Basic GPS Concept - Fundamentals of Global Positioning System Receivers: A Software Approach - page 7

CHAPTERTWO

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. 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 )

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.