Research On Ship Imaging Approach For Airborne SARs Based On Location Of Target’s Echo

Airborne synthetic aperture radar(SAR)has the advantages of high signal-to-noise ratio,high resolution and flexible maneuverability.Therefore,the use of airborne SAR technology to monitor ship targets on the sea has obvious advantages.It is of great significance to study the imaging technology of ship targets for airborne SARs in the areas of maritime rights enforcement,oil spill surveillance,wrecked ship rescue,maritime transportation management and control.However,the imaging mechanism of SAR requires that the target is in a static state,and the motion of the target will cause the SAR image defocused.Ships usually have a certain speed,and the hull will appear three-dimensional rotation of yaw,pitch and roll due to the combined effect of sea waves and other factors.In addition,the airborne platform deviates from the ideal trajectory due to the influence of airflow disturbance and other factors,which will also affect the SAR imaging quality.In view of the above problems,and based on the literature review on the research status of ship imaging technology for airborne SARs at home and abroad,the following research work has been carried out in this paper:Firstly,in view of the current situation that there are more qualitative and less quantitative studies on the impact of ship motion on SAR imaging quality,quantitative analysis of the impact of ship translation and three rotations on SAR imaging quality is carried out,and simulation experiments were designed to verify the effect.Secondly,data processing such as motion compensation,image focusing and image merging were carried out to generate the whole image of the marine scene in view of the acquired airborne SAR real data.According to the high accuracy of the acquired inertial navigation data,the motion compensation method based on inertial navigation data is adopted to improve the processing efficiency.In the aspect of focusing,the de-chirp processing method is adopted,which further improves the processing efficiency.After a series of processing including motion compensation,de-chirp imaging and image merging,a whole image of the marine scene with good quality is obtained.Thirdly,aiming at the shortcomings of ISAR(Inverse Synthetic Aperture Radar)refocusing method based on ship sub-image in airborne SARs,ISAR refocusing method based on the location of target’s echo is studied.Above all,the situation that the ISAR refocusing method based on ship sub-image may encounter the challenge of azimuth ambiguity when it is applied in airborne SAR is analyzed theoretically,and the according simulation results are given.Next,the echo of a ship is located and extracted according to the azimuth and range position of ship target in the complete SAR image.After that,the echo of the ship is processed by range alignment and phase compensation procedures.In view of the shortcoming of existing time windowing methods,an improved time windowing method based on time-frequency analysis technology is proposed,which improves the efficiency of windowing processing.Afterwards,a series of processing such as time windowing,rotation compensation,azimuth FFT were applied to the real data after phase compensation to obtain clear ship images.By calculation,the contrast of the SAR images of the two refocusing vessels is 59.2,61.7,the image entropy is 8.3,7.4,and the signal-to-noise ratio is 44.3,57.5respectively.The numerical values of the above technical indicators verify the target echolocation based on the target.The effectiveness of the ISAR refocusing method.In the process of windowing,the processing time of the improved windowing method is 1022 seconds and 1202 seconds respectively,and the windowing processing time of the existing windowing method is 1326 seconds and 1445 seconds respectively,and the improved windowing method has a processing speed of about 20%,which verifies the efficiency of the improved windowing method.