## 3 Materials and Methods
### Sensitivity Phenomenon and Criterion on Navigation Receiver  
In studies on navigation receivers, the C/N0 [26; 27] is always used as the sensitivity criterion for the satellites' loss of lock. In the test, a certain type of navigation signal simulator is utilized to generate navigation signals, and the transmitting power is set as \(-90\) dBm and \(-110\) dBm, respectively. When \(9\) satellites are turned on, the initial C/N0 of all satellites is \(54\) dB-Hz and \(50\) dB-Hz, separately, which is not proportional to the transmitting signal power. By observing the output data of the satellites' status in receiver software, the C/N0 of each working satellite can be obtained. The receiver could not receive any signal from the satellites when the number of positioning satellites is less than \(5\). In that case, the C/N0 of each satellite which is losing its positioning is less than \(38\) dB-Hz under \(-90\) dBm working power and it is less than \(34\) dB-Hz for \(-110\) dBm working power. Hence, the critical interference C/N0 of each satellite is defined as \(38\) dB-Hz when the working power is \(-90\) dBm, while the C/N0 needs to be reduced to \(34\) dB-Hz when the working power is \(-110\) dBm. Therefore, it is inaccurate to employ a fixed C/N0 value as the sensitivity criterion of the navigation receiver. In this study, the requirement of positioning is that there must be at least \(5\) positioning satellites for the navigation receiver. During the interference process, the satellite's C/N0 is constantly changing. When the navigation receiver is interfered by single-frequency electromagnetic radiation at a certain frequency, satellites C/N0 may experience the process of \"decreasing-jumping-increasing\", and the system is still in positioning in the end. Therefore, it is difficult to decide the critical interference state of the navigation receiver only by using the C/N0 as the sensitivity criterion. By observing the positioning longitude, latitude and altitude in the navigation system software, it is found that the positioning altitude is more sensitive to electromagnetic interference. In our experimental environment, the altitude usually keeps at \(66\) m without any electromagnetic radiation interference. This value is related to the geographical position of receiver. It is rapidly reduced to \(0\) m without any intermediate value appearing when the electromagnetic interference reaches a certain level. In this case, the navigation receiver loses its positioning function. It is easier to tell if the system is working properly by using the altitude. Therefore, the positioning altitude is taken as the sensitivity criterion will be helpful to test the accurate sensitivity data. Each sensitive phenomenon obtained by observing the positioning altitude of the navigation receiver in the experiment is shown in Table 1. In order to define the accurate sensitivity criterion, it is necessary to examine the effect phenomenon so as to ensure the stability of the sensitivity criterion. As can be seen from Table 1, the loss of positioning duration is different in each phenomenon level. To accurately define the sensitivity criteria, not only must be the navigation receiver be effectively interfered by the minimum power, but also the time parameter is a prerequisite. In phenomenon level 1, the loss of positioning only is shown for a second, which cannot lead to effective interference to the navigation receiver, and even this phenomenon does not occur at some frequency points. Both of the phenomenon level 2 and level 3 have great uncertainties on measurement, the measured threshold of sensitivity is not stable. The measurement of sensitivity according to the first three sensitivity phenomena in Table 1 is extremely unstable, the maximum error is up to 6 dB, which exceeds the acceptable tolerance defined in GJB-151B-2013 [13].