Research On Key Techniques In Shallow Water Multibeam Bathymetry Sonar

Multibeam bathymetry sonar is one of the important devices for submarine topographical survey in modern time.With the constant progress of modern science and technology,the technique and device of shallow water multibeam bathymetry sonars are also constantly developing and improving,and are widely applied to national defense and national economic construction sectors such as submarine pipeline inspection,submarine cable laying,ocean engineering survey,submarine resource survey,submarine sediment detection,undersea environmental survey,reservoir storage survey,power station dam monitoring,embankment monitoring and guiding port and channel dredging,safe water navigation,etc.According to the current development trend of the device and technique of shallow water multibeam bathymetry sonars both home and abroad as well as the actual demands for the development of domestic shallow water multibeam bathymetry sonars,this paper mainly carries out research in four respects of wide coverage and high resolution multibeam bathymetry technique,multibeam near-field focusing technique,multi-ping and rolling stabilizing bathymetry technique and robust detection of seabed topographical technique in order to improve the comprehensive performance of shallow water multibeam bathymetry sonars.The main contents are as follows:Firstly,research was carried out on high resolution wide coverage multibeam bathymetry technique based on pulse compression.On the one hand,Linear Frequency Modulation(LFM)signal pulse compression technique was introduced.The loss of coherence in two sub-array signalswas affected by the high sidelobe level in interferometric bathymetry method.In order to reduce the loss of coherence,the LFM multibeam echo signal pulse compression method of variable window function were introduced.On the other hand,given the problem of large operand for LFM signal pulse compression,research was carried out on the property of the autocorrelated sidelobes of the coded signals,and a high resolution multibeam bathymetry technique based on low autocorrelated sidelobe code was put forward by making use of the low operand of coded signal pulse compression.To further reduce the operand,a fast pulse compression structure for coded signal based on Field-Programmable Gate Array(FPGA)was proposed.Then simulation analyses and lake experiments were carried out with three signals of Continuous-Wave(CW),narrow pulse,LFM and low autocorrelated sidelobe code as the sounding signals respectively.The results show that compared with CW narrow pulse,LFM and low autocorrelated sidelobe code are improved qualitatively in terms of bathymetry quality;the LFM multibeam echo signal pulse compression method of variable window functioncan reducing the loss of coherent for all beam;when the sounding signal is low autocorrelated sidelobe coded signal,it's easier to calculate the pulse compression in a real-time manner,which is conducive to engineering realization,therefore,it can be used in low-cost sonars which requires less for interferometric bathymetry precision.Secondly,research was carried out on multibeam near-field focusing technique.On the one hand,far-field approximation model is adopted for conventional multibeam bathymetry sonars to receive signal processing,which significantly reduces the resolution formed by beams received in near-field.Given this problem,research was carried out on Real-time Dynamic Focused Beam-forming(RT-DFBF)method of multibeam based on FPGA.Firstly,research was carried out on the fundamental of phase shifting focused beam-forming,and the feasibility for solving near-field dynamic focused beam-forming by it was demonstrated.Secondly,the real-time performance for each calculation step in the algorithm was further analyzed to reasonably balance calculation accuracy and real-time performance.Then a real-time processing structure based on FPGA was proposed.Finally,the real-time performance,effectiveness and practicability of this method were verified by pool experiment.This method was proved to have certain engineering application value.On the other hand,unfocused transmit and transmit focusing with single focus in shallow water multibeam bathymetry sonarsnear-field bathymetry will reduce the resolution for the track direction.Given this problem,the method of transmit focusing with multiple focuses for shallow water multibeam bathymetry sonarsnear-field based on Kasami code was proposed.Firstly,the principles of focusing with single focus were discussed,and Kasami code signal with superior correlation was introduced to solve the problem of mutual interference among signals with multiple focuses.Secondly,its transmitting and receiving principles were derived by a formula.Then comparison under the three conditions of unfocused transmit,transmit focusing with single focus and focusing with multiple focuses by means of simulation.The results show that the method of focusing with multiple focuses has narrower transmitting beam width and higher Integrated Side Lobe Ratio(ISLR).Finally,pool experiment verifies that this method can effectively improve the resolution of shallow water multibeam bathymetry sonarsnear-field bathymetry for the track direction.Thirdly,multi-ping and rolling stabilizing bathymetry technique was researched.On the one hand,in conventional multibeam bathymetry,sounding signal cannot be transmitted again before the target signal reaches the reception array,which may reduce the bathymetry frame rate.Given this problem,shallow water multibeam bathymetry method with multi-ping based on Kasami code was proposed.Firstly,the researchers discussed the principles of frequency division multiplexing bathymetry method with multi-ping as well as parallel and serial bathymetry methods with multi-ping based on Kasami code.Then the performances of band interference and bathymetry resolution of the three methods were compared by means of simulation analyses.The results show that serial bathymetry method with multi-ping based on Kasami code not only has low band interference,but also has high bathymetry resolution.Finally,pool experiment verifies that this method can effectively increase the measurement bathymetry efficiency under the condition of ensuring measurement resolution.On the other hand,the constant change of rolling angle leads to the asymmetry of the covering line on the right and left in case of single survey line,which requires higher overlapping rate of the survey lines,thus reducing bathymetry efficiency.Given this problem,real-time rolling stabilizing technique based on FPGA was proposed.Firstly,research was carried out on the fundamental of real-time rolling compensation,and the real-time performance for each calculation step in the algorithm was analyzed.Then a FPGA real-time calculation structure was proposed and was applied to shallow water multibeam bathymetry sonar with wide-coverage to verify the real-time performance of this technique.Finally,the efficiency was verified by means of the on-site rolling data in Songhua Lake.The results show that this technique can compensate the deviation of the covering line in a real-time manner according to the change of rolling angle,which can make the covering line symmetrical on the left and the right,ensure the effective coverage width of the survey line,and avoid reduction of survey efficiency.Finally,research was carried out on the technique of robust detection of submarine topography.On the one hand,given the problem of false topography caused by "tunnel effect",tunnel effect removal technique based on Order-Statistics Constant False Alarm Rate(OS-CFAR)and Local Mean Dichotomy K-FINDER(LMD-K-FINDER)were proposed.Firstly,research was carried out on OS-CFAR and the principles for removing tunnel effect.Secondly,the fast computing methods for K-FINDER,LMD-K-FINDER principle and LMD-K-FINDER under relaxed k condition and sliding condition were researched sequentially.Then simulation research shows that this method can not only effectively remove "tunnel effect",but also has faster computation speed than similar fast methods.Finally,verification and analysis were carried out on the data from lake and sea experiments.The results show that this method can effectively avoid the false topography caused by"tunnel effect" and has smaller operand.On the other hand,in conventional detection methods,the target echoes from the water other than the seabed will lead to abnormal bathymetry values,thus causing false topography.Given this problem,terrain following technique based on direction prediction was proposed.Firstly,research was carried out on the search technique for intra-frame terrain following origin and terrain following technique based on direction prediction.Then simulation analysis shows that the intra-frame terrain following technique is generally stable as the information such as terrain continuity,SIR and range,etc.is reasonably used and direction prediction technique is adopted.Setting a reasonable time window by means of its trajectory tracking can effectively reduce the abnormal values from conventional WMT and interferometric bathymetry method caused by interference of non-terrain targets in the water.Finally,verification was carried out on the data from lake and sea experiments.The results show that this method has good terrain following ability against interference of conventional water targets.As the upper and lower threshold information is not used during the implementation of this method,this method does not rely on operator as far as possible and has certain automaticity,which can reduce the operator's workload.As the first several frames of data are not used,the mutual dependency on bathymetry information among frames can be reduced.In case of a wrong frame caused by extreme terrain,it can quickly track it again without being affected by the previous wrong terrain.