| Summary: | 碩士 === 國立成功大學 === 民航研究所 === 102 === SUMMARY
In order to confirm existence of GPS (Global Positioning System) interference and try to identify possible interference sources, this work respectively used the AGC (Automatic Gain Control) theory and the modified energy detector measurement with IF (Intermediate Frequency) signal on the GPS receiver frontend for real time GPS RFI (Radio Frequency Interference) monitors. Based on the results of experiments, the existence of GPS interference was confirmed and some possible sources of interference were identified near our university. In the future, the proposed GPS interference monitor scheme could be implemented at airport vicinity to protect the GPS spectrum from possible interferences to harden aviation satellite navigation service.
INTRODUCTION
The civil aviation organizations worldwide are implementing next generation communication, navigation, surveillance, and air traffic management (CNS/ATM), and the enabling technology of this CNS/ATM system is the GPS, for instance, the time synchronization of the air data network and the air communication network, and aircraft navigation for approach and landing. However, the transmitting power of GPS signal is extremely low and thus vulnerable to intentional or unintentional RFIs. Because GPS is the enabling technology of the modern civil aviation applications, there would be a significant impact on the aviation operations if GPS service is degraded. Thus, the objectives of this work are to confirm the existence of GPS interference and try to identify possible sources of GPS interference sources in our daily life.
METHODS
The AGC theory and the modified energy detector measurement with IF signal were applied on the GPS receiver frontend for real time GPS RFI (Radio Frequency Interference) monitors.
Any signal that was closed to the GPS L1 band might be able to pass the band-pass filter of one GPS frontend, and the noise floor would be therefore raised such that the receiver could not acquire or track the GPS satellite. Based on the prior work, the AGC in the receiver frontend gave an indication of the noise level that was received by the frontend. Therefore, with the calibration of daily satellites AGC patterns at the same location, the large changes in the AGC could be identified as possible RFI events. The use of AGC for RFI monitoring was attractive because no additional component was needed to add to the receiver hardware. The AGC function is a simple mechanism that be used on detecting interference. But only few receivers could output this AGC information, the other method based on the signal energy measurement which called the modified energy detector measurement method was proposed. In addition, our GPS RFI monitor system had the real-time processing part and the post-processing part. The real-time processing part was developed by the applications of multi-thread and was presented in the implemented GUI (Graphical User Interface). The post-processing part included the analyses of time-frequency characteristics and user positioning. The time-frequency analysis applied the WELCH algorithm to the PSD (Power Spectral Density) measurement, and the user positioning analysis presented the acquisition and tracking result in baseband.
RESULTS AND DISCUSSION
In this work, the focus was on detecting and analyzing the moving vehicles which might carry possible GPS interference sources on the roads nearby our university. The monitor system first recorded the IF data while the possible GPS interference event took place, and then the system generated the corresponding time-frequency plots to evaluate the characteristics of the possible GPS interference. In addition, the possible intentional GPS interference source was the major interest of this work, so the detection scheme included a video camera to capture the pictures of the surrounding area when the detection was triggered. These pictures as well as the corresponding time-frequency plots would be very helpful to classify the possible GPS interference source. Finally, the GPS SDR (Software Defined Radio) receiver was used to change the receiver parameters and structures to verify the impacts of the various interference on GPS signal processing and the resulting user positioning performance. Figures 1 and 2 were the experimental results by the monitor system with AGC detector and the modified energy detector. Figure 1 showed the interference event that blocked all GPS satellite signals for at least 80 seconds and Figure 2 illustrated the possible source of interference of interest on the road.
CONCLUSION
The objectives of this work were the design of GPS RFI detection scheme that could identify some moving vehicles which might carry possible GPS interference sources on the roads. A GPS RFI monitor system was developed which included a GPS SDR receiver with the AGC based detector and the energy based RFI detector to detect possible interference events and a video camera recorded the corresponding video stream simultaneously with RFI events. As shown in the experimental results, the developed GPS RFI monitor system successfully detected and identified the possible GPS RFI sources on the roads around our university. Finally, the impacts of the detected GPS RFI event on the GPS signal processing and the resulting positioning performance were presented in this thesis as well.
|
|---|
