Research On Range Ambiguity Suppression Method Of Space-borne Radar

Space-borne radar is mounted on satellites,which overcome the limitation of the earth's curvature and airspace,and can perform long-distance and large-scale target detection and tracking.It is widely used in the fields of economy and national defense,such as communications,environmental remote sensing,and military reconnaissance.However,space-borne radar usually adopts a pulse system,as well as its wide beam coverage and high platform speed,which makes it impossible to avoid range ambiguity and Doppler ambiguity at the same time.When high pulse repetition frequency(PRF)is used to avoid Doppler ambiguity,serious range ambiguity will occur.Range ambiguity makes the target signal not only compete with clutter in different directions at the same distance,but also compete with clutter in multiple ambiguity distances,which seriously affects clutter suppression,target detection performance and imaging quality.In view of the above problems,the main research contents of this thesis are as follows:(1)Research on the model,characteristics and influencing factors of spaceborne radar range ambiguity echo signal.Firstly,the space-time two-dimensional signal model and the multi-channel synthetic aperture radar(SAR)signal model are established according to the geometric observation model.Secondly,the clutter range Doppler spectrum,space-time two-dimensional spectrum,characteristic spectrum and space-time adaptive processing are analyzed.Then,the PRF design constraints of spaceborne radar system are introduced.Finally,the typical indexes to measure the performance of ambiguity suppression are given.(2)Research on the multi-channel ambiguity suppression algorithm based spatial freedom,including the elevation multi-channel ambiguity suppression algorithm and the azimuth multi-channel ambiguity suppression algorithm.First,the principle and applicability of three elevation multi-channel ambiguity suppression algorithms are introduced:beamforming null algorithm based on compressed sensing,Capon beamforming based on spatial smoothing,and Toeplitzized adaptive digital beamforming(Adaptive Digital Beamforming,ADBF)null broadening algorithm,experiments prove that this type of algorithm can effectively suppress the range ambiguity echo from the side lobe area,but cannot suppress the main lobe and near main lobe ambiguity.Secondly,the principle and applicability of two typical azimuth multi-channel ambiguity suppression algorithms,transfer function method and STAP reconstruction method are introduced.Finally,in order to solve the problem of performance degradation caused by the insufficient number of training samples in the range dimension,a spectral reconstruction algorithm based on iterative adaptive approach(IAA)is studied.The algorithm reconstructs the covariance matrix through IAA algorithm,which can overcome the problem of inaccurate estimation of the covariance matrix caused by the insufficient number of disdance dimension training samples.Simulation results show that the algorithm can recover the unambiguous Doppler spectrum under the condition of a small number of range training samples or a single range sample.(3)Research on the single-channel ambiguity suppression algorithm that uses the degree of freedom of the pulse waveform to suppress the range ambiguity.First of all,to address the problem of the clutter range sidelobe modulation(RSM)effect caused by the inconsistent pulse pressure response of pulse to pulse waveform diversity in clutter background will lead to the decrease of clutter suppression ability,and even the failure of coherent processing,the performance analysis models of target coherent accumulation,single-channel moving target indicator(MTI)processing,space-time processing and SAR imaging for pulse to pulse waveform diversity in clutter background are established,and the quantitative performance indicators are given.Then,the principle and applicability of frequency domain filtering method and time domain energy dispersing method are introduced.Aiming at the time domain energy dispersion method that requires multiple orthogonal waveforms to be transmitted between pulses,the time-domain energy dispersing method based on pulse width frequency modulation polarity agility waveform and matched filter and the time-domain energy dispersing method based on cyclically shifted LFM signal and mismatched filter are compared and studied.Theoretical analysis and experimental results show that two methods can effectively suppress range ambiguity while performing coherent processing to obtain better clutter suppression performance.Finally,the range ambiguity suppression schemes suitable for point targets and small-scene distributed targets are given.