加利略衛星導航系統與其訊號擷取

Abstract

全球導航衛星系統(Global Navigation Satellites System; GNSS),乃泛指所有以衛星為基礎的導航系統。目前在太空中運行的衛星導航系統除美國GPS衛星系統、俄國GLONASS衛星系統外，歐洲加利歐(Galileo)衛星系統也正如火如荼的開始設置系統，預計在2008年底系統架設完畢。 加利歐(Galileo)衛星系統是EC和ESA所規劃設計發展的一個全球性、全天候，多項服務的三維空間即時定位系統。其技術部份是由JU(Joint Undertaking)所控管。利用三角定位原理以取得接收機之位置; 並藉由定位系統提供多樣性的服務。 加利歐系統所廣播的星曆資料係採用GTRF(Galileo Terrestrial Frame System)座標系統與GST(Galileo System Time)時間系統; 在訊號傳送上除採用分碼多工(CDMA)方式外，訊號並採用特殊的BOC調變與3G系統的引導頻道(Pilot Channel)概念; 而同一服務目的的訊號傳送頻帶少則兩個，多則三個，可以藉此抵消掉電離層與對流層效應。其衛星群共分三個軌道面，可涵蓋高、中、低緯度的地區。 加利歐衛星訊號因有特殊的BOC調變與較長的衛星碼設計，所以在訊號擷取與追蹤上有其困難性。因為較長的衛星碼導致接收機擷取訊號所能容忍的頻率誤差變小; 從GPS系統原本的每500Hz做一次訊號擷取，變成在加利歐系統擷取L1B與L1C頻道，每125Hz要做一次訊號擷取的動作。簡言之，即訊號擷取上，加利歐系統所運算的次數是擷取GPS訊號次數的四倍。這對接收機而言是一個沉重的負擔。 故我們欲探討書上所說，訊號擷取搜尋頻率為正負5k的合理性。對GPS而言，因為每500Hz做一次訊號擷取，所以對搜詢的頻帶範圍多1~2kHz不在意; 但對加利歐系統而言，每多1kHz的搜尋範圍，則需得增加8次訊號擷取運算量。這將會大大增加訊號擷取時間。所以欲藉由計算衛星相對定點接收機運動，實際都卜勒頻率變化範圍，來討論接收機在訊號擷取上，其上下限應為何才合理。也將在論文最後，討論其它可能影響到接收機訊號擷取運算次數的因素。

GNSS (Global Navigation Satellites System) is the general meaning of the satellites-based navigation system. The available two systems include GPS (Global Positioning System) of the United States and GLONASS (Global Navigation Satellites System) of the Russia right now; and another system, Galileo, will be set up well in 2008. Galileo is a global, all-weather and twenty-four-hours, precise three-dimensional navigation system, which developed by JU (Joint Undertaking) of ESA.GTRF (Galileo TeRrestrial Frame System) coordinate and GST (Galileo System Time) time system are adopted in Galileo. The signal transmitted by the techniques of CDMA (Code Division Modulation Access), BOC (Binary Offset Carrier) Modulation, and pilot channel. And there are two or three channels existed for one particular service; user can recover signal from these channel to reduce ionosphere and troposphere effects. The Galileo space segment is composed of a constellation of 27 satellites with 3 operational in-orbit spares. It is difficult to acquisition of and tracking Galileo signal because of BOC modulation and longer code length. We do GPS signal acquisition each 500Hz in 5k Hz of carrier frequency; but we must do Galileo signal acquisition each 125Hz in L1B and L1C channel. In other words, in GPS system, the acquisition times is 21 in 5k Hz, but it is 80 in Galileo system; it is a large sum. Then we think about why we must search the frequency area in 5k Hz? For GPS system, we do acquisition each 500Hz in GPS signal, so we careless of search frequency area. But we do acquisition each 125Hz in Galileo signal; if we reduce search frequency area, such as 1k Hz, we can decrease acquisition 8 times. We calculate doppler frequency which cause by Galileo satellite moves in it’s orbit relative to fixed position in earth. And we want to find the reasonal search frequency area in GPS and Galileo system. Finaly, we drew some conclusions about reducing search frequency area.