GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 315

is correlated with sections 2 and 3 of the input signal. Finally, section 200 of
C/A code is correlated with sections 200 and 1 of the input signal. Overall, 200
125-point output data are obtained.
The C/A code is repeated every ms and there are only 20 (not 200) 250 point
C/A codes in one ms. Since the correlation is only needed to perform in one
ms, the preceding operation must be repeated 19 more times. These additional
operations are shown in Figure 12.7. In this figure the operation is close to that
shown in Figure 12.6. The only difference is that the C/A is shifted one section
each time. In this figure the first two shifts and the last shift are shown. Because
each operation generates 125 of the 200-point outputs, the total output is a 2500
(20 × 125) by 200-point matrix. Both the row and column of the matrix are in
the time domain.
FFT is performed on the output data to obtain the frequency information. A
200-point FFT is performed 2500 times, and the results are 2500 by 200 outputs,
which are the same results as shown in Figure 12.4. The amplitude of the outputs
is obtained from the real and imaginary components. There are 200 frequency
bins and 2500 time bins. These outputs are searched to find the highest value. If
the highest output also crosses a certain threshold, the acquisition is complete.
If the output does not cross the threshold, noncoherent integration is needed to
improve the S / N .
From the 200-point FFT one can obtain 200 frequency bins, and each one
is separated by 100 Hz. For signals whose frequency falls in the center of two

section 1 section 2 section 3 section k section 200

250 pts

Input signal in time domain

section 2 section 3 section 4 section k+1 section 1

C/A code

125 pt 125 pt 125 pt 125 pt 125 pt

section 1 section 2 section 3 section k section 200

250 pts

Input signal in time domain

section 3 section 4 section 5 section k+2 section 2

C/A code

125 pt 125 pt 125 pt 125 pt 125 pt

section 1 section 2 section 3 section k section 200

250 pts

Input signal in time domain

section 20 section 1 section 2 section k-1 section 19

C/A code

125 pt 125 pt 125 pt 125 pt 125 pt

FIGURE 12.7 Correlation with C/A code shifted.