GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 68

CHAPTERFIVE

GPS C/A Code Signal Structure

5.1 INTRODUCTION ( 1,2 )

In the previous chapters user positions are calculated. In order to perform the user
position calculation, the positions of the satellites and pseudoranges to the satel-
lites must be measured. Many parameters are required to calculate the positions
of the satellites and they are transmitted in the satellite signals.
This chapter provides the details associated with the GPS signals. Spilker ( 1 , 2 )
not only gives a very good discussion on the signal, it also gives the reasons these
signals are selected. The discussion in this chapter is limited to the fundamentals
of the signals, such that a receiver design can be based on the signals.
There are basically two types of signals: the coarse (or clear)/acquisition (C/A)
and the precision (P) codes. The actual P code is not directly transmitted by the
satellite, but it is modified by a Y code, which is often referred to as the P(Y)
code. The P(Y) code is not available to civilian users and is primarily used by
the military. In other words, the P(Y) code is classified. The P(Y) code has
similar properties of the P code. In order to receive the P(Y) code, one must
have the classified code. Therefore, only the fundamentals of the P code will
be mentioned in this topic. The discussion will be focused on the C/A code. In
general, in order to acquire the P(Y) code, the C/A code is usually acquired first.
However, in some applications it is desirable to acquire the P(Y) code directly,
which is known as direct Y acquisition.
The radio frequency (RF) of the C/A code will be presented first, then the C/A
code. The generation of the C/A code and its properties will be presented because
they are related closely to acquiring and tracking the GPS signals. Finally, the