近年來由於全球定位系統(GPS)快速的發展與技術的成熟，已廣泛地應用在導航定位系統上。一般大眾在使用定位系統裝置時，從螢幕地圖中可知道身在何處，但卻往往不清楚定位的精度為何。在本論文研究中，利用一般GPS接收器的標準輸出格式-NMEA資料來達成位置精度的判斷，以提供信號品質的資訊給使用者及其它接收端。我們使用加速儀與電子羅盤來分別對GPS計算的載具速度(SOG)、角度(COG)進行比對，並根據環境產生的遮蔽效應與多路徑效應，找出GPS信號的特性。其中較顯著的四項因子：仰角40度以上衛星的C/No平均值、SOG穩定度、COG解出之向心加速度以及PDOP值。最後我們提出利用模糊邏輯法的演算法完成GPS信號可信度的判斷。

With the rapid development and maturity in technology, Global Positioning System (GPS) has been widely used in navigation and positioning systems. One issue in GPS positioning is the lack of positioning accuracy information and people may use the GPS fix without any idea of its quality. In this thesis, the standard GPS NMEA output format is being used to determine the accuracy of the positioning and the information can then be provided to other applications or users. The Speed over Ground (SOG) and Course over Ground (COG) measured by the GPS are compared with the outputs from an accelerometer and a compass respectively. Based on the compared results, the characteristics of the GPS under the influence of the shadowing and multi-path effects of the environment are investigated. Four most important factors, including the average C/No values of the satellites with an elevation angle which is more than 40 degrees, the stability of SOG, the centripetal acceleration derived from COG and Position Dilution of Precision (PDOP) can be used to describe the GPS characteristics in these environments. Finally, a Fuzzy Logic based algorithm is proposed to determine the reliability and quality of the GPS fixes.

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