Research On Calibration And Compensation Method Of Airborne Fluxgate Magnetic Tensor Gradiometer

Magnetic tensor gradiometer is an instrument that measures the spatial derivatives of three magnetic field components and can be loaded into the flight platform for aeronautical geophysical exploration.It has a high spatial resolution,rich information and other characteristics,especially for the discovery of shallow buried mines and magnetic moving targets,etc.Magnetic gradient tensor is generally composed of sensors that can measure magnetic vectors by a certain spatial distribution,such as superconducting and fluxgate,etc.Magnetic tensor gradiometers,using the fluxgates as the core components,have the advantages of wide temperature range,low cost and high resolution.However,there are two key issues that greatly reduce the data quality of the fluxgate magnetic tensor gradiometer.One is that the fluxgate magnetic tensor gradiometer suffers a variety of system errors,such as scale-factor,non-orthogonal,misalignment,zero-offset,dynamic,and nonlinear errors in a fluxgate,along with the inconsistency among the errors of fluxgates.The other is that maneuvering an aircraft flying in the geomagnetic field can generate magnetic interference effects on a tensor gradiometer,such as permanent magnetic interference,induced magnetic interference and eddy current magnetic interference.When the fluxgate magnetic tensor gradiometers is very far away from the aircraft,the data collected by the fluxgate magnetic tensor gradiometers is only affected by its own system error.In view of this application,this paper first proposed a system error correction method for fluxgate magnetic tensor gradiometer.This method is based on the static error model and the dynamic characteristics of single fluxgate.The system is calibrated in two steps: the first step to complete the correction with the feature that the gradient tensor of the geomagnetic field at high altitude is approximately zero.The second step to complete the correction on the phenomenon that the unchanged tensor rotation as a result of non-uniform magnetic field on the ground can indicate the inconsistency of the scale factors among different tensor components.Simulation experiments and the actual flight results using a helicopter have verified the effectiveness of the proposed method.The improvement ratios of the field tensor components are from 359.6 to 1765,and the RMS of each component has reached to the level of 1n T/m.Considering the situation that the aircraft will produce magnetic interference when the distance between the fluxgate and the aircraft is very close,this paper presents a correction and compensation method for airborne fluxgate magnetic tensor gradiometer.Firstly,the magnetic interference model of the aircraft and the error model of a single fluxgate has been proposed.Then we have seamlessly combined two models into the unified calibration model of a tensor gradiometer by a recursive method.Finally,we have simultaneously elicited the correction coefficients and the magnetic properties of the aircraft by a calibration flight at high altitude in an area of low magnetic gradient.We have evaluated the performance of the proposed method through simulation and actual flight results using a microlight.The Root-Mean-Square noise of each component has reached the level of less than 2n T/m,and the improvement ratios are from 4087.4 to 17526 in terms of the measured field data of tensor components.The two methods proposed in this paper are very easy to extend to other structures of the fluxgate magnetic tensor gradiometer and the SQUID tensor gradiometer.