Acquisition of Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 268

1 1

Inputs

0 0

− 1 − 1

0 0 0 0 0 0 20 40 60 80

(a) Input with transition.

(b) Input without transition.

2

Reference

1

(c) Reference without transition.

0

0 10 20 30 40

40
20

40
20

0 0

Outputs

− 20
−

− 20
− 40 40

0 10 20 30 40 0 10 20 30 40

(d) Output with transition. (e) Output without transition.

FIGURE 10.23 Correlation results with no transition in reference.

Output with data transition Output without data transition

40 40

20 20

0 0

− 20 − 20

40 40

− −

0 10 20 30 40 0 10 20 30 40

(a) Input with transition. (b) Input without transition.

FIGURE 10.24 Combined outputs from Figures 10.22 and 10.23.

The difference of the two correlation peaks is 40. Similar results are shown
in Figure 10.23. In this figure the reference does not contain a phase transition.
These outputs will be used with the outputs from Figure 10.22 to determine
if a phase transition exists. Figure 10.24 is obtained by subtracting the absolute
values of the Figure 10.23 outputs from the absolute values of Figure 10.22.
When there is a transition in the data, the peak output is 40; otherwise, 40 and

the difference is 80. Compared with the outputs from Figure 10.22 the difference
is doubled. Thus the detection of a data transition can be improved.

−