近年來，由於通訊技術日趨蓬勃發展，各種具有通訊功能的科技產品不斷地推陳出新，也使電波之使用環境日趨複雜，然而頻譜(spectrum)資源數量有限，因此開始頻繁出現違法設立無線電台之情況。而違法電台所形成之電波會對合法電台使用者產生干擾，進而可能造成通訊中斷或是原本頻段之電波使用者被覆蓋。然而現有之頻譜監測儀器大多數向國外購買，導致我國現有之頻譜監測儀器價格昂貴。
有鑒於此，本實驗透過軟體定義無線電（Software Defined Radio, SDR）以及軟體無線電周邊裝置(Universal Software Radio Peripheral, USRP)架設簡易的時間到達差（Time Difference of Arrival, TDOA） 定位系統，這種方式可以用幾萬元來實現。其中為了使每個監測站（Monitoring station）的時間精準同步，本研究將在各監測站台安裝全球定位系統自律振盪器（Global Position System disciplined oscillator, GPSDO），並且搭配網路時間協定（Network Time Protocol, NTP），來有效驗證 SDR 設備是否有正確得到衛星的訊號，以確保時間之精準度。其實驗結果顯示，透過GPSDO雖然無法讓各個監測站的啟動時間達到精準的同步，但可藉由自定義的校正啟動時間方法，減少訊號比對之誤差。然而利用此校正啟動時間之方法，仍舊無法運用在實際之定位，因此本研究對此結果逐一分析與討論。

In recent years, wireless communication technology has been widely used. A wide variety of telecommunication device as well as service, such as smart phone and over-the-top service is increasingly being introduced to the market, resulting in a huge demand of radio waves. However, radio spectrum is a limited resource. There are many illegal radio stations that may radiate electromagnetic waves causing interference with licensed radio stations. Hence, telecommunication regulators in the world need a radio monitoring system to identify and find the location of the unwanted radio stations.
However, in Taiwan, the major equipment of the existing spectrum monitors is often procured and imported from oversea, resulting in an inflated price for the existing spectrum monitoring system. In view of this, the thesis is to design a radio positioning system with Time Difference of Arrival (TDOA) algorithm. Software Defined Radio (SDR) and Universal Software Radio Peripheral (USRP) devices are employed in this system. The primary advantage of the proposed system is its low cost as compared with hardware-based TDOA technology. In fact the cost can be as low as a hundred thousands of NT dollars.
In order to synchronize the time of each monitoring station accurately, this study installed a Global Position System Disciplined Oscillator (GPSDO) at each monitoring station and integrated it with the Network Time Protocol (NTP) to effectively verify whether the SDR equipment has correctly obtained the satellite signals to ensure the time accuracy. The experimental results indicated that correcting the activation time of each station by GPS timing is not practical because there is still a slight error in the time of each station. However, this study revealed that the error of signal synchronization can be reduced to a certain level by a self-defining correction of the start-up time. Finally, we discussed further in regards to signal synchronization.

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