Research On The Key Imaging Technology Of High-Resolution Synthetic Aperture Sonar

In the field of underwater target detection,it requires more and more higher sonar imaging resolution.In order to get higher imaging resolution,the method of improve the working frequency and enlarge the size of the array can be adopted.However,the method of improve the working frequency will reduce the detection range,and the size of the array is limited by the space of the carrier(such as AUV and towed body).Synthetic aperture sonar(SAS)can synthesize the virtual large aperture based on the movement of the small-aperture array,and it will obtain high azimuth resolution images of the underwater target by imaging algorithms.Synthetic aperture technology makes it possible for sonar to achieve high-resolution imaging based on low frequency and small aperture physical array.The imaging algorithms are the key technology of SAS system.These imaging methods can be divided into the time domain and frequency domain algorithms according to the different signal processing domain.The time delay and sum algorithm can synthetic sonar data of the any track in the time domain.The more common frequency algorithms have RD,CS,?K algorithm,and the FFT(Fast Fourier Transform)is widly used in these algorithms,which can make the signal processing more efficient.In order to achieve both high azimuth resolution and high mapping rate,the multi-receiver technology has widely been used in SAS imaging.At the same time,in the actual engineering application,the waves and ship routes would cause the sonar platform deviates from the ideal straight line tracking.The phase error which caused by these motion of sonar platform will make the image quality decreased,the target defocused,and the shadow object arisen in the image.So the SAS must adopt motion compensation measures to achieve high quality image.The thesis studied on SAS imaging algorithm and motion compensation algorithm of small size array under the conditions of broadband and wide beam.In order to obtain high-quality and high-resolution image for underwater objects,the research contents include the improved RD algorithm and CS algorithm in large squint angle imaging mode,the multi-array SAS imaging algorithm and improved CS algorithm.In the motion compensation of SAS,overlapping phase center algorithm,phase gradient estimation autofocus algorithm,and non space variant phase error compensation processing are studied.Based on these algorithms,an improved gradient gradient autofocus algorithm is proposed for strip mode.1.The traditional Range-Doppler algorithm is mainly used in side looking or small squint angle mode of synthetic aperture imaging.In application of synthetic sonar,the large squint angle imaging is often required and the range-azimuth coupling is serious in the condition.On the basis of studying the principle of the classical range Doppler algorithm,it improved the imaging algorithm in large squint mode.A more accurate range hyperbolic model is proposed and then the algorithm is deduced in the condition of abandoning the Fresnel assumption.The new secondary range compression method proposed in the paper could eliminate effectively the range-azimuth coupling.Any of point target is simulated in imaging region by using of the improved algorithm.The simulation conclusion showed that the improved Range-Doppler algorithm had higher resolution and moderate computational complexity than the traditional algorithm.2.An improved CS algorithm is proposed to solve the serious coupling problem between range and azimuth in large squint angle mode.By analyzing and deducing the spectrum structure and phase factor compensation of target echo signal,a more precise compensation factor is designed,and more than third-order phase coupling is compensated in the swath.The improved CS algorithm is applied to the imaging simulation of any point target in the mapping zone.The simulation results show that the improved CS algorithm can achieve ideal imaging focusing,and it is more suitable for large squint angle imaging than the traditional CS algorithm.3.The current multiple-array synthetic aperture sonar are almost based on the displace phase center approximation,and the method had always ignored the influence of nonstop-hop-stop and azimuth spatial variation approximation,it will result in the blurred image under the condition of broadband and wide-beam.The multi-array synthetic aperture sonar improved nonlinear Chirp Scaling algorithm(NCS)is proposed in this paper,and it can take into account both imaging accuracy and efficiency.Any point target in imaging region had be simulated by using the algorithm,and the simulation conclusion showed that the improved algorithm had higher resolution than the traditional algorithm.The new method can satisfy the need of the wide-beam multiple-receiver synthetic aperture sonar imaging.4.The method of motion compensation is summarized on the basis of analyzing the model of motion error.The estimation accuracy and the limitation of the classical overlapping phase center algorithm is analyzed.In view of the limitation of traditional motion error algorithm,an extending stripmap phase gradient autofocus algorithm is proposed.The method divides the full aperture data into several overlapping sub apertures,and estimates the range non-spatial phase error of each sub aperture.The high precision Doppler center is estimated for each sample point,and the local linear phase at the overlapped aperture is compensated.The simulation results show that the focusing effect of the algorithm after compensating motion error is better than the traditional algorithm.5.The imaging algorithms of multiple-array SAS are tested in water tank.The synthetic aperture imaging experiment is carried out with the method of simulating the imaging target.In the experiment,the array of six hydrophones is used to collect imaging target signals.Using the data collected from the experiment and simulation data,the improved CS algorithm and the ?K algorithm of range migration correction and imaging focusing effect are compared and analyzed.According to the analysis results,both the multiple-array SAS improved CS algorithm and multiple-array SAS ?K algorithm can correct the range migration well,and they could be completely applied to the SAS imaging.