on the square or the in-ter-pulse correlation operations, the proposed algorithm does not suffer the cross term problem. The constraints among the unambiguity Doppler frequency rate estimate, the pulse repetition frequency, the sampling frequency and the time duration of the observa-tion is given.