全球定位系統(Global Positioning System, GPS)衛星的訊號強度非常微弱，加上訊號傳輸的衰減，以致接收端所收到訊號的功率極低，容易受到環境的影響和其他訊號的干擾。更令人擔憂的是目前民用航空現代化的趨勢越來越仰賴GPS所提供的服務，且非法的人為干擾裝置也逐漸增加，倘若GPS訊號干擾裝置出現在機場周遭，將使飛航安全受到潛在的威脅，嚴重時更可能造成意外的發生。為了證實在日常生活中GPS干擾的存在，並試著找出可能的GPS干擾源，本論文分別以接收器前端(Frontend)內皆設有的自動控制增益(Automatic Gain Control, AGC)，以及藉由中頻(Intermediate Frequency, IF)訊號改良後的 能量觀測法(Modified Energy Detector Measurement)作為偵測GPS干擾訊號的主要原理，搭配的影像擷取技術設計出不同的干擾訊號檢測機制，並於不同電腦作業系統下以圖形化介面(Graphical User Interface, GUI)建立出兩套GPS干擾監測系統。而該系統中分為即時處理端和後處理端，在即時處理端主要是著重在GUI的方式實現，探討系統在多工時多執行緒(Multi-Thread)的應用；而後處理端則分別由時頻分析(Time-Frequency Analysis)和軟體定義無線電(Software Defined Radio, SDR)的方式，於訊號處理的階段做詳細的分析，試著找出干擾訊號在接收端所影響之部分，以驗證干擾訊號的存在和對使用者定位影響之程度大小；最後，再透過系統所擷取的影像記錄干擾發生前後環境之變化情形，並與同時收下之GPS訊號相關資訊進行比對，嘗試找出在日常生活可能的GPS干擾源。

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.

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