Research On Echo Separation And System Error On Distributed Multiple Input Multiple Output Synthetic Aperture Radar

Synthetic Aperture Radar(SAR)has been widely used in military,civil and many other fields with the ability to obtain high-resolution images of the ground in all-day and all-weather conditions.It’s an important technology of space-to-ground remote sensing.Due to the constraint of "minimum antenna area",traditional SAR systems cannot achieve high resolution wide swath(HRWS)imaging.Distributed multiple input multiple output(MIMO)SAR arranges multiple small satellites along the heading in space with large baselines.It multi-transmits and multi-receives to obtain a large number of equivalent phase centers which are used to eliminate the azimuth ambiguity.High resolution in azimuth can be obtained under low PRF.The improvement of the spatial sampling rate makes up for the shortage of the time sampling rate and realizes HRWS imaging.Satellites in distributed arrangement of small satellites achieves different tasks with independent transceiver systems through mutual cooperation,and has the advantages of standardization,mass production and high cost-effective in hardware production.Based on HRWS imaging requirements,this paper conducts research on distributed MIMO SAR system modeling and signal reconstruction,echo separation method and performance analysis,system error modeling and correction methods.The main contents of this paper are summarized as follows.Firstly this paper introduces the model and theoretical basis of distributed MIMO SAR.Firstly,the system model of distributed MIMO SAR is established,and the expression of the echo at receiving end is deduced.Based on the system model,the basic principle of range echo separation is analyzed,and the basic principle of azimuth signal reconstruction is introduced in detail.From the equivalent phase center,the cause of Doppler ambiguity,the specific methods of signal reconstruction and evaluation indicators are mainly carried out.Then this paper studied the echo separation under distributed MIMO SAR.Aiming at the different characteristics of two waveform systems of linear shift signal and pulse cyclic shift signal,the use of least squares filter and linear constrained minimum variance filter for echo separation are proposed respectively,and the simulation of point target and surface target is carried out to verify.We proposed range ambiguity index to evaluate the echo separation performance,and the range ambiguity formulas corresponding to the two kinds of echo separation methods are deduced.The simulation results shows that the range ambiguity was controlled between-20 d B and-50 d B,which verified the effectiveness of two methods.At last this paper analyzes and corrects the errors according to the features of distributed MIMO SAR system.Firstly,the system model of the satellite under three attitude errors of roll,pitch and yaw is established,and it is pointed out that the attitude error causes the angle of arrival error,which reduces the echo separation performance.In order to ensure echo separation and imaging performance under positive side view SAR,the rolling attitude error should be controlled within small range.To contain the influence caused by large attitude errors,two methods were proposed.The first method uses the multiple point linear constrained minimum variance filter to broaden the beam pattern,and the second method uses the orthogonal subspace algorithm to estimate the attitude error caused by attitude error.The system model under the satellite position error is established,and it is pointed out that the position error affects the azimuth reconstruction by causing additional phase,and the phase error is estimated and compensated by the orthogonal subspace algorithm,and used the measured data to verify it.Finally,the signal model under the frequency synchronization error is established,and it is pointed out that the frequency synchronization error affects the signal azimuth reconstruction in two forms: fixed frequency offset and noise frequency offset.Then this paper analysed error influence by simulation experiment is carried out.Through this study,the threshold range of fixed frequency bias and noise frequency error can be given for different system parameters.