Research On Key Technology Of Low Angle Radar Target Detection And Elevation Measurement

Low-altitude penetration is one of the important factors that threaten the radar detection performance.In modern warfare,enemy aircraft and missiles often use terrain as a cover,adopt low-altitude flight to approach the target position,use terrain occlusion to avoid radar and air defense weapons,and therefore reduce radar measurement and tracking performance.When detecting a low-angle target,the radar receives the direct wave signal reflected from the target,and in addition it also receives specular reflected and diffuse scattered echoes by the ground or sea.Then a multipath effect is formed.At this time,the direct wave signal and the multipath signal appear as coherent superposition at the receiver,so that the radar received signal changes in amplitude and phase.In severe cases,the reflected signal and the direct signal can cancel mutually,and greatly reduce the low angle target detection probability and measurement accuracy.This thesis focuses on the key problems and technical difficulties in low-altitude multipath target detection and elevation measurement.The main contents include multi-frequency fusion low-altitude target detection,joint estimation of low-altitude target height and ground reflection height,target altimetry estimation method based on terrain reflection coefficient estimation,and low-angle target elevation estimation in multiple-input multiple-output mode.The specific content is as follows:1.Firstly,the reflection coefficient of multipath propagation of low-angle target is introduced.The reflection coefficient is a crucial parameter in low-angle target detection and elevation measurement.The main influencing factors for the ground reflection coefficient is analyzed.Then,the low-altitude target’s four-path multipath echo detection model is established,and the characteristics of the multi-path echo amplitude are studied,which can change with the antenna height,target height and transmission frequency periodically.Finally,the low-altitude target elevation measurement model of multi-channel digital array is established.The influence of multi-path echo on the target elevation measurement is analyzed.The commonly used low-altitude elevation measurement methods are studied,including conventional low-altitude angle measurement methods and array angle super-resolution methods.And the limitations of these methods are pointed out.2.A low-angle target detection method based on sequential statistics and non-coherent accumulation is proposed.The amplitude of the signal received by radar is enhanced or faded with a near-period features.That is caused by the multipath reflection.At this time the detection performance will decrease significantly.Even the target can not be detected.By using an ultra-wide working bandwidth radar operation mode,and making full use of sufficient frequency diversity,the CFAR detection probability in average can be achieved.The commonly used frequency diversity technique is frequency agility.If K frequency echoes in all the N frequency echoes are detected,the target is detected.According to the require of the detecting probability and the false alarm rate,appropriate K and N can be selected.A good CFAR detection performance of low altitude target can be achieved.Compared with the K/N detector,the non-coherent accumulation method is expected to obtain better detection performance.However,in the low elevation angle target detection,serious multipath cancellation can be observed at some frequency points,and it can result in the low SNR of these echoes.These echoes can not contribute to the system detection performance but reduce the gain of the entire system.This chapter proposes a new detection method based on sequential statistics and non-coherent accumulation.Only some frequency echoes with strong energy are used for non-coherent accumulation,and the optimal number of accumulation echoes can be accurately obtained by simulation,then a good low angle target detection performance can be obtained.3.A joint estimation method of target and reflection surface height based on projection gradient is proposed.Using traditional array super-resolution methods,such as the maximum likelihood algorithm or the subspace method,the obtained measurement accuracy can not meet the needs of modern radar.The high-determined maximum likelihood algorithm has a high angular estimation accuracy,but it requires a priori knowledge of surface height and ground reflection coefficient,and the algorithm is very sensitive to the height error of the reflecting surface.In view of the shortcomings of the above existing algorithms,this chapter proposes a joint estimation method for the target and reflection surface height of low-angle radar.The algorithm can eliminate the dependence of the current multipath altimetry algorithm on the height of the a priori reflection surface,and improve the radar angle measurement accuracy in multipath environment.At the same time,the height information of the reflection surface can be obtained,so as to achieve stable tracking of the low-altitude flying target.4.A robust low-altitude target angle measurement method based on composite reflection coefficient is proposed.At present,the traditional array super-resolution methods do not meet the radar’s high-altitude requirements for low-altitude targets.Although the ground-accurate model can obtain high measurement accuracy under ideal conditions,it requires a priori information such as antenna height,reflection surface height,and reflection coefficient,which is often difficult to meet in practice,so the low-angle measurement method for terrain stability is very important.Firstly,the composite ground reflection coefficient is estimated based on the received data,and then the composite steering vector is set for the elevation angle measurement by using the relationship between the target angle and the mirror angle.The accuracy of the elevation measurement is closely related to the composite reflection coefficient.Since the composite scattering coefficient is obtained by the received data,it is not sensitive to the error of the height of the reflection surface.In addition,the difference between the target angle and the mirror image angle caused by the terrain height accuracy is almost negligible.Therefore,although the algorithm of this chapter also uses the information of the satellite digital elevation model,it is not sensitive to the height error of the reflector surface,and the robustness of the algorithm can be achieved.5.A low-angle target polynomial rooting algorithm for MIMO radar with arbitrary array structure is proposed.In order to improve the angular resolution of array radar,the MIMO system is used to expand the aperture to improve the target angular resolution,which improves the elevation measurement accuracy of low-altitude targets.The algorithm transforms the steering vector of the MIMO antenna into a virtual low-dimensional linear array structure steering vector by using array interpolation technique.Then,a coherent spatial smoothing algorithm is used to extract the coherent source from the equivalent virtual uniform linear array.Finally,the polynomial root algorithm is used instead of the angle search for further processing,thereby reducing the computational complexity.