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Fundamentals of Global Positioning System Receivers A Software Approach - Chapter 2

Basic GPS Concept Chương này sẽ giới thiệu các khái niệm cơ bản của một máy thu GPS xác định vị trí của nó. Để hiểu rõ hơn về khái niệm, yêu cầu GPS hiệu suất sẽ được thảo luận đầu tiên. Những yêu cầu này xác định việc bố trí của chòm sao vệ tinh. Từ chòm sao vệ tinh, vị trí người sử dụng có thể được giải quyết. Tuy nhiên, các phương trình cần thiết để giải quyết vị trí người sử dụng bật ra được phương trình phi tuyến đồng thời, rất khó để giải quyết. | Fundamentals of Global Positioning System Receivers A Software Approach James Bao-Yen Tsui Copyright 2000 John Wiley Sons Inc. Print ISBN 0-471-38l54-3 Electronic ISBN 0-471-20054-9 CHAPTER TWO Basic GPS Concept 2.1 INTRODUCTION This chapter will introduce the basic concept of how a GPS receiver determines its position. In order to better understand the concept GPS performance requirements will be discussed first. These requirements determine the arrangement of the satellite constellation. From the satellite constellation the user position can be solved. However the equations required for solving the user position turn out to be nonlinear simultaneous equations which are difficult to solve directly. In addition some practical considerations i.e. the inaccuracy of the user clock will be included in these equations. These equations are solved through a linearization and iteration method. The solution is in a Cartesian coordinate system and the result will be converted into a spherical coordinate system. However the earth is not a perfect sphere therefore once the user position is found the shape of the earth must be taken into consideration. The user position is then translated into the earth-based coordinate system. Finally the selection of satellites to obtain better user position accuracy and the dilution of precision will be discussed. 2.2 GPS PERFORMANCE REQUIREMENTS 1 2 3 Some of the performance requirements are listed below 1. The user position root mean square rms error should be 10-30 m. 2. It should be applicable to real-time navigation for all users including the high-dynamics user such as in high-speed aircraft with flexible maneuverability. 3. It should have worldwide coverage. Thus in order to cover the polar regions the satellites must be in inclined orbits. 7 8 BASIC GPS CONCEPT 4. The transmitted signals should tolerate to some degree intentional and unintentional interference. For example the harmonics from some narrow-band signals should not .