## 2 Related Work  
### T-L Model  
The airborne magnetic survey system consists of multiple modules that are mainly composed of an optical pump magnetometer (OPM), fluxgate magnetometer, radar altimeter, GPS navigation module and other units. Among them, OPM is a scalar total field magnetometer that provides high-precision measurement of space magnetic fields [18]. Its measurement is the sum of the modulus of the geomagnetic field vector and other magnetic field vectors (magnetic interference, target signal, etc.). It is numerically equivalent to the sum of the modulus of the geomagnetic field vector and the projections of other magnetic fields in the direction of the geomagnetic field. The obtained signal is the superposition of other interference fields and magnetic target signals after filtering the slowly changing geomagnetic field. So, it is important to eliminate the magnetic interference of the UAV platform for aeromagnetic measurement. A fluxgate magnetometer can measure the vector of a space's magnetic field. It can obtain the information of the included angle between the coordinate system of the flight platform and the geomagnetic field vector through the fluxgate magnetometer fixedly connecting to the flight platform, so as to measure the attitude of the flight platform. The space of the manned vehicle is large enough, so the magnetometer can be installed far away from the magnetic interference sources such as electronic equipment to eliminate magnetic interference. Therefore, the traditional compensation method considers the magnetic interference caused by the interaction between the airframe material and the background geomagnetic field during aircraft movement. In 1950, [PERSON] and [PERSON] proposed that the aircraft interference field can be divided into three parts: residual magnetic field, induction field and eddy current field [19; 20; 21]. The magnetic field is a vector, and it is necessary to establish a coordinate system for analysis. As shown in Figure 1, the position of the magnetometer on the aircraft is taken as the origin. The X-axis is parallel to the fuselage axis and points to the nose, the Y-axis is perpendicular to the profile of the fuselage and points to the right wing and the Z-axis is perpendicular to the plane. e represents the geomagnetic field. \(\alpha\), \(\beta\), \(\gamma\), respectively, represent the angle between the geomagnetic field vector and the triaxial coordinate system of the body. (1) Residual magnetic field: It is generated by ferromagnetic materials in the aircraft and generally considered to be a constant vector in the coordinate system of the body. The measured value of a scalar magnetometer is the superposition of the projection of the geomagnetic field and magnetic anomaly field in the direction of the geomagnetic field, so the residual magnetic field interference can be expressed as  
\[H_{rd}=\frac{H_{e}}{|H_{e}|}\cdot H_{r}=H_{rx}cos\alpha+H_{ry}cos\beta+H_{rz} cos\gamma= \begin{bmatrix}cos\alpha\\ cos\beta\\ cos\gamma\end{bmatrix} ^{T}\cdot \begin{bmatrix}H_{rx}\\ H_{ry}\\ H_{rz}\end{bmatrix} \]  
where \(H_{r}\) represents the residual magnetic field vector. \(H_{rx}\), \(H_{ry}\) and \(H_{rz}\) are the three components of XYZ of the constant magnetic field in the body coordinate system. (2) Induced magnetic field: It is mainly generated by the magnetization of the soft magnetic material on the aircraft by the geomagnetic field. The magnetic material on the aircraft has various shapes and complex distribution. In the modeling, it is equivalent to three soft magnetic rods that are only magnetized in the X, Y and Z directions. Due to demagnetization, the induced magnetic field of the soft magnetic bar is in different directions from that of the magnetized magnetic field. Therefore, after the soft magnetic bar in the X direction is magnetized by the X component of the geomagnetic field, it induces magnetic fields along the X, Y and Z directions. Similarly, Y and Z direction soft  
Figure 1: Aircraft coordinate system diagram.