GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 79

FIGURE 5.6 Auto and cross correlation of C/A code.

In Figure 5.6a, the maximum of the autocorrelation peak is 1023, which equals
the C/A code length. The position of the maximum peak is deliberately shifted
to the center of the figure for a clear view. The rest of the correlation has three
values 63, − 1, and − 65. The cross-correlation shown in Figure 5.6b also has

three values 63, 65.

These are the values calculated by using equations in Table 5.4. The dif-
ference between the maximum of the autocorrelation to the cross correlation
determines the processing gain of the signal. In order to generate these figures,
the outputs from the C/A code generator must be 1 and

− 1, −

1, rather than 1 and
0. The mathematical operation to generate these figures will be discussed in the
Section 7.7.

−

5.8 NAVIGATION DATA BITS ( 2,3,7 )

The C/A code is a bi-phase coded signal which changes the carrier phase between
0and π at a rate of 1.023 MHz. The navigation data bit is also bi-phase code,
but its rate is only 50 Hz, or each data bit is 20 ms long. Since the C/A code
is 1 ms, there are 20 C/A codes in one data bit. Thus, in one data bit all 20
C/A codes have the same phase. If there is a phase transition due to the data bit,
the phases of the two adjacent C/A codes are different by π . This information

is important in signal acquisition. One can perform signal acquisition on two

±