GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 314

section 1

section 2

Input data 500 points

FFT

Frequency domain

Keep first 250 points

Keep first 125 points

Multiply

Keep first 250 points and take complex conjugate

Frequency domain

FFT

C/A code 500 points

section 1 zeros

FIGURE 12.5 Operation of 250-point C/A code and 500 input data.

The C/A code only has values in the first half, since the second half of the C/A
code is padded with zero. Therefore the output only has information in the first
half. Thus, in all, only 125 points of data are kept.
In effect, one uses 250 points of C/A code to generate 125 points of output
data. This is equivalent to sliding 250 points of the C/A code over the input data
in two-point steps. At each step the C/A multiplies the input data, which are
summed into one point.
The same operation is performed on the rest of the input data (the remaining
199 sections), as shown in Figure 12.6. In this figure, section 1 of C/A code
is correlated with sections 1 and 2 of the input signal. Section 2 of C/A code

section 1 section 2 section 3 section k section 200

250 pts Input signal in time domain

section 1 section 2 section 3 section k section 200

C/A code

125 pt 125 pt 125 pt 125 pt 125 pt

1

2 3 200

FIGURE 12.6 Correlation of the first 250 points of the input and the C/A code.