Research On Correction Methods Of Errors In Multi-beam Sounding Resulted From Water Flow

With multi-beam sounding and integrated navigation technology combined,multi-beam sounding system has quick and accurate access to data of underwater terrain and boasts advantages such as high precision and high efficiency.At present,improving the accuracy of multi-beam sounding,the overriding approach for oceanographic survey,in sounding is the top priority of the surveying industry.There are numerous factors with impact on the accuracy of multi-beam sounding results,such as errors in sound velocity profile,tide level measurement,inertial navigation system attitude measurement,positioning of GNSS positioning system and installation,etc..The existence of these errors affects the accuracy and aesthetics of multi-beam sounding data.Although in most cases these errors are still within the tolerance range of 1%,they may lead to discrepancy in elevation of adjacent bands in multi-beam measurement,difficulty in splicing of data for stratification and affect the ultimate performance of the mapping product.Although declination of transducer installation is corrected in the study of multi-beam error processing,the mounting bracket of transducer has always been regarded as a rigid body for this correction.The mounting bracket,which is regarded as a non-rigid body in this paper,undergoes a reversible elastic deformation due to the impact of water flow during measurement.The amount of elastic deformation is time-varying at different speed of ship and water velocity.Shifting deflection of transducer caused by elastic deformation of the bracket is beyond correction for installation deviation,resulting in the inclusiveness of the impact of this error in the final sounding results.Multi-beam bracket deformation is caused by factor like water flow and speed of ship.Such deformation also leads to the axle offset of relative attitude indicator of transducer,which may influence the accuracy of results.As for this phenomenon,the paper proposes a method that calculates water velocity and output speed of ship according to the navigation data,in order to fit the attitude deviation and correct the results of depth measurement.The achievements of this research include:(1)A navigation-data-based calculation method of water velocity and output speed of ship,which can be used in cases without loading ADCP,is proposed.(2)A method to work out the rolling deviation according to sounding results of adjacent stripes is proposed,and it can be used to fit the attitude deviation and correct the sounding results with speed information.First of all,water velocity and output speed of ship of two adjacent stripes without abnormal sounding results in the testing zone in good sea condition are calculated respectively according to navigation data,and then the attitude deviation of these two stripes in different time are worked out respectively based on discrepancy in elevation of the overlapping portions of the stripes;secondly,a modifier formula of water velocity,output speed of ship and attitude deviation is formulated;last but not least,this formula is applied to solving attitude deviation and correcting the sounding data of stripes in the testing zone.According to the experimental results,the proposed method can eliminate attitude deviation effectively,with the average of discrepancy in elevation reduced from 0.188m(before)to 0.032m(after).The stripe splicing effect is improved and the data accuracy is enhanced enormously.