## 3 Theoretical Analysis and Experiment
### Experiment
#### 3.2.3 Experimental Results and Analysis  
Figure 13 shows the corresponding relations between the power supply current and the engine state during the experiment. The power supply current increases sharply when the engine is started. While the engine is running smoothly, the current is about 30A and there are fluctuations of peak value of 10A. The current decreases rapidly after engine stops, which proves that the engine is the main electrical equipment on the UAV and the engine operation can bring large current. The changes in magnetic field and current during the experiment are shown in Figure 14. The large current can cause great magnetic interference when the engine is running. As the engine is power equipment; it needs to work continuously as long as the aircraft is flying in the air, so if no measures are taken, the magnetic interference of UAV electronic equipment can always exist. Figure 12: Schematic diagram of placement orientation of experimental equipment. Figure 13: The corresponding relationships between the power supply current and the engine state. Figure 15 shows the changes in power supply current and total magnetic field when the engine is not working. According to the figure, the power supply current is less than 0.85A when the engine is not running, and the time-domain magnetic field has no obvious fluctuation. The magnetic field's peak-to-peak value is less than 0.2 nT after filtering. This proves that the experimental environment is stable and the influence of small current on magnetic measurement is not obvious. Figures 16 and 17, respectively, depict the changes in power supply current and total magnetic field when the UAV is placed in the north-south direction and the east-west direction. The graph, from top to bottom, is the current curve, the original magnetic field curve and the magnetic field curve after bandpass filtering. Comparing the current curve in Figures 16 and 17, the amplitude of the power supply current is basically the same when the engine is running, which is about 30 A. Combining the three curve graphs, it was found that the engine can generate large magnetic interference when it is running, and the magnetic interference has a strong correlation with the current change in time. Figure 14: Changes in magnetic field and current during the experiment. Figure 15: Changes in power supply current and total magnetic field when the engine is not working. It can be found that the total magnetic field is reduced by about 200 nT when the engine is running and the UAV is placed in the north-south direction but the total magnetic field is increased by about 100 nT when the UAV is placed in the east-west direction. Comparing the current curve and the original magnetic field curve in Figures 16 and 17, it was found that the characteristics of magnetic interference caused by the same current in different placement directions are different. According to the vector synthesis relationship, the included angle between the magnetic interference of UAV electronic equipment and the geomagnetic field vector should be obtuse when the UAV is placed in the north-south direction. When placed in the east-west direction, the angle between the magnetic interference of UAV electronic equipment and the geomagnetic field vector should be acute. That is to say, the magnetic interference vector of UAV electronic equipment should be similar to the characteristics of permanent magnet materials, regardless of the magnetic field's magnetization. Figure 16: Changes in power supply current and total magnetic field when the UAV is placed in the north-south direction. Figure 17: Changes in power supply current and total magnetic field when the UAV is placed in the east-west direction. It can be found from the magnetic field curve, after bandpass filtering, that the waveform of the magnetic interference that is caused by the suddenly changed current is similar to the magnetic target signals. This causes serious interference in target recognition. To sum up, three conclusions can be drawn:  
1.