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Acquisition of GPS C/ A Code Signals Để theo dõi và giải mã thông tin trong các tín hiệu GPS, một phương pháp mua lại phải được sử dụng để phát hiện sự hiện diện của tín hiệu. Sau khi tín hiệu được phát hiện, các thông số cần thiết phải đạt được và truyền cho một chương trình theo dõi. Từ các chương trình theo dõi thông tin như các dữ liệu điều hướng có thể thu được. Như đã đề cập tại mục 3.5, phương pháp mua lại phải tìm kiếm trên một dải tần số ± 10. | 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 SEVEN Acquisition of GPS C A Code Signals 7.1 INTRODUCTION In order to track and decode the information in the GPS signal an acquisition method must be used to detect the presence of the signal. Once the signal is detected the necessary parameters must be obtained and passed to a tracking program. From the tracking program information such as the navigation data can be obtained. As mentioned in Section 3.5 the acquisition method must search over a frequency range of 10 KHz to cover all of the expected Doppler frequency range for high-speed aircraft. In order to accomplish the search in a short time the bandwidth of the searching program cannot be very narrow. Using a narrow bandwidth for searching means taking many steps to cover the desired frequency range and it is time consuming. Searching through with a wide bandwidth filter will provide relatively poor sensitivity. On the other hand the tracking method has a very narrow bandwidth thus high sensitivity can be achieved. In this chapter three acquisition methods will be discussed conventional fast Fourier transform FFT and delay and multiplication. The concept of acquiring a weak signal using a relatively long record will also be discussed. The FFT method and the conventional method generate the same results. The FFT method can be considered as a reduced computational version of the conventional method. The delay and multiplication method can operate faster than the FFT method with inferior performance that is lower signal-to-noise ratio. In other words there is a trade-off between these two methods that is speed versus sensitivity. If the signal is strong the fast low-sensitivity acquisition method can find it. If the signal is weak the low-sensitivity acquisition will miss it but the conventional method will find it. If the
