Research On Digital Beamforming In Range Of Spaceborne SAR And Its Application

High resolution wide swath(HRWS)imaging is an important research direction for synthetic aperture radar(SAR).Hampered by the restriction of the minimum antenna area,the conventional single-channel spaceborne SAR system can not achieve the HRWS images.Digital beamforming(DBF)can make full use of the spatial signal information obtained by each element of the array antenna to form multiple independent and controllable beams without loss of signal-to-noise ratio,so as to achieve target tracking and zero setting of spatial jamming signal,and effectively improve the receiving gain of expected signal.Therefore,it is widely used in modern radar applications.Multi-aperture SAR system with DBF is one of the hotspots of earth observation research in microwave remote sensing,it can overcome the contradiction between geometric resolution and range swath,achieve high resolution and wide swath imaging for spaceborne SAR.This paper mainly focuses on DBF.The main work and innovations of the thesis are listed as follows:1.In order to solve the problems of channel mismatch and pulse extension loss(PEL)of the multi-channel SAR system in range,the DBF processing method based on multi-group of time delayers is proposed.According to the different echo time,it could provide more accurate time delay for all targets in the wide imaged swath,signals from all elevation channels can be better coherently combined,and the influence of PEL can be greatly relieved.To solve the above problems,the DBF processing method based on multi-frequency is proposed.The transmitted pulse with a large bandwidth could be divided into multiple sub-pulses with a small bandwidth to reduce the range offset,which can relieve the influence of PEL in this mehtod.Due to the more accurate delay compensation and smaller bandwidth are related to range offset,the DBF processor with multi-frequency and multi-group time delays is proposed.This method has the advantages of the DBF processing method based on multi-frequency and the DBF processing method based on multi-group of time delayers,and effectively solves the problems of channel mismatch and relieves the influence of PEL.Simulation results on point targets validate the proposed DBF processing method.2.To overcome the range ambiguity caused by aliased echo in multiple subpulses SAR system,the DBF processor with null steering based on multi-group time delays is proposed.This method divides the conventional null beamforming into two steps,and the multi-group time delays are introduced to compensate range offsets between different elevation channels after the first step,and the weight vector in the second step is improved to effectively separate the aliasing echo.To solve the above problems,the DBF processor with null steering based on multi-frequency is proposed.In this method,every transmitted sub-pulse with a large bandwidth is divided into multiple sub-pulses with a small bandwidth to improve the performance of multiple sub-pulses SAR system.Those method also can improve the separation of aliasing echoes and effectively suppress range ambiguity.Simulation results on point targets validate the proposed DBF processing method.