Target Direction Detecting Method Based On Time Reversal Technique

This thesis proposes a method of sparse array detection target direction based on electromagnetic time reversal for the practical applications of the time reversal method.First,this paper focuses on the problem of grating lobes when sparse antenna arrays are used to detect far-field targets,and it deeply investigates the principle of time reversal to detect the target direction.Then we propose two methods of the main lobe recognition by sparse antenna arrays.Second,considering the difficulties when the time reversal method is applied to the detection of far-field targets in the real environment,a new time reversal operator by combining radar signal processing is proposed in this paper which is suitable for a single transmitting antenna.An experimental scenario is also designed for the actual experiments.On one hand,this paper introduces the basic theory of time reversal technology and two methods of target direction estimation based on time reversal,time inversion mirror(TRM)and time inversion operator decomposition method(DORT).Then we analyze the problem of grating lobes when sparse arrays detect the target direction,and we also propose two methods,time reversal window function method and the interpolation one,to identify the main lobe.The time reversal window function method is directed to the broadband antenna,which does not require any limitation of the array distribution.This method uses the invariance of the main lobe direction of the antenna array pattern at different frequencies and the variability of the grating lobe direction as the frequency changes.Its implementation involves multiplying the time reversal results at different frequencies to identify the direction of the main lobe.The interpolation method is aimed at narrowband antennas.It does not have high requirements on the antenna bandwidth,but having special requirements on the distribution of antenna arrays.This method requires a small dense part in the middle of the sparse array,and continuously inserting virtual array elements between the antenna array elements.Thus,it transforms the sparse antenna array to a virtual dense antenna one,which can identify the main lobe direction by the time reversal result of the sparse antenna array.On the other hand,this paper studies the method of detecting the target direction by the time reversal technology in practice,and a time reversal operator is proposed to detect moving targets in the far field.First,this paper analyzes the problems when time reversal detects targets direction in reality.In the existing theory of time reversal,the problem of signal envelope detection and compensation of Doppler effect of moving targets are not well considered.Second,this paper combines the time inversion method with the radar signal processing scheme.The pulse compression method in radar signal processing solves the problem of signal envelope detection,which improves the signal-to-noise ratio of the received data through coherent accumulation.We also compensate for the Doppler effect of the target through Doppler frequency estimation and compensation.Third,we propose a method of constructing a time inversion operator using transmission functions in different time periods.This new type of time inversion operator uses the movement of the target to replace the change of the transmitting antenna.Therefore,this method only requires one transmitting antenna to detect the direction of the moving targets.Finally,an experimental scenario is designed for the new time reversal operator and conducted actual experiments.