Research On Linearly Modulated Frequency Stepping Technique In Wideband Phased Array Radar

When developing phased array radar in the early stage,in order to simplify,replace the time delayer which should be used with the phase shifter,at this time the peak position of the pattern is related to the frequency,when the signal bandwidth is wide,the adjacent antenna unit receiving signal has the time difference,waveform distortion occurs when the antenna unit receives the signal.When the signal is incident from a non-vertical angle,the phase difference from the plane phase wavefront to each unit is a function of the frequency,this limits the working bandwidth of the phased array antenna system,in order to extend the instantaneous working bandwidth of the phased array antenna system,the ideal method is to use the time delayer instead of the phase shifter,the phase shifter of each antenna unit is replaced by the time delayer.When all antenna units are connected to the corresponding delay amount,the plane phase wavefront is completely matched with the incident wave,so the instantaneous working bandwidth will not be limited.However,compared with phase shifter,the cost of the time delayer is higher,this ideal alternative is often impractical,so it is impractical to use each unit in the wideband phased array radar to transmit and receive wideband signals.Wideband bandwidth synthesis technology came into being,that is,the radar emitting linearly modulated frequency stepping signal,through the signal processing of the target echo to achieve the wideband bandwidth synthesis,thus realizing the replacement of the wideband system with narrowband system,reducing the requirement of instantaneous bandwidth.In this paper the following research is carried out,aiming at the problem of linearly modulated frequency stepping technique,that is,WBWS:1.Firstly,fundamentals on linear phased array antennas,such as pattern,beamwidth,grating lobes,sidelobes and instantaneous bandwidth,are introduced.Then the limitation of narrowband characteristics of phased array antennas to wideband phased array is discussed.At last,the principle of pulse compression,which is very important to realize stepping FM technology,is discussed in detail.Through the detailed theoretical derivation of the matched filter,it is proved theoretically that the pulse compression can improve the signal-to-noise ratio by sending the pulse with a long pulse and compressing it in the receiver,and it will not degrade the range resolution,and the effective suppression of the sidelobes produced by the pulse compression with Taylor?s filter,The simulation analysis of the latter is carried out,and through the theoretical analysis of the ambiguity function,it is concluded that the Doppler effect has an adverse effect on the performance of the matched filter.2.Secondly,the Ambiguity function of linearly modulated frequency stepping signal and wideband linear FM signal is studied,and it is concluded that the range resolution of linearly modulated frequency stepping signal is the same as that of Linear FM signal,but the velocity resolution is better than that of the latter.Then,the time domain WBWS method and frequency domain WBWS method are discussed in detail,and finally,the direct emitting linear FM signal is compared with the linearly modulated frequency stepping signal with the same total transmission bandwidth as the target range profile,and the frequency domain WBWS method is used to get the range profile,and the one-dimensional range profile made by frequency domain WBWS method is obtained,It is concluded that the one-dimensional range profile made by frequency domain WBWS method is basically in agreement with the direct emission,The main side lobe ratio of direct emission is about-35dB,and the main side lobe ratio of frequency domain WBWS method is about-25dB,so the frequency domain WBWS method will cause the side lobe level to increase obviously.3.Finally,the influence of Doppler effect on linearly modulated frequency stepping signal is discussed in detail,and it is concluded that the influence of Doppler effect can be divided into the chirp pulse part and frequency stepping part,and the specific constraint conditions are deduced.It is concluded that the main difficulty of Doppler compensation for linearly modulated frequency stepping signal lies in the compensation of the primary phase item error of the frequency stepping part,this paper applies the least burst error principle which was originally applied to the frequency stepping signal velocity compensation,and applies it to the velocity compensation of the linearly modulated frequency stepping signal.Burst error function uses 1-norm to measure error after velocity compensation,it achieves the global minimum value when the compensating speed achieves the real radial velocity of the target.In this way,we transform the problem of motion velocity parameter estimation into the optimal parameter search problem of real radial velocity based on the least burst error principle on the compensation velocity axis.The velocity value of a selected rough estimate is used as the center of the range of values,and according to the empirical value,the range of values is set,and then we can choose the compensation precision of the first phase as the minimum search unit value,and the actual velocity of the target is obtained when the least burst error function obtains the global minimum value.Finally,the one-dimensional range profile of the target is obtained by using the actual velocity obtained to compensate the velocity and carry out the inverse IDFT.It is concluded that this compensation method can be applied to the linearly modulated frequency stepping signal,and the speed measurement accuracy is high,which can compensate the offset 2n v/f_p of the three scatters and the distortion of the waveform caused by the target motion well.