Tracking GPS Signals Người ta có thể nghĩ rằng phương pháp cơ bản theo dõi một tín hiệu là để xây dựng một bộ lọc băng hẹp xung quanh một tín hiệu đầu vào và làm theo nó. Nói cách khác, trong khi tần số của tín hiệu đầu vào thay đổi theo thời gian, tần số trung tâm của bộ lọc phải tuân theo tín hiệu. Trong quá trình theo dõi thực tế, tần số trung tâm của bộ lọc băng hẹp là cố định, nhưng một tín hiệu được tạo ra tại địa phương theo tần số của. | Fundamentals of Global Positioning System Receivers A Software Approach James Bao-Yen Tsui Copyright 2000 John Wiley Sons Inc. Print ISBN 0-471-38154-3 Electronic ISBN 0-471-20054-9 CHAPTER EIGHT Tracking GPS Signals INTRODUCTION One might think that the basic method of tracking a signal is to build a narrowband filter around an input signal and follow it. In other words while the frequency of the input signal varies over time the center frequency of the filter must follow the signal. In the actual tracking process the center frequency of the narrow-band filter is fixed but a locally generated signal follows the frequency of the input signal. The phases of the input and locally generated signals are compared through a phase comparator. The output from the phase comparator passes through a narrow-band filter. Since the tracking circuit has a very narrow bandwidth the sensitivity is relatively high in comparison with the acquisition method. When there are phase shifts in the carrier due to the C A code as in a GPS signal the code must be stripped off first as discussed in Section . The tracking process will follow the signal and obtain the information of the navigation data. If a GPS receiver is stationary the expected frequency change due to satellite movement is very slow as discussed in Chapter 3. Under this condition the frequency change of the locally generated signal is also slow therefore the update rate of the tracking loop can be low. In order to strip off the C A code another loop is needed. Thus to track a GPS signal two tracking loops are required. One loop is used to track the carrier frequency and is referred to as the carrier loop. The other one is used to track the C A code and is referred to as the code loop. In this chapter the basic loop concept will be discussed first. Two tracking methods will be discussed. The first one is the conventional tracking loop. The only unique point of this method is that the tracking loop will be presented in .
