# Secure Indoor Localization Based on Extracting Trusted Fingerprint
## 3 Localization Model
### Effective Screening Model  
In the underground coal mine localization environment, the length and width of the coal mine, the number of mines and the position of obstacles are dynamically changing. The size of the RSSI will change in real time due to the environmental factors. The traditional static fingerprint database localization technology is not suitable for the scene whose environment changes frequently. In order to adapt to the variable environment of coal mines, we deploy some reference anchor nodes of known positions in the localization environment and update the fingerprint database according to the localization results of the anchor nodes. While updating the fingerprint database, this paper analyzes and compares the changes of RSSI received by reference anchor nodes to identify impaired APs. In the underground coal mine localization environment, the physical equipment and the personnel concentration areas are variable, so there are some difficulties in distinguishing the impaired AP from the environment-affected APs. In order to choose a better filter mechanism, we analyzed the characteristics of RSSI, which is affected by the environment. We collected RSSI of environment-affected APs and normal APs in four different directions as shown in Figure 5. The four images in Figure 5 represent RSSI obtained in four different directions, each of which represents normal RSSI and  
Figure 4: Localization error analysis. (**a**) Description of the normal position area. (**b**) Description of the position area when an AP is impaired. (**c**) Description of the position area when an AP is broken. disturbed RSSI. It can be seen from Figure 5 that the impact of environmental factors is regional, which is that only the areas affected by the environment have a change of RSSI, while RSSI in other regions are not affected. From Figure 5, we can draw the conclusion that the obvious change of RSSI can only occur in the direction of environmental impact, whose change do not affect all the reference anchor points. In order to find the difference between the impaired AP and the environment-affected AP, we measured the RSSI of the AP in four different directions when the AP was impaired as shown in Figure 6. Figure 6 shows the comparison of RSSI regarding impaired AP and normal AP in four different directions. As you can see from Figure 6, the RSSI around the AP change when the AP is impaired; the surrounding reference anchor nodes can detect the change of the RSSI and can obtain the fluctuation degree according to the RSSI collected over a period of time. However, when the environment changes, only the reference anchor nodes in a certain direction may detect the change of the fluctuation degree and the size of RSSI. Therefore, the method of voting by reference anchor nodes can be used to identify the impaired AP. Moreover, as can be seen from Figure 6, the RSSI fluctuation value of the normal AP is small, and the fluctuating RSSI value of the impaired AP is large. Furthermore, the normal AP's RSS value is greater than the value of the impaired AP. Due to the existence of RSS heterogeneity, such as RSS offset, there may be some error if comparing the average value of RSS before and after updating the database to determine whether the AP is normal. In addition, using the change of standard deviation before and after the update database as a voting standard, we can screen out APs with relatively stable RSSI values. Using these APs for positioning can make the positioning result more accurate. Therefore, this paper compares the standard deviation of RSSI before and after updating the database to vote on the impaired AP. Figure 5: Environmental impact on RSSI. (**a**) RSSI values that are measured in the east. (**b**) RSSI values that are measured in the west. (**c**) RSSI values that are measured in the south. (**d**) RSSI values that are measured in the north. When an AP is impaired, if the reference anchor node around it votes on the AP using the voting mechanism, the number of votes of the impaired AP will increase.