應用於GPS系統之可適性多路徑追蹤及抑制迴路

全球定位系統(GPS)可全天候提供精確的位置、速度與時間的資訊給全球的使用者。在GPS系統中，其導航數據資料是利用直接展頻 (DS-SS) 分碼多工的方式進行傳遞。數種誤差來源影響了GPS量測的精確度，包括衛星時脈偏移，電離層延遲，對流層延遲，接收機動態追蹤誤差，多路徑效應與熱雜訊誤差等。理論上而言，經過差分法的技術可以消除所有兩個接收機的共有項誤差。然而多路徑效應因為接收機位置的不同而是無法加以消除，其成為高精度接收機中的主要誤差來源。一般而言，在GPS參考站與遠端接收機上所發生的多路徑效應是不一樣的。儼然已成為差分式GPS中最顯著的定位誤差來源。在本研究中，我們提出一個動態GPS應用中具有多路徑抑制的接收機系統架構。它包括四個部分: (1)可適性路徑估測器(Adaptive Path Estimator; APE)，(2)多路徑干擾重建器(Multipath Interference Reproducer; MPIR)，(3) 耙式碼延遲鎖相迴路(Rake-based Delay Locked Loop; RB-DLL)，(4) 耙式載波相位鎖相迴路(Rake-based Phase Locked Loop; RB-PLL)。在此僅考慮短路徑延遲所造成之效應(延遲時間在1.5 chip內)。為了在相關領域(Correlation Domain)估測反射路徑參數，我們採用快速傅利葉進行循環相關運算(Circular Correlation)來減少計算複雜度。同時利用可適性路徑估測器來估測多路徑效應中延遲路徑的各項參數。根據前項的預估參數，相對的多路徑成分啟動來完成延遲信號重建的功能。再將複製的延遲信號與具有多路徑效應的信號分別在載波鑑別器與碼鑑別器內部作相減的運算，如此一來便可將已消除多路徑干擾的信號送入耙式碼延遲鎖定迴路與耙式載波相位鎖定迴路中來完成信號同步的功能。本論文中，我們先對GPS展頻信號及多路徑效應信號特性進行了解，進一步設計可適性多路徑預估器，來估測短延遲時間參數，並將其放入耙式延遲相位鎖定迴路將多路徑干擾予以消除。我們採用Matlab模擬工具來驗證多路徑抑制系統的成效。在GPS室外操作時，在使用一般右手圓極化和半球形態的天線下，可假設所接收到的最小訊號功率約為-154.6 dBW。在一般狀況下，GPS接收機的有效雜訊溫度約為513K，相對應在2-MHz的中頻(IF)頻寬下所生成的熱雜訊功率約為-138.5 dBW。在此假設下針對不同的訊雜比(SNR)，即不同的IF頻寬下，利用可適性耙式延遲相位鎖定迴路，估測(1)反射延遲路徑時間(Reflection Delay Time)，(2)穩態追蹤誤差(Steady-state Tracking Error)，驗證其抑制干擾的成效與表現。

關鍵字

全球定位系統機收機；可適性路徑估測器；多路徑干擾重建器；耙式碼延遲鎖相迴路；耙式載波相位鎖相迴路

並列摘要

The global positioning system (GPS) provides accurate positioning and timing information useful in many applications. The GPS satellites broadcast ranging codes and navigation data with the technique of direct sequence spread spectrum (DS-SS). A wide variety of error sources affect the GPS measurement of pseudorange (also known as code-phase) and integrated Doppler (also known as carrier-phase). Among these are satellite user range error, ionospheric delay, tropospheric delay, receiver dynamic tracking error, multipah and thermal noise. The use of differential techniques theoretically eliminates all error sources which are common to both receivers. The error which remains is multipath, and it becomes the dominant error source in high precision GPS applications. Multipath errors are not identical to the GPS reference station and remote receivers. Thus, it becomes the significant error source in differential GPS. In this research, a multipath mitigation tracking system is presented for dynamic GPS applications. It is comprised of four function blocks, those being (1) adaptive path estimator (APE), (2) multipath interference reproducer (MPIR), (3) Rake-based delay locked loop (RB-DLL), and (4) Rake-based phase locked loop (RB-PLL). Only the short delay condition with delay less than 1.5 PN chip is considered here, because GPS pseudorange error envelope decreases to zero for delay time greater than 1.5 PN chip. In order to estimate reflection profile in the correlation domain, the FFT-based circular correlation and block average method (BAM) are utilized to offer significant savings in computational complexity. The APE estimates the delayed profiles and coefficients of the reflection signals. With the path parameters from APE, the corresponding multipath arms are activated to accomplish the multipath reproduction. These replica profiles are used for subtracting the reflection components from carrier and code discriminators before sending it into the Rake-based carrier/code tracking loops. In this thesis, we first introduce the characteristic of GPS spread-spectrum signal and multipath effect. Then, we design the APE and estimate the short-delay path parameters to perform multipath interference cancellation in the RB-DLL. The simulation results of the multipath mitigation system are obtained by using Matlab tool to verify the performances. In outdoor condition, the received signal power is assumed to be -154.6 dBW, because the minimum received signal power is about this value by using a typical GPS antenna with right-hand circular polarization and a hemispherical pattern. The noise power is assumed to be -138.5 dBW in a 2-MHz IF bandwidth, because a typical effective noise temperature for a GPS receiver is 513K. The reflection delay estimation and steady-state tracking error are conducted at different IF band SNR environments (i.e., different IF bandwidth) via extensive simulation to demonstrate the performances of our proposed adaptive rake-based multipath technique.

並列關鍵字

Global Positioning System (GPS) Receiver ； adaptive path estimator (APE) ； multipath interference reproducer (MPIR) ； Rake-based delay locked loop (RB-DLL) ； Rake-based phase locked loop (RB-PLL)

參考文獻

[1]E. D. Kaplan, ed., Understanding GPS: principles and

Introduction to spread spectrum communications, New

York: Prentice Hall International, 1985.

[3]Wern-Ho Sheen and Gordon L. Stuber, “A New Tracking

Loop for Direct Sequence Spread Spectrum Systems on

國際替代計量

應用於GPS系統之可適性多路徑追蹤及抑制迴路

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