TABLE OF CONTENTS The last few years there has been increase interest in the commercialization of Global Navigation Satellite Systems (GNSS), which are also often referred to as Global Positioning Systems (GPS) in various applications. A GPS system uses spread spectrum signals (BPSK) emitted from satellites in space for position and time determination. Until recently, the use of GPS was essentially reserved for military use. Now there is great interest in using GPS systems for navigation of commercial aircraft. The FAA is looking into the possibility of using a wide area augmentation system (WAAS) to cover the whole United States with one system. There are also applications in the automotive industry, surveying, and personal use for hikers and recreationalists. Due to the increase in popularity of GPS, and since it is a spread spectrum communication system using BPSK, a brief introduction is included in this text.
The NAVSTAR GPS satellite transmits a direct sequence BPSK signal at a rate of 1.023 Mbps using a code length of 1023 bits. The time between code repetition is 1 ms. This is known as the coarse/acquisition (C/A) code which is used by the military for acquisition of a much longer precision code (P-code). The commercial industry uses the C/A code for the majority of the applications. There are 36 different C/A codes that are used with GPS which are generated by modulo-2 adding the C/A code with a different delayed version of the same C/A code (see Figure 9-1).
