# Analysis of Electromagnetic Field Characteristics of Wave Glider
## 4 Analysis of Magnetic Field Noise Characteristics
### Measurement Result Analysis  
Place the magnetic field sensor 30 cm directly below the steering engine for measurement, the measurement results are shown in Figure 12. As the action of the steering engine must be caused by changes in the angle of the tractor, the significant increase or decrease in the magnetic field in the figure is due to changes in the position of the tractor, while the rapid change in the magnetic field is caused by the action of the steering engine.  
Figure 11: Measurement method for magnetic field of underwater gliders. (**a**) Measurement method for magnetic field of underwater towed steering engines. (**b**) Measurement method for magnetic field of underwater towed body shaking.  
As can be seen from Figure 13, when the magnetic field sensor is mounted on the metal at the bottom of the traction machine, although there is a certain distance from the rudder, it will still be affected by the magnetic field interference generated by the rudder action. However, the amplitude is much smaller than that mounted directly below the rudder, which is due to the fact that the magnetic field signal is inversely proportional to the distance third power.  
As can be seen from the analysis in Figure 14, in order to simulate the magnetic field generated when the tractor is swaying under the action of waves, the peak value of the magnetic field associated with the swaying is measured to be about 20 nT when the sensor is placed at a location 80 cm directly below the tractor, and the magnetic field generated by the swaying is relatively stable with a small range of variation in the amplitude of the signal, unlike that caused by the rudder operation.  
Figure 12: The magnetic field generated by the action of the underwater towing steering engine (sensor located directly below, with a distance of 30 cm). (**a**) Three-axis magnetic component; (**b**) magnetic modulus.  
Figure 13: The magnetic field generated by the action of the underwater towing steering engine (sensors installed on metal trailers). (**a**) Three-axis magnetic component; (**b**) magnetic modulus.  
By analyzing the measurement results (as shown in Figure 15), it can be concluded that the sensor is located 80 cm directly below the underwater tractor, and the operation of the steering engine will still produce a certain impact on magnetic field measurement.  
Measure the magnetic field intensity of the glider underwater towing machine (as shown in Figure 16), and the peak-to-peak value of the magnetic field when passing is about 80 nT. Due to the length of the towing machine being about 2 m, the magnetic moment of the towing machine is roughly calculated to be about 0.5 A\(\cdot\)m\({}^{2}\).  
According to the analysis of the measured magnetic field data, it can be seen that the amplitude of the magnetic field signal generated by the underwater towing body swaying of the wave glider is small, and the change is relatively smooth, while the amplitude of the magnetic field signal generated by the rudder work is large and the speed of change is fast, so it can be known that the underwater towing body rudder is the main source of the magnetic field interference when the wave glider is underwater.  
Figure 14: The sensor is located 80 cm directly below the tractor (the tractor shakes left and right, while the steering engine does not move). (**a**) Three-axis magnetic component; (**b**) magnetic modulus.  
Figure 15: The sensor is located 80 cm directly below the tractor (underwater towing steering engine action). (**a**) Three-axis magnetic component; (**b**) magnetic modulus.