Research On Anti-jamming And Coherent Processing Technology For Agile Radar

In the modern warfare,the electromagnetic environment faced by radar is becoming gradually complex.The shapes and forms of the jamming threat are constantly changing.Especially,digital radio frequency memory(DRFM)technology is applied in the field of electronic countermeasures,which makes jammer to accurately copy and rapidly forward the radar signal to generate a train of realistic false targets.As a result,the target and jamming overlap in the space-time-frequency domain and the authenticity of the target is difficult to determine,which brings severe threats and challenges to the battlefield survivability of radar.As an “active” anti-jamming technology,agile radar has unique advantages in anti-jamming.Through the parameter agility in a random/pseudo-random manner between intrapulses/inter-pulses,agile radar can greatly improve the low interception and anti-jamming capability,which has a wide application in the future.However,agile radar is a double-edged sword.It improves the low interception and anti-jamming capability of radar while bringing the difficulties to the coherent processing of radar,which seriously impedes the application and development of agile radar technology.This thesis uses the “active” anti-jamming advantages of agile radar,aims to improve the capability of radar to combat typical active jamming,focuses on the technical problems faced by agile radar in the process of antijamming,and researches anti-jamming and coherent processing technology for frequency agile and pulse repetition frequency(PRF)agile radar,sparse frequency agile-orthogonal frequency division multiplexing(SFA-OFDM)radar and intra-pulse parameter agile radar.The main contents of this thesis are summarized as follows:1.Analysis on the anti-jamming principle and the coherent processing difficulties of the agile radar.First,the mathematical model and mechanism of the typical active jamming are established and analyzed.On this basis,the anti-active jamming mechanism of the interpulse frequency agile radar,intra-pulse frequency agile radar and pulse repetition frequency agile radar are thoroughly analyzed,which lays a theoretical foundation for the anti-active jamming of the agile radar.Then,the essential reason for the incompatibility between the agile radar and the moving target detection(MTD)technology of the traditional radar is analyzed,which provides a theoretical basis for the agile radar coherent processing.2.Aiming at the dense false target jamming,the anti-dense false target jamming technology and the target parameter estimations of the frequency agile and PRF agile radar are proposed.First,the frequency agile and PRF agile radar model is established.Then,according to the analysis of radar echo signal,the target range-velocity estimation problem of radar is equivalent to the two-dimensional parameter estimation problem of a linear array.Finally,the super resolution signal processing method in array signal processing is applied to the parameter estimation of the target,and the super resolution estimation of range and velocity is realized by using the multiple signal classification algorithm(MUSIC).Furthermore,the dense false target interference scenario is constructed under the agile radar system,and the“active” anti-jamming performance of the frequency agile and PRF agile radar is verified by comparing with the traditional parameter fixed radar.3.Aiming at the narrowband aiming jamming,anti-narrowband aiming jamming and the coherent processing of the SFA-OFDM radar are proposed.First,the SFA-OFDM radar signal model is established.Then,the SFA-OFDM radar is equivalent to the traditional frequency agile radar through the intra-pulse subcarrier synthesis processing.And two kinds of coherent signal processing methods are proposed.1)The minimum variance distortionless response(MVDR)algorithm is used to estimate the range and velocity of the radar target.By constraining the response of the desired signal to a constant and minimizing the total power of the output signal,the optimal weighting vector is obtained.By using this weighting vector,the super-resolution estimation of radar target parameters is realized while the jamming is suppressed.Finally,the Cramér-Rao lower bounds(CRLBs)on target range and velocity estimations are analyzed and derived,which is used to measure the SFA-OFDM radar parameter estimation performance.2)In the framework of compressed sensing theory,the super resolution range and velocity reconstruction of SFA-OFDM radar is achieved by solving a nonlinear optimization problem based on a sparse priori information.Meanwhile,aiming at the problem that the traditional orthogonal matching pursuit(OMP)algorithm has the large computational complexity,an improved OMP algorithm is proposed.In this algorithm,the atom that is the most relevant to the measured vector is selected by updating the local dictionary matrix.Using the selected atom,the range and velocity are reconstructed.Besides,for the typical narrowband aiming jamming,the numerical simulations are provided to verify the jamming suppression capability of the SFA-OFDM radar.4.Aiming at the interrupted-sampling repeater jamming(ISRJ),a method against the ISRJ of the intra-pulse parameter agile radar is proposed.Considering the discontinuous and periodic characteristics of the ISRJ,a radar waveform named intra-pulse parameter(carrier frequency and chirp rate polarity)agile waveform is designed.The radar pulse is divided into multiple sub-pulses and the characteristic differences between target echo and jamming signal are enhanced under the cover of sub-pulses.On the basis of the proposed intra-pulse parameter agile waveform,the interfered sub-pulse is transformed into the fractional transform domain according to the aggregation characteristics of linear frequency modulation signal in the fractional Fourier domain and the characteristic differences between target and jamming in the time-frequency domain.According to the location of the jamming,a filter is constructed to suppress the jamming in the fractional transform domain.And the jamming parameter sensitivity of the proposed jamming suppression method is analyzed using the signal to jamming ratio improvement factor as measured.