Research On Coarse Alignment For Strapdown Inertial Navigation System Of Underwater Vehicles

Inertial navigation system(INS)is widely used in aerospace,aviation and navigation as it can provide the accurate attitude,velocity and position of the carrier.Owing to its autonomy,continuity,and comprehensiveness of navigation,inertial navigation system(SINS)plays an irreplaceable role in the navigation area of the underwater vehicles.With the development of modern computer technologies and inertial devices,the strapdown inertial navigation system has the characters of lower cost,smaller size,simpler structure and higher reliability,and it has gradually become the mainstream and trend of navigation system.Initial alignment is one of the key technologies for SINS,because the performance of the SINS is largely determined by the accuracy and rapidity of the alignment process.Initial alignment is divided into coarse alignment and fine alignment,and coarse alignment need to provide a rapid and accurate alignment result for the fine alignment.The existing mature algorithms are suitable for static base and swaying base,but the problems of low alignment accuracy and poor robustness are still exist in in-motion conditions.Underwater vehicles are widely used in underwater survey,theater investigation,anti-submarine detection and other underwater tasks.Due to the limitation of working environment,the underwater vehicles have to finish the alignment task in the swaying,oscillation or even in-motion base,thus,the research of alignment algorithm which is suitable for underwater vehicles is vital.Firstly,this paper introduces the basic working principle and the mechanization of the SINS.At the same time,the necessity of initial alignment is analyzed,and the influence of the initial attitude error angles on attitude,velocity and position is derived and simulated.Then,the analytic coarse alignment and inertial frame coarse alignment based on the TRIAD algorithm is introduced,and the applicable condition of these coarse alignment algorithms is analyzed.Meanwhile the external speed aided SINS coarse alignment(ESASCA)algorithm under the condition of low speed is derived.In order to avoid the eternal speed difference process,the traditional ESASCA algorithm has been simplified in this paper and simulated.Next,according to the limitation of working environment,the external speed aided optimization-based coarse alignment(EOBCA)for underwater vehicles is proposed.Compared with TRIAD algorithm,EOBCA algorithm can calculate the initial attitude matrix by using sufficient measurement vectors to set up the attitude matrix equations,which improve robustness of the coarse alignment,then EOBCA algorithm process turn to problem of Wahba in order to obtain the initial attitude matrix,and the simulation results show that EOBCA algorithm can meets the needs of underwater vehicles' SINS coarse alignment in no-speed,low-speed and high-speed motion.Meanwhile the Doppler velocity log provides the external speed which may contain outlier practically,in this case,improved median filter based optimal coarse alignment(IMFBOA)algorithm has been proposed to shield the influence of outliers in alignment process.A variety of motion states of underwater vehicles are simulated and compared with the EOBCA methods.Finally three-axel table swaying experiment,vehicle experiment,and Anti-outlier semi-physical simulation experiment are carried to verify the performance of the proposed underwater vehicle's coarse alignment method.