GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 72

be divided into 37 different P codes and each satellite can use a different portion
of the code. There are a total of 32 satellite identification numbers although only
24 of them are in the orbit. Five of the P code signals (33 - 37) are reserved for
other uses such as ground transmission. In order to perform acquisition on the
signal, the time of the week must be known very accurately. Usually this time
is found from the C/A code signal that will be discussed in the next section.
The navigation data rate carried by the P code through phase modulation is at a
50 Hz rate.
5.5 C/A CODE AND DATA FORMAT ( 1,2,5 )
The C/A code is a bi-phase modulated signal with a chip rate of 1.023 MHz. There-
fore, the null-to-null bandwidth of the main lobe of the spectrum is 2.046 MHz.
Each chip is about 977.5 ns (1/1.023 MHz) long. The transmitting bandwidth of
the GPS satellite in the L1 frequency is approximately 20 MHz to accommodate
the P code signal; therefore, the C/A code transmitted contains the main lobe and
several sidelobes. The total code period contains 1,023 chips. With a chip rate of
1.023 MHz, 1,023 chips last 1 ms; therefore, the C/A code is 1 ms long. This code
repeats itself every millisecond. The spectrum of a C/A code is shown in Figure 5.2.
In order to find the beginning of a C/A code in the received signal only a
very limited data record is needed such as 1 ms. If there is no Doppler effect
FIGURE 5.2 Spectrum of a C/A code.