## 4 Discussion
### Discussion on Spoofing Experiments Results  
This section discusses the results obtained in Section 3.2.3.  
The spoofing experiments were conducted according to the following description. First, the GPS L1 data of the satellite is normally received through the four antennas. After a period of time, turn on the spoofer device and broadcast spoofing signals to the four antennas. In the spoofing state, all the receiver receives are spoofing signals. After some time, turn off the spoofer device, and the four antennas will receive normal satellite signals again. The experiments mainly focus on the change of the SSE test metric under the spoofing state. The results of two spoofing experiments are shown in Figure 17a,b, respectively. During the time period without spoofing, the SSE test metric is smaller than the threshold. At the moment when the spoofer device is turned on or off, the SSE test metric has a large jump, and its value has a sudden change of about \(10^{2}\) magnitude. The SSE test metric is much larger than the threshold during the time period when the receiver is spoofed (the time period between the red dotted lines in Figure 17a,b). Each epoch can accurately determine whether the receiver has received spoofing signals, and the detection rate reaches 100%.  
In the case of spoofing, the elevation angle and azimuth angle of each spoofing signal is the direction information of the spoofer. It can be seen from Figure 18 that before the receiver is spoofed, the elevation and azimuth angles of G07 and G14 are different, and the satellite signal is related to the real position of the satellite in the airspace. After the receiver is spoofed, the elevation and azimuth angles of the two satellites suddenly change to near the true value of the direction of the spoofer. This is consistent with reality because, at this time, the signals received by the receiver are all sent by the transmitting antenna of the spoofer. After the spoofer device is turned off, the receiver receives satellite signals normally. The elevation angle and azimuth angle of the two satellites are no longer close to the direction of the transmitting antenna and return to normal. In this experiment, under spoofing conditions, since all satellite signals have the same DOA, and the calculated value of DOA is far from the expected value, it can be identified that the received signals are all spoofing signals.  
Figure 19 shows the changes in the orientation of all satellites in the airspace for normal and spoofing situations. Before the spoofer device is turned on, the satellites are at various locations in the airspace. After turning on the spoofer device, the number of satellites simulated by the spoofer is reduced compared with the normal situation, and the elevation and azimuth angles of the satellites are suddenly changed to the direction of the transmitting antenna of the spoofer, which is consistent with the reality. Table 4 shows the different information \(\Delta g_{n}\) on the DOAs of each signal broadcast by the spoofer. It can be seen from Table 4 that because the spoofing signals broadcast by the same spoofer reach the receiver through the same propagation path, the differences in DOAs between different signals are very small. This is consistent with reality. The direction of the transmitting antenna of the spoofer is \(Ele=41^{\circ},\ Azi=33^{\circ}\), and the error between the calculated values and the real value is within \(5^{\circ}\).