GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 78

of Table 5.3 as inputs and checks the time delay. If the correct data are used as
inputs, the output will show “OK,” otherwise, it will show “not match.”
A program (p5 2) can be used to generate the C/A code. The program is an
extension of the program (p5 1) to include the two maximum-length sequence
generators. In the program, the delay time listed in Table 5.3 is used as input to
generate the G2 signal rather than using the code phase selections in column 3.
The first 10 bits of the generated C/A code should be compared with the result
listed in the last column of Table 5.3.

5.7 CORRELATION PROPERTIES OF C/A CODE ( 1,2 )

One of the most important properties of the C/A codes is their correlation result.
High autocorrelation peak and low cross-correlation peaks can provide a wide
dynamic range for signal acquisition. In order to detect a weak signal in the
presence of strong signals, the autocorrelation peak of the weak signal must be
stronger than the cross-correlation peaks from the strong signals. If the codes are
orthogonal, the cross correlations will be zero. However, the Gold codes are not
orthogonal but near orthogonal, implying that the cross correlations are not zero
but have small values.
The cross correlation of the Gold code is listed in Table 5.4. ( 1 )
For the C/A code n = even = 10, thus, P = 1023. Using the relations in this
table, the cross-correlation values are: − 65/1023 (occurrence 12.5%), − 1/1023
(75%), and 63/1023 (12.5%). The autocorrelation of the C/A codes of satellite
19 and the cross correlation of satellites 19 and 31 are shown in Figures 5.6a
and 5.6b respectively. These satellites are arbitrarily chosen.

TABLE 5.4 Cross Correlation of Gold Code

Number of Shift
Register Stages

Normalized Cross
Correlation Level

Probability of
Level

Code Period

2 (n + 1 )/ 2

+ 1

−

0.25

P

1
P

= 2 n P − 1 n = odd − 0.5

2 (n + 2 )/ 2 − 1

0.24

P

2 (n + 2 )/ 2 + 1

− 0.125

P

1
P

= 2 n

P − 1 n = even − 0.75

2 (n + 2 )/ 2

− 1

0.125

P