| Geomagnetic navigation has become one of the hot study points in navigation domain because of its characteristics of passivity, good concealment and so on. But many interferential magnetic fields of the vehicle, such as magnetic fields of ferromagnetic materials and electric equipments, will be added to the geomagnetic field, which lead to distortion of the magnetometer’s measurements and decrease navigational precision. So it is necessary to study the characteristics of vehicle magnetic field and find methods to compensate it.This paper studies on the characteristics of vehicle interferential magnetic fields, restraining method of random interferential magnetic fields in measured geomagnetic signal, vehicle magnetic fields compensation method based on linear measurement model and vehicle magnetic fields compensation method based on non-linear model, tests the effects of presented methods by conducting an experiment on an underwater vehicle. The main work and results are listed as following:1. Aiming at the problems of numerous sources and complex characteristics of vehicle interferential field, this paper put forwards magnetic vehicle analysis method based on finite-element method and studies on the characteristics of vehicle induced field and eddy current field. The finite-element model of a certain vehicle is set up. Characteristics of vehicle induced field and eddy current field are studied in electromagnetic analysis software Ansoft Maxwell. The induced field is studied using three-dimensional static analysis. The results show that the three components of induced field are proportional to geomagnetic field, and the method that determine induced field coefficient matrix is found. The eddy current magnetic field is studied using three-dimensional transient analysis; the magnetic field distribution and influences of geomagnetic field, vehicle velocity, acceleration on it are analyzed. The results show that the magnitude of eddy current field in vehicle reaches the max at vehicle middle part and the minimum at ends, and it is proportional to geomagnetic field and vehicle acceleration but has nothing to do with vehicle velocity. These results are important in instructing magnetometer configuration and interferential magnetic field modeling.2. Aiming at the noises and random interferential magnetic fields in geomagnetic measurements, a method based on morphological filter and Hilbert-Huang transformation is put forward. The noises and its characteristics in the signal measured by the magnetometer are analyzed; a generalized combined morphological filter is designed using structure elements with different shapes and sizes. The principal and application of Hilbert-Huang transformation in signal analysis is studied, and the geomagnetic signal analysis and preprocessing method based on morphological filter and HHT is presented. The morphological filter is used to delete the instantaneous sharp interferential noise, so that spectrum overlapping during empirical mode decomposition can be reduced. After decomposition, a proper threshold is set according to start time and duration of the noises to process the signal. The results of experiment show that the presented method can not only be used to analyze the characteristics of different kinds of noises in geomagnetic signal but also do well in eliminating the instantaneous sharp interferential noises and noises which is close in frequency to that of the geomagnetic signal.3. In order to eliminate the permanent and induced interferential fields, a vehicle compensation method based on linear measurement model are presented. Based on the characteristic that geomagnetic magnitude at any point near the earth is constant and independent of magnetometer attitude, a mathematical formula for vehicle interferential field compensation is deduced and the compensation procedure is also given. At the same time, the fact that solving coefficients of interferential fields is the key for compensation is pointed out. The trust-region method and Unscented Kalman filter method are both used to evaluate the parameters in the model. Simulations and experiment are conducted to analyze performances and effects of the methods. Comparison of the two methods with the elliptical fitting based method is also drawn. Results show that trust-region method has higher parameter estimation precision and better robustness, unscented Kalman filter method is sensitive to original parameters, but has advantage in real-time processing. In calibration maneuver, vehicle should encounter as many as its real attitude angles to improve estimation precision of the parameters.4. In order to further improve the compensation precision, after analyzing on errors of vehicle magnetic field linear measurement model, a nonlinear semi-parametric model based method is presented aiming at the interferential fields that can’t be described with definite physical model. Non-parametric model is used to describe the influence of non-parametric components in vehicle interferential fields and nonlinear semi-parametric model is set up for compensation. Two-step kernel estimation method is used to estimate parameters in the model. Results of simulation and experiment show that nonlinear semi-parametric model can describe the influences of vehicle magnetic fields better. With this model, parameter estimation precision and compensation effect are both improved. Aiming at the conditions when eddy current magnetic field can’t be ignored, a method using vectorial magnetometer measurement for compensating geomagnetic magnitude is presented. The method first turns differential equation into algebraic equation by parameter expansion, and then solves the equation using least square method. Attitude measurement equipment is used to provide attitude angles. A simulation is done to test the effect of the method. The result shows that the presented method can compensate permanent, induced and eddy current magnetic field, and it is also simple and easy to realize.5. An underwater carrying experiment is designed and conducted to test the vehicle interferential fields compensation method presented before. Basic information of the experiment, such as subjects and environment, is introduced. Overall solution and maneuver solution in calibrating area are designed separately. A land test is conducted to test the magnetic field character of vehicle and inertial navigation system. The result is used to instruct the magnetometer configuration. The measurements got after experiment are preprocessed first, and then vehicle magnetic field parameters are evaluated using traditional measurement model and nonlinear semi-parametric model separately. Finally the measurements are compensated using estimated parameters. The results show that in calibration area, after compensation with traditional measurement model, the max absolute measurement error is 75 n T, which reduces to 60 n T after processed with the semoparametric model. This result shows that the analysis and compensation method presented before can eliminate the influence of vehicle interferential magnetic field and improve measurement precision o f geomagnetic field effectively. As is reported publicly, this is the first carrying experiment on real equipment in China. |
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