| Twenty-first century is the century of the ocean,underwater vehicles are critical support for countries to explore mysteries of the oceans,develop marine resources and compete for the command of the sea,and are widely used in the fields of marine science,commerce and military.Underwater navigation system provides the information of position,velocity and attitude for underwater vehicle,which are important for sailing safety and executing task.Inertial navigation system with advantages of all-weather and anti-jamming is the main positioning method of underwater vehicle,and is suitable for military underwater vehicle.Due to the error of inertial sensor,calculation error and gravity disturbance,the error of inertial navigation system accumulates over time,which seriously restricts the ability of long-endurance navigation.With the continuous improvement of inertial sensor accuracy,gravity disturbance has gradually become one of the major error sources of inertial navigation system,and compensation for the effect of gravity disturbance on inertial navigation system is an important way to improve covert navigation ability of underwater vehicle.This thesis focuses on the impact of gravity disturbances on inertial navigation system,studies the compensation method of gravity disturbance.Firstly,the error mechanism of gravity-induced error is analyzed,and compensation methods are proposed based on the analysis conclusions,and two key issues involved in the application of gravity disturbance compensation method are studied.The main research results are summarized as follows.(1)The error mechanism of gravity-induced error is analyzed.From the perspective of coordinate definition and vector calculation law,the effects of horizontal gravtiy disturbance on initial alignment and navigation calculation are systematically analyzed.The analysis indicates that due to the horizontal gravity disturbance the navigation coordinate frame built in initial alignment is not consistent with the coordinate frame in which the navigation calculation equation is derived.Then the vectors used in navigation equation are not projected into the same coordinate frame,which violates the law of vector calculation and causes error of inertial navigation system.(2)Based on the conclusion of error mechanism analysis,it is clear theoretically that horizontal gravity disturbance needs to be compensated in both initial alignment and navigation calculation,not only in one of the phases.For two different navigation coordinate frames,the method of compensation in velocity calculation and the method of compensation in attitude calculation are proposed.(3)The effect of accelerometer bias on horizontal gravity disturbance compensation is analyzed.The presence of accelerometer bias will bring uncertainty on the horizontal gravity disturbance compensation effect.In certain circumstances it may even happen that the accuracy of inertial navigation system decreases after compensation which is a critical issue in practice.The reason why the accelerometer bias causes the uncertainty of the compensation effect is analyzed,the model of gravity vector measurement noise is built and a new estimation method of accelerometer bias is proposed based on the model.(4)The calculation degree and order of gravity spherical harmonic model is investigated.The requirement of horizontal gravity disturbance compensation is analyzed,and it's indicated that the low-frequency component of gravity disturbance is the main source of gravity-induced error.According to the relationship between degree and frequency,the formula for calculating the cirtical calculation degree is derived,and a reduced-degree method of horizontal gravity disturbance compensation is proposed. |
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