| In aeromagnetic anomaly detection, the aircraft fixed magnetic field produced by its hard magnetic materials, the inductive magnetic field produced by the magnetization of soft magnetic materials, and the eddy current magnetic field generated by the aircraft cutting the geomagnetic field, are interferencing magnetic fields that influence the magnetometer reading. The signal magnetic field in the flight height producted by the target to be detected is usually much smaller than the interferencing magnetic fields. As a result, eliminating or compensating the interferences is a key technique in aeromagnetic anomaly detection.The compensation algorithm developed in this thesis is based on the total magnetic field measured by a scalar magnetometer and the geomagnetic directions measured by a fluxgate magnetometer. It is a soft compensation procedure, realized by software.First, by analyzing and characterizing the interference properties, mathematical models are established for each type of intereferences, and complex collinearity between the basic interferencing functions are discussed. To acquire the data to carry out the compensation algorithm, a group of standard calibration flights is followd,which is usually a clockwise square, i.e. geomagnetic heading north, heading east,heading south, and heading west. The aitcraft performs roll, pitch and yaw maneuvers on each of the four meagntic headings. The scalar and vector magnetometers synchronously record the total magnetic field and the three components of the ambient field.Next, the compensation coefficents are extracted by using a weighted matching approach in conjunction with the ridge estimation method. The approach is reduced to the traditional Least Square Method(LSM) or the Galerkin method if the interferencing characteristics functions are used as the weighted functions. The ridge estimation method is usually giving better solution than that obtained by the LSM. Furthermore, an iterative scheme that solve for the background magnetic field and the interferening magnteic field is proposed. Numerical simulation results show that the proposed algorithm is feasible.Finally, in order to verify the compensation algorithm, a lot of outdoor simulation experiments are conducted. The proposed method is well validated by the measureddata. Usually, a quality factor less than 100 pT or an improved ratio great than 20 can be achieved, which meet most engineering requirements.The mathematical modeling of the aircraft magnetic interferences, the extracting method for the compensation coefficients, and the simulation experiment validations reported in the present thesis is useful to instruct real aeromagnetic compensation experiments at next stage. |
PDF Full Text Request