應用於全球定位系統之多路徑干擾偵測及消除演算法

Multipath Detection and Mitigation Algorithm for Global Positioning System

陳民培(Min-Pei Chen)

指導教授 ： 曹恆偉

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.01210

摘要

隨著衛星全球定位系統(Global Positioning System, GPS)的成熟，許多共同的定位誤差來源能夠藉由差分技巧(Differential Technique)加以消除，像是時脈誤差(Clock Error)、對流層誤差(Tropospheric Error)和電離層誤差(Ionospheric Error)。然而多路徑干擾(Multipath Interference)會隨時間和環境的不同而改變，無法使用差分技巧來消除，所以成為接收機定位誤差的主要來源之一。
依據多路徑訊號的特性，本論文分別提出多路徑干擾偵測(Detection)和多路徑干擾消除(Mitigation)的演算法。藉由偵測演算法，接收機能夠判斷是否存在多路徑訊號；當偵測到多路徑訊號，則執行減低定位誤差的方法。設計的議題，包含相關器電碼相位間距(Correlator Spacing)的決定、移動平均濾波器(Moving Average Filter)的tap數和臨界值(Threshold)的設定，並分析前端射頻濾波器(Front-End RF Filter)效應和基頻電路中定點(Fixed-Point)計算的影響。模擬結果顯示偵測器能夠達到即時(Real-Time)偵測的要求，並且其誤判率(False Alarm Probability)和錯失率(Miss Detection Probability)都很低。消除演算法是在電碼鎖相迴路(Delay Lock Loop, DLL)的鑑別器(Discriminator)上作設計，藉由提高鑑別器增益(Discriminator Gain)，能夠達到減少多路徑干擾效應的好處。由模擬結果，與短相關器(Narrow Correlator)相比，在射頻濾波器頻寬為8.184 MHz下，能夠減少約20%的虛擬距離(Pseudorange)誤差；當頻寬為16.368 MHz時，則能減少約56%的虛擬距離誤差；而其抖動(Jitter)則稍增為短相關器的1.6倍。

關鍵字

多路徑干擾偵測 ； 交相關函數失真 ； 臨界值 ； 多路徑干擾消除 ； 鑑別器

並列摘要

In Global Positioning System (GPS), many positioning errors such as clock error, tropospheric error and ionospheric error can be eliminated by using differential techniques. But characteristics of multipath interference depend on time and environment, which means that it can’t be canceled by differential technique. Therefore, multipath becomes one of the main error sources in GPS.
According to the characteristics of multipath, this thesis proposes the multipath detection and multipath mitigation algorithm. The purpose of multipath detector is to determine whether a multipath is present. If multipath detector decides that a multipath is present, then the receiver may take some actions to mitigate multipath effect. The design issues of multipath detector contain selection of correlator spacing, taps of moving average filter and threshold setting. The influences of front-end RF filter and fixed-point effect are also analyzed. Simulation results show that the false alarm and miss detection probability of multipath detector are quite low. Moreover, the ability of real-time detection is very robust. For multipath mitigation, this thesis proposes a code discriminator to reduce multipath effect by means of increasing discriminator gain. In comparison with conventional narrow correlator, our proposed discriminator provides a 20% reduction in pseudorange error when the front-end RF bandwidth is 8.184 MHz and 56% when it is 16.368 MHz. However, the jitter when using this proposed discriminator is about 1.6 times larger than narrow correlator.

並列關鍵字

multipath detection ； cross-correlation distortion ； threshold ； multipath mitigation ； discriminator

參考文獻

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