Research On Phase-coded Signal For Improving SAR Imaging Performances

Synthetic aperture radar(SAR)imaging is a kind of active microwave remote sensing technology which uses electromagnetic wave as detection carrier.Compared with conventional optical imaging,it has the advantages of all-day and all-weather work,longdistance and wide swath.It has extensive research and application in military battlefield surveillance and reconnaissance,land and resources investigation,marine target detection and recognition.The traditional SAR system uses linear frequency modulation(LFM)signal as the transmitting signal,the characteristics of the transmitting signal limit the improvement of SAR imaging quality to a certain extent.Phase-coded is a kind of commonly-used pulse compression signal,which has many kinds and possesses excellent characteristics.The purpose of this paper is to select and optimize the appropriate phase-coded signal as the transmit signal,and to provide a new idea for the improvement of SAR imaging quality.The research contents of this paper include:(1)This paper introduces the basic signal theory of phase-coded signal,such as spectrum,ambiguity function and aperiodic autocorrelation function,analyzed the characteristics of Barker code,compound Barker code and m sequence and studies the feasibility of encoding phase-coded signal as SAR system.The SAR imaging results of the phase-coded signal are analyzed through simulation,and compared with the traditional LFM signal imaging results.(2)In order to meet the needs of low sidelobe SAR imaging,reduce the impact of strong scattering point targets on SAR imaging quality,and avoid the sidelobe of strong point targets submerge weak targets,it is necessary to study the phase-coded waveform with ultralow sidelobe.Complementary phase-coded signal has ideal zero sidelobe characteristics,so this paper studies a SAR imaging processing method based on complementary phase-coded signal.Firstly,the signal timing of complementary phase-coded SAR system is analyzed.In order to solve the problem that the echo receiving time of two groups of codes is inconsistent,an accurate echo delay compensation function is given.The range sidelobe of the signal can be greatly reduced by the superposition of the compensated two groups of pulse compression signals.Then,the CZT algorithm is used to correct the spatial rangevariant migration accurately.Finally,the azimuth pulse compression is used to obtain the focused SAR image.The point target simulation of complementary phase-coded signal is completed,and the quantitative analysis of the results shows that the SAR system with complementary phase-coded can achieve the effect of ultra-low range sidelobe.(3)In order to solve the problem of range ambiguity in wide swath SAR imaging,this paper proposes a range ambiguity suppression method based on orthogonal phase-coded signal.Firstly,multiple population genetic algorithm(MPGA)was used to optimize the orthogonal phase-coded signals,which overcomes the shortcoming that sequential quadratic programming(SQP)algorithm and standard genetic algorithm(SGA)can not give consideration to the optimization effect and optimization speed at the same time.Then,according to the distribution of sub swaths,the transmitting time sequence of orthogonal phase encoding signals is designed,and the ambiguity of the range is suppressed by orthogonality between signals.Finally,the simulation results of point target and ship target using orthogonal phase-coded signal optimized by MPGA verify that the proposed method can effectively suppress the range ambiguity in SAR imaging.