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Introduction to GPS The Global Positioning System - Part 2

GPS Details 2,1 cấu trúc tín hiệu GPS Như đã đề cập trong Chương 1, mỗi vệ tinh GPS truyền một tín hiệu vô tuyến lò vi sóng bao gồm hai tần số sóng mang (hoặc sóng sin) được điều chế bởi hai mã kỹ thuật số và một thông báo chuyển hướng (xem Hình 2.1). Hai tần số sóng mang được tạo ra tại 1,575.42 MHz (gọi tắt là nhà cung cấp dịch vụ L1) và 1,227.60 MHz (gọi tắt là nhà cung cấp dịch vụ L2). . | 2 GPS Details Positioning or finding the user s location with GPS requires some understanding of the GPS signal structure and how the measurements can be made. Likewise as the GPS signal is received through a GPS receiver understanding the capabilities and limitations of the various types of GPS receivers is essential. Furthermore the GPS measurements like all measurable quantities contain errors and biases which can be removed or reduced by combining the various GPS observables. This chapter discusses these issues in detail. 2.1 GPS signal structure As mentioned in Chapter 1 each GPS satellite transmits a microwave radio signal composed of two carrier frequencies or sine waves modulated by two digital codes and a navigation message see Figure 2.1 . The two carrier frequencies are generated at 1 575.42 MHz referred to as the L1 carrier and 1 227.60 MHz referred to as the L2 carrier . The corresponding carrier wavelengths are approximately 19 cm and 24.4 cm respectively which result from the relation between the carrier frequency and the speed of 13 14 Introduction to GPS light in space 1 2 . The availability of the two carrier frequencies allows for correcting a major GPS error known as the ionospheric delay see Chapter 3 for details . All of the GPS satellites transmit the same L1 and L2 carrier frequencies. The code modulation however is different for each satellite which significantly minimizes the signal interference. The two GPS codes are called coarse acquisition or C A-code and precision or P-code . Each code consists of a stream of binary digits zeros and ones known as bits or chips. The codes are commonly known as PRN codes because they look like random signals i.e. they are noise-like signals . But in reality the codes are generated using a mathematical algorithm. Presently the C A-code is modulated onto the L1 carrier only while the P-code is modulated onto both the L1 and the L2 carriers. This modulation is called biphase modulation because the carrier