Global Positioning Systems, Inertial Navigation, and Integration, Second Edition

3

SIGNAL CHARACTERISTICS ANDINFORMATION EXTRACTION

Why is the GPS signal so complex? GPS was designed to be readily accessible to millions of military and civilian users. Therefore, it is a receive-only passive system for a user, and the number of users that can simultaneously use the system is unlimited. Because there are many functions that must be performed, the GPS signal has a rather complex structure. As a consequence, there is a correspondingly complex sequence of operations that a GPS receiver must carry out in order to extract desired information from the signal. In this chapter we characterize the signal mathematically, describe the purposes and properties of the important signal components, and discuss generic methods for extracting information from these components.

3.1 MATHEMATICAL SIGNAL WAVEFORM MODELS

Each GPS satellite simultaneously transmits on two L-band frequencies denoted by L₁ and L₂, which are 1575.42 and 1227.60 MHz, respectively. The carrier of the L₁ signal consists of an in-phase and a quadrature-phase component. The in-phase component is biphase modulated by a 50-bps (bits per second) data stream and a pseudorandom code called the C/A-code consisting of a 1023-chip sequence that has a period of 1 ms and a chipping rate of 1.023 MHz. The quadrature-phase component is also biphase modulated by the same 50-bps (bits per second) data stream but with a different pseudorandom code called the P-code, which has a 10.23-MHz chipping rate and a one-week period. ...