伽利略系統信號搜尋架構研究

全球導航衛星系統(GNSS)在現今已廣泛的使用在各個領域中，以美國的全球定位系統(GPS)最讓人熟知，而歐洲聯盟也自行開發下一代全球導航衛星系統，稱之為伽利略(Galileo)衛星定位系統。本論文採用低中頻接收機架構，可接收伽利略在E1頻帶的信號。並針對伽利略衛星碼與訊號調變上作處理。伽利略衛星訊號因有較長的衛星碼設計與特殊的二進制偏置載波(BOC, Binary Offset Carrier)調變，所以在訊號搜尋與追蹤上有其困難性。為了縮短伽利略衛星搜尋時間，所提出的方法是修改展頻碼的結構和縮短展頻碼週期，以降低所需的運算複雜性。然而，將展頻碼縮短的代價是，修改過後的展頻碼正交性，將不會和原始展頻碼一樣好，造成在最大相關增益的衰減；而修改過後的展頻碼再經過BOC的調變，會產生多個峰值的複雜搜尋訊號過程。本論文以非模稜兩可的搜尋演算法於分析BOC調變。為了避免模稜兩可的自相關函數(ACF, Autocorrelation Function)資料，使用非模稜兩可的搜尋演算法，如BPSK-like與SCPC架構，其在導航資料與副載波頻率的條件下，可以視為是一種兩個BPSK(Binary Phase Shift Keying) 疊加起來的調變訊號，以調製成明確可辨的訊號成份。最後，模擬環境為伽利略訊號中採用取樣頻率為38.192 MHz和中心頻率位(IF, Intermediate Frequency)於9.548 MHz。使用三種CSC不同結構的碼長在BPSK-like、SCPC架構下模擬。結果顯示，本論文所提出的搜尋演算法，除了降低硬體複雜度，且當訊號雜訊比(SNR)低、弱的時候，能提供更好的訊號正交特性與避開模稜兩可的訊號峰值所產生的誤警報。

關鍵字

全球導航衛星系統； BOC調變； BPSK-like方法； SCPC方法； CSC ；模稜兩可的擷取訊號

並列摘要

Nowadays GNSS (Global Navigation Satellite System) has been widely used in many places. Among all the navigation systems, GPS is widely used and well-known to people. The European Union will develop its own next generation GNSS, called Galileo Positioning System. In the thesis, the Low-IF receiver architecture is adopted to receive Galileo E1 signal. And modulation processing effect is analyzed for Galileo satellite signal. It is difficult to acquisition and tracking Galileo signal because of longer code length and BOC modulation. The proposed method is to modify the code structure and shorten the code period such that the acquisition complexity can decrease the search time of Galileo satellite. The orthogonality of the modified code will not be as good as the original code when there is degradation of maximum correlation gain. And modify the spreading code and then through the BOC modulation have multiple peaks, which complicate signal acquisition process. In this thesis, the impact of unambiguous acquisition algorithms for BOC modulation was analyzed. In order to avoid the ambiguities of the absolute value of ACF, unambiguous acquisition algorithms, namely, BPSK-like and SCPC, are used, which can be considered as a superposition of two BPSK modulated signals, located at negative and positive subcarrier frequencies. Final, the Galileo signal is sampled at a frequency of 38.192MHz and the signal is down-converted to an IF of 9.548MHz. Using three kinds of different CSC structural length is conducted in BPSK-like or SCPC architecture simulation. It is shown that our proposed search algorithm can provide better performance with low hardware complexity for acquiring these satellite signals and possible wrong peak selection in the low Signal-to-Noise Ratio (SNR) conditions.