Research On Initial Alignment Method Without Latitude For Strapdown Attitude And Heading Reference System

The strapdown attitude and heading reference system(SAHRS)requires to implement the initial alignment process to determine the initial attitude before entering normal working mode.Thus,the alignment speed and accuracy of alignment process will significantly affect the startup time and attitude accuracy of SAHRS.Currently,traditional alignment methods need to obtain the latitude information from external devices.However,it is difficult to obtain the latitude in some cases,such as the lock-loose or denial of satellite signals and under deep water environments,where traditional alignment methods will fail to work.It is highly required to investigate the alignment methods without using latitude.At present,the research on initial alignment methods without using latitude information is still on the primary stage,and the influential mechanism of angular motion and linear motion on alignment process also needs to be further studied.The existing alignment methods without using latitude information focus on the alignment of stationary base and swaying base,and they suffer from poor resistance to sensor noise disturbances.Besides,the in-sailing alignment problem without latitude is still not solved at present.The subject “Research on initial alignment method without latitude for SAHRS” is investigated to analyze the influential mechanism of angular motion and linear motion on initial alignment process,and to fulfill the fast startup and accurate attitude measurement requirements of SAHRS without using latitude.Aimed at solving the alignment problem without latitude on stationary base,swaying base and in-sailing condition,corresponding initial alignment methods with strong disturbance resistance ability,which transform the initial alignment without latitude as a Wahba problem of attitude determination,are proposed to achieve the initial alignment without latitude of SAHRS in complicated environments.The main contents of this dissertation are summarized as follows:Firstly,the accelerometer and gyro information and relevant constraint relationships,instead of using latitude information,are used to construct the model of earth rotation angular vector in navigation frame.Transforming the initial alignment without latitude as a Wahba problem,then the eigenvalue decomposition-based stationary self-alignment method and gradient descent optimization-based fast stationary alignment method are proposed successively.The error model shows the proposed methods achieve the same alignment errors,which reach the limit caused by sensor bias errors,as traditional OTRIAD method depending on latitude information.And it indicates that the latitude-free condition doesn’t increase extra alignment errors to the proposed methods.In addition,the simulation results demonstrate that the alignment results of the proposed methods are better than the exiting alignment method without latitude ONTRIAD1 by 12.31%,12.31% and2.83%,which are the same to the data-averaging preprocessed OTRIAD2.The proposed methods reach better result in yaw around 180°~ 360°than data-averaging preprocessed ONTRIAD2 without using latitude by 2.02%.Secondly,a gravity vector in earth frame-based self-alignment method without latitude for swaying base is proposed,which uses the apparent motion of gravity in inertial frame and relative constraints to construct the model of gravity in earth frame instead of using the latitude.It optimizes the model by replacing the initial measurement with multi-measurements.The objective function without latitude is established,and then the velocity-based and position-based objective functions are also constructed to improve the alignment accuracy on swaying base without latitude.The results show that the alignment results of proposed method are better than the exiting alignment method without latitude TVSA by 1.45%,13.43% and 27.85%,respectively,and approach to the traditional alignment method TRIAD.Thirdly,dealing with the alignment problem in-sailing condition without using latitude,a scheme contains the coarse estimation of attitude quaternion and its correction is proposed.It simplifies in-sailing alignment as a swaying base alignment problem by ignoring linear motion,builds the objective function from specific force equation,and then obtains the coarse attitude quaternion.After analyzing the influential mechanism of linear motion to alignment process without latitude,the gyro output is compensated using coarse attitude quaternion,which eliminates the linear motion influence and accomplishes in-sailing alignment without the latitude.Furthermore,a velocity integration formula-based in-sailing alignment method without the latitude is proposed.This method avoids the process of calculating acceleration by differencing the velocity and effectively solves the alignment problem in-sailing condition without using the latitude.The simulation results indicate that the alignment results of the proposed method are better than the traditional alignment method ADIA,which verifies the effectiveness of the proposed method to solve the in-sailing alignment problem without using latitude.Furthermore,the stationary and swaying alignment experiments of four turntable positions,the mooring experiment,the in-sailing alignment experiments on constant speed and maneuver conditions are implemented,which are used to validate the performances and effectiveness of the proposed methods,utilizing the self-designed fiber optic gyro(FOG)-based SAHRS(gyroscope drift: 0.01°/h and accelerometer bias: 100g).The results show that the proposed methods on stationary base achieve the horizontal maximum errors less than 0.01°and yaw maximum error less than 0.05°within 60 s.Compared with existing method without using latitude ONTRIAD,the yaw maximum error is reduced by8.31%.The proposed method on swaying base achieves the horizontal maximum errors less than 0.02°and yaw maximum error less than 0.07°within 600 s,which approaches that of traditional TRIAD method.The proposed method in-sailing condition achieves the horizontal maximum error less than 0.02°and yaw maximum error less than 0.07°within 1800 s.Therefore,the experiment results verify the effectiveness of the proposed methods to solve the practical alignment problems without using the latitude.