Research On Key Problems Of AMTI Processing For Spaceborne Bistatic Radar

The spaceborne receiver/transmitter of bistatic radar system is placed on two satellites respectively,which adopts the mode of single satellite transmission and multi-channel processing by the receiving satellite.The receiver of the system is passive,and the probability of being detected is very small.At the same time,because of its strong forward scattering characteristic of anti-stealth target,it is easy to find and track the stealth target,and has good capability of AMTI and electronic warfare.The system can also use synchronous orbit and low orbit satellite models for network observation,and can realize global imaging,mapping and ground/air moving target detection applications,which makes them have great application potential.In this dissertation,bistatic radar system detection of spaceborne source is taken as the research object,the spaceborne bistatic radar clutter model and configuration optimization are analyzed、direct path signal suppression and utilization algorithm is proposed,and the method to solve off-grid problem based on sparse recovery system is studied.The main work is as follows.1.Clutter model and performance model of bistatic spaceborne radar were established and configuration optimization for same track height system was completed.Bistatic spaceborne system have the characteristic of large orbital configuration difference and heterogeneous space scale,which makes the clutter spectrum structure of bistatic systems have great differences under different orbital configurations.Combined with the bistatic spaceborne geometric model,the effects of spatial variation of resolution and topographic relief on clutter covariance matrix are considered comprehensively,and the clutter range dependence and adaptive processing loss of the quasi-single base configuration and the remote bistatic configuration at the same orbital altitude are analyzed.At the same time,the resolution、spatial and temporal coverage models of the bistatic radar in different orbital configurations are analyzed according to the motion characteristics of bistatic satellite in different orbits.Finally,based on the analysis of resolution、capability of space-time coverage and spectrum structure,constraints on bistatic spaceborne configuration and observation mode are proposed,which can support the configuration optimization design of bistatic spaceborne radar.2.The signal modeling and characteristic analysis of direct path signal between spaceborne bistatic radar are completed,and a suppression algorithm of direct path signal is proposed.On the one hand,when the orbital structure of satellites is very different,due to the rapid motion between satellites,the direct wave signal will be added with a second-order even higher-order on the basis of the first-order Doppler phase,and the direction of the spatial incoming wave is also changing rapidly with time,so that the direct wave signal after pulse compression has Doppler and spatial broadening.On the other hand,the direct wave as the reference signal will cause the scene signal to be affected by direct signal phase.Moreover,due to the short distance between satellites,the strong energy of direct path signal will cover the surveillance channel and drown the scene echo signal.So we analyzed the characteristics of direct path signal,and proposed a space-time second-order broadening suppression algorithm.This method makes a second-order correction to the traditional spatial angle broadening formula,and designs a time-frequency coupled filter based on characteristics of direct path signal.Compared with the traditional method,it reduces the broadening width,an can lose less time degree of freedom to improve the suppression performance.Finally,the influence of direct signal to surveillance channel is compensated by the estimated parameters.The result of simulation experiment shows that compared to the traditional algorithm,the proposed method can effectively suppress the second-order direct path signal and obtain narrower notch of improvement factor curve,and eliminate the impact of direct wave on scene signal.3.The sparse recovery algorithms by dictionary division are usually introduced a off-grid problem to estimate the clutter subspace and spectrum,and deteriorate the performance of clutter suppression when facing the variability problem of spaceborne bistatic clutter.However,the real clutter frequency of space and Doppler is usually not consistent with the frequency meshed by dictionary of sparse recovery.On the basis of SRCN-STAP,the proposed methods approximate the maximum value of the objective function through grid splitting in the local dictionary,and then solve the position of the clutter atom.The core of the grid-splitting method is to use the local iterative grid splitting method to reduce the number of grid division points,so as to recover the clutter spectrum more robustly and accurately.The experimental results of simulation data show that the proposed method can accurately estimate the clutter covariance matrix under single/small sample conditions,and obtain better detection performance.