【正文】 is classified under GNSS and was conceptualized by the United States Department of Defense () in , this is the only fully functional and globally available satellite navigation system since GPS satellite navigation system,consists of:1.‘Space Segment’ which has 24 operational satellites that continuously transmit a ranging signal containing a navigation message stating the current position with time corrections.2.‘Control Segment’ which tracks each satellite and periodically uploads to satellites its clock time corrections with the prediction of future satellite positions. 3.‘User Segment’ with a user receiver which tracks the ranging codes in the received signal and calculates the threedimensional position and local time. The GPS signals are transmitted on three radio frequencies (RF) in the Ultra High Frequency (UHF) band,which includes all frequencies between 500 MHz to frequencies are referred to as L1, L2 and L5 and are derived from a mon clock frequency,. These signals prise of the Carrier, Ranging Codes, and the Navigation Data. NAV Data The NAV data D(t) is a 50 bps data stream containing the Satellite Vehicles (SV) ephemerides,SV clock behavior data, status messages, handover information, etc. D(t) is modulo2 added (ExclusiveOR)ed to ranging codes and this resultant bit trains are modulated onto the L1 and L2 carriers. In Block IIRM and IIF SV’s, Civil Navigation data (CNAV)is used which includes all the parameters of D(t).is a 25 bps data stream which is coded by a rate1/2convolutional encoder. During the initial period of Block IIRM SV’s operation,the SV may modulo2 add the NAV data D(t) instead of CNAV data. Ranging Codes / Spreading Sequences Ranging codes or spreading sequences are referred to as PseudoRandom noise (PRN) sequences because of their random but yet deterministic I, II, IIA, IIR SV’s have two unique spreading sequences or first one is the Coarse Acquisition (C/A) code which is a sequence of 1023 chips and repeats itself every 1ms giving it a chip rate of second code is the Precision (P) code or the Y code which is used in place of Pcode whenever the antispoofing mode of operation has been ranging code is the modulo2 sum of two subsequences referred to as X1 and X2 with 15,345,000 and 15,345,037 chips is a 7 day long sequence with a chipping rate of Mbps. In Block IIRM, IIF and subsequent block of SV’s,two additional ranging codes are transmitted which are referred to as L2 CivilModerate (L2 CM) and L2 CivilLong (L2 CL).L2 CM code is 10230 chips in length and repeats itself every 20ms,giving it a chipping rate of CL being a longer code repeats itself every seconds retaining the Kbps chip rate resulting in 767250 chips. Carrier The carrier frequency,oris typically modulated by one or more bit trains which is a posite generated by modulo2 addition of PseudoRandom noise (PRN) ranging code and the system data referred to as NAV, D(t) / CNAV, data. Other Navigation Systems Since GPS was developed by .,other countries too were involved in the race for having a positioning system of their of them are briefly described below. GLONASS The Global Navigation Satellite System (GLONASS) is a Russian spacebased radio navigation system that provides Position,Velocity and has two L band navigation signals one for Civil and other for Military transmits a unique PRN code pair (C/A and P(Y)) but on different frequencies unlike GPS where the code pair is transmitted on the same frequency in a Code Division Multiple Access (CDMA) GLONASS satellite transmits a unique PRN code pair but on different frequencies. This is denoted as Frequency Division Multiple Access (FDMA).A GLONASS receiver tunes into the particular satellite in the same manner like tuning to a radio station. GALILEO GALILEO is currently being developed by the European Union (EU) and European Space Agency (ESA). It is expected to be fully operational by will have 30 satellites orbiting in 3 medium earth orbital planes with two control centers that would be setup in Europe to monitor and manage the navigation system。對于捕獲的硬件實現(xiàn)文中只對核心部分進行了講解。在北斗接收機中，捕獲算法的研究和實現(xiàn)是很重要的一個方面，主要研究成果如下： 本文先介紹了衛(wèi)星導(dǎo)航定位系統(tǒng)的信號模型，包括信號的產(chǎn)生原理，衛(wèi)星信號的特點，信號結(jié)構(gòu)以及調(diào)制解調(diào)等。 第5章 結(jié)論 北斗衛(wèi)星導(dǎo)航系統(tǒng)﹝BeiDou Navigation Satellite System，英文縮寫B(tài)DS﹞是中國正在實施的自主發(fā)展、獨立運行的全球衛(wèi)星導(dǎo)航系統(tǒng)。下面采用內(nèi)插的方式直接利用粗略捕獲得到的譜線進行內(nèi)插以得到更加精確的載波頻率估計。因而改進后的算法只需要一次FFT就可以同時完成頻率和碼相位的并行搜索，捕獲算法運算量將大大減少。 傳統(tǒng)基于FFT的并行捕獲算法假設(shè)收到北斗和GPS的信號經(jīng)過射頻前端下變頻變?yōu)橹蓄l信號，并在中頻進行采樣，則采樣后的信號的表達式為： （41） 其中 為第K顆衛(wèi)星的PN碼， 為調(diào)制數(shù)據(jù)信息， 為中頻頻率， 多普勒頻率偏移， 為載波相位， 為高斯白噪聲。根據(jù)偽隨機碼( PN)的自相關(guān)性直接序列擴頻( DSSS)信號的捕獲過程就是在( PN)碼碼相位和載波多普勒頻率域上進行的二維搜索的過程。FFT快捕電路功能框圖如下圖所示: 圖35 典型的FFT快捕電路功能框圖 基于FFT快速捕獲的多通道信號捕獲流程 四路測距通道和一路遙控通道共用一個處理模塊，逐個通道進行捕獲，當前搜索通道由其捕獲狀態(tài)和跟蹤鎖定狀態(tài)共同決定，當正在搜索的通道狀態(tài)為未捕獲到或跟蹤狀態(tài)為失鎖時，繼續(xù)搜索當前通道，否則轉(zhuǎn)入下一通道。10KHz,以500Hz的步長掃頻，則最大捕獲時間為： （315） 最小捕獲時間為： （316） 平均捕獲時間為： （317） 其中，TD為系統(tǒng)做一次FFT所需要的時間。時域滑動相關(guān)捕獲方法是最常容易的一種捕獲方法，當前大部分北斗和GPS接收機都采用這種方法實現(xiàn)C/A碼的捕獲。這種算法不便于簡化接收機的實現(xiàn)，而隨著現(xiàn)在科學(xué)技術(shù)的日益更新，現(xiàn)代通信需要實時的，高效益的，精密的測量和傳遞，時域捕獲是不符合科學(xué)發(fā)展的前提的。但是實際實現(xiàn)過程中需要注意一個問題：導(dǎo)航電文數(shù)據(jù)的傳輸速率是50bit/s，每20個C/A碼就有一個數(shù)據(jù)位，如果K大于20就會跨越數(shù)據(jù)位，使得累加值變小從而判斷到?jīng)]有捕獲到信號。根據(jù)C/A碼的強自相關(guān)性，當接收機所復(fù)現(xiàn)的C/A碼與輸入的衛(wèi)星C/A碼的碼相位在任何一邊的偏移超過一個基碼時，有最小的相關(guān)值，其間是線性的變化。 并行FFT快速捕獲算法總結(jié) 該方法采用并行搜索碼相位的同時利用FFT進行頻譜分析將傳統(tǒng)的二維搜索變成一維搜索,節(jié)省了捕獲時間,是比較合適的方法。引入了快速傅利葉變換將時域中進行的相關(guān)運算變換到頻率域處理,基本思路是利用時域相關(guān)等價于頻域相乘的關(guān)系,在頻域計算偽碼的相關(guān)函數(shù),將本地碼序列和接收到的一個周期的信號做FFT變換到頻域,相乘后再反變換到時域,將各碼相位的最大相關(guān)值與門限比較判斷。采用M個載波相關(guān)器,每個載波相關(guān)器的載波NCO分別產(chǎn)生不同頻率的載波。 碼并行載波串行:采用N個獨立的碼相關(guān)器,相關(guān)器之間相差半個碼片,它們共用一個載波NCO,載波多普勒